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Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia, Argentina
Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina Frankie D. Jackson, James G. Schmitt, Sara E. Oser PII: DOI: Reference:
S0031-0182(13)00273-3 doi: 10.1016/j.palaeo.2013.05.031 PALAEO 6520
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date: Revised date: Accepted date:
20 February 2013 21 May 2013 23 May 2013
Please cite this article as: Jackson, Frankie D., Schmitt, James G., Oser, Sara E., Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.05.031
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ACCEPTED MANUSCRIPT Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina
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Frankie D. Jackson*, James G. Schmitt, Sara E. Oser
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Department of Earth Sciences, Montana State University, 226 Traphagen Hall, Bozeman,
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Montana, 59717, USA.
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*Corresponding author: Tel.: +1 406 994 6642; fax: +1 406 994 6923. E-mail addresses: [email protected] (F. Jackson); [email protected] (J. Schmitt); [email protected] (S. Oser)
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ACCEPTED MANUSCRIPT ABSTRACT At the Auca Mahuevo locality in the Upper Cretaceous Anacleto Formation in Patagonia,
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Argentina pedogenic processes associated with Vertisol development affected changes in both
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individual titanosaur egg morphology and three-dimensional egg distribution. These changes resulted primarily from vertical and lateral movement within fluvial overbank sediments due to
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clay mineral expansion and contraction in alternating wet and dry seasonal conditions. At the
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scale of individual sauropod eggs, pedogenic sediment movement produced egg shearing, eggshell fracture and displacement, mechanical abrasion of egg ornamentation, and alteration of
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egg size and shape. Movement of either individual eggs or subsets of eggs along slickensided surfaces (1) modified the number and relative position of eggs within individual clutches, (2)
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combined eggs of one of more clutches produced by different females, and (3) combined eggs from one or more nesting horizons, producing a time-averaged fossil assemblage. These
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mechanisms of egg rearrangement suggest that accurate interpretation of dinosaur reproductive behavior using fossil egg assemblages preserved in fine-grained fluvial overbank deposits require careful assessment of pedogenic processes.
Keywords: sauropod eggs, Auca Mahuevo, paleo-Vertisols
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ACCEPTED MANUSCRIPT 1. Introduction
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In alluvial basins episodic flood events produce thick fine-grained overbank sedimentary
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sequences that comprise a volumetrically dominant and important component of many fluvial systems (Kraus, 1987). Only major flood events typically deposit more than a few centimeters of
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sediment and therefore sediment accumulation rates on the floodplain are commonly low and
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result primarily from vertical accretion through suspension settling of fine grained materials (e.g. Walling et al., 1992; Aalto et al., 2003; Pizzuto et al., 2008). As such, these fluvial overbank
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sequences often provide an excellent record of post-depositional pedogenesis, with paleosols better preserved during periods of slower subsidence or at greater distances from active channels
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(Kraus, 1999). Furthermore, floodplain environments are subject to relatively low rates of erosion beyond that caused by laterally migrating or incising fluvial channels and, therefore, the
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overprint of pedogenic processes is recorded with high resolution (Kraus, 1999). The resulting paleosols provide important information relative to rates of basin subsidence, paleoclimate, and paleoecology of floodplain ecosystems (Retallack, 1990) Many dinosaur eggs are preserved in pedogenically modified fine-grained fluvial floodplain deposits that record episodic flooding in overbank environments (Varricchio et al., 1999; Chiappe et al., 1999; Sander et al., 1998; 2008, Lopez-Martinez, 1999; Bravo et al., 2000; Cojan et al., 2002; Therrien, 2005; Jackson, 2007; Jackson et al., 2008; Grigorescu and Csiki, 2008; Vila et al., 2010a). In addition, in situ embryos are occasionally preserved within eggs when water and fine-grained sediment fill the pores that traverse the eggshell, prohibiting gas exchange between the embryo and atmosphere. Inundation of these nesting grounds by floodwaters also leads to further interment of eggs and clutches by mud settling from suspension 3
ACCEPTED MANUSCRIPT (e.g. Chiappe et al., 2004). These deposits provide new surfaces for colonization by plants and animals that contribute to subsequent soil development. However, taphonomic studies of
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stratigraphic intervals containing dinosaur eggs typically focus on egg and clutch distribution,
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mode of incubation, and nest structure, while often disregarding pedogenic processes that may influence site taphonomy. As a consequence, the taphonomic role of pedogenesis in post-
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depositional modification of egg-bearing strata remains poorly documented.
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The Upper Cretaceous (Campanian) Anacleto Formation of the Neuquén basin, Argentina represents one of the best preserved and extensive dinosaur egg- and clutch-bearing
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units in the fossil record (Fig. 1). A series of papers describe the Auca Mahuevo locality, including sedimentology and stratigraphy (Dingus et al., 2000, 2009; Garrido, 2010), sauropod
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embryonic remains (Chiappe et al., 1998, 2001; Salgado et al., 2005), eggshell microstructure (Grellet-Tinner et al., 2004, 2006), excavated nests preserved as trace fossils (Chiappe et al.,
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2004), reproductive behavior (Chiappe et al., 1999, 2004, 2005), and other aspects of sauropod biology (Jackson et al., 2004, 2008; Grellet-Tinner, 2005; Grellet-Tinner et al., 2006; Jackson, 2007; Schweitzer et al., 2005; Garcia and Cerda, 2010). These sauropod egg-bearing deposits were subject to Vertisol development, characterized by episodic periods of saturation and drying of the substrate (Chiappe et al., 1998; Loope et al., 2000; Chiappe and Dingus, 2001; Jackson, 2007), leading to extensive vertical and lateral movement of sediments, driven by shrinking and swelling of clay minerals during soil development. The relatively extreme nature of material displacement typical of Vertisols in general, combined with the well-documented distribution of sauropod egg-bearing layers in the Anacleto Formation, provides an opportunity to examine the role of Vertisol development in affecting changes in sauropod egg distribution within a developing soil. Here, we document specifically the role of physical soil processes on individual 4
ACCEPTED MANUSCRIPT eggs and clutches, and the resulting influence of pedogenesis on interpretation of dinosaur
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reproductive biology.
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2. Geology
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The Auca Mahuevo locality lies approximately mid-way between Neuquén and Rincón
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de Los Sauces, just west of Highway 8 in the northeast corner of Neuquén Province, Argentina (Fig. 1). The 86-m thick section exposed at the Auca Mahuevo locality consists of Upper
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Cretaceous terrestrial fluvial deposits of the Anacleto Formation (Dingus et al., 2000; 2009; Garrido, 2010) (Fig. 1A). The contact with the underlying Bajo de la Carpa Formation (the lower
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formation in the Río Colorado Subgroup) is absent at Auca Mahuevo, although these outcrops are exposed about 10 km to the west (Dingus et al., 2009). An erosional contact separates the
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Anacleto Formation from the overlying 10 m-thick, estuarine and shallow marine deposits of Allen Formation (Ardolino and Franchi, 1996). At the Auca Mahuevo study area the Anacleto Formation consists primarily of reddish brown sandstone, siltstone, and mudstone. Four titanosaur egg-bearing stratigraphic intervals (termed “egg beds” in Chiappe et al., 1998, 1999) are present within the finer-grained siltstone and mudstone intervals (Dingus et al., 2000, 2009). Garrido (2010) interpreted these mudrocks and associated sandstones as fine-grained, mixedload, meandering fluvial floodplain and channel deposits, respectively, developed under semiarid climatic conditions with well-differentiated wet and dry seasons. Most egg clutches are preserved in levee and proximal overbank facies. In these areas rare flooding events during the rainy season covered extensive areas with muddy standing water from which silt and clay settled from suspension (Dingus et al., 2000, 2009; Chiappe et al., 1999; Garrido, 2010). 5
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3. Materials and methods
At least four egg-bearing layers (numbered 1–4, from stratigraphically lowest to highest)
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are present at the Auca Mahuevo locality (Fig. 1A). The quarry and adjacent “flats” of egg bed 3
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have yielded abundant fossil eggs containing titanosaur embryonic bone and skin (Chiappe et al.,
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1998, 1999, 2004; Grellet-Tinner et al., 2005; Garcia, 2007; Schweitzer et al., 2005). The study reported here focuses on a quarry excavated in egg bed 3 because the mudrock provides
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abundant evidence of Vertisol development (Loope et al., 2000; Chiappe and Dingus, 2001). Strike and dip of the strata and slickensides, and trend and plunge of slickenlines were measured
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with a Brunton compass. The low stratal dip (3°) requires no correction to the original bed orientation to accurately orient features. To document these paleosol features, the eggs within a
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27 m2 area were mapped using a metric grid and graph paper, with the azimuth of slickenlines recorded on the map. Measurements of the distance and depth of eggs, relative to a datum established adjacent to the quarry, were recorded in order to determine their location stratigraphically and within the grid. These measurements allowed 3-D reconstruction of egg positions, using MATHLABS (Mathworks, 2011; see also Storrs et al., in press). Taphonomic data (e.g., presence of embryonic remains, egg physical attributes, relationship of eggs and clutches to pedogenic features) were also noted and specimens photographed during excavation. X-ray diffraction was used to identify bulk and clay mineral composition in six mudrock matrix samples (three from egg bed 1 and three from egg bed 3). The samples were finely ground into powder using a mortar and pestle, and passed through a 210-mesh (63 µm) sieve. The samples were analyzed using an XGen-4000 x-ray diffractometer at 1 kV with CuKα 6
ACCEPTED MANUSCRIPT radiation (λ= 1.5418 Å) and acceleration voltage of 1 kV. Data were collected at 2θ values ranging from 15°to 75° for bulk mineral identification and 3° to 18° for clay mineral analysis.
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Clay minerals were separated from calcite with glacial acetic acid, and then mounted as an
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oriented aggregate mount so that the incident x-ray beam was directed along the crystallographic z-axis of the minerals. A series of treatments including air drying, glycolation with ethylene
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glycol, and heating to 400°C for overnight were used to ascertain the presence of expandable
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layer clay minerals.
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4. Description
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The reddish to reddish-brown color (10R 5/4) of the siliciclastic mudstone exposed in egg bed 3 is relatively uniform throughout the study area (Fig. 1), and clay mineral assemblages
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identified by x-ray diffraction are consistent with the presence of illite and mixed layer illite/montmorillonite. This mudstone and that of the stratigraphically higher egg bed 4 contain abundant slickensides, whereas these features are absent in underlying egg beds 1 and 2 (Dingus et al., 2009: fig. 7). These polished, randomly oriented slickensides served as surfaces for slickenfibre (mineral) growth, primarily calcite or fibrous gypsum crystals (Fig. 2A). Exposures of the fine-grained massive mudstone in the egg bed 3 quarry occasionally exhibit medium subangular to angular blocky peds from two to several centimeters in diameter. These aggregations of soil particles separate along planes of weakness, giving a hackley appearance to the quarry walls. Compaction and absence of obvious mechanically infiltrated or chemically precipitated cutans around the peds contribute to the difficulty of their identification. Abundant drab, blue-gray, irregular mottling occurs throughout the profile and includes spots (sensu 7
ACCEPTED MANUSCRIPT Retallack, 1990) with sharp or diffuse borders up to 8 cm in diameter (Fig. 2B) and cylindrical tubules (~ 1.0 to 2.5 cm diameter) that sometimes taper downward. A thin ( 5 cm) lens of fine-
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grained reddish-tan sandstone is intercalculated with mudstone in the egg bed 3 quarry. This
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sandstone exhibits faint relic trough cross lamination and moderately abundant root traces,
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including a 1.5 cm cylindrical root cast abruptly truncated by an overlying egg-bearing stratum (Fig. 2 C, D). Although rare, a shallow, v-shaped fissure (1–1.5 cm wide) in-filled by slightly
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coarser, lighter colored reddish brown mudstone is also present within the quarry. A 10–15 cmthick bed occurs at the stratigraphically lowest excavated level within the quarry and includes a
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zone of pale green mudstone containing red, thread-like branching root traces. These rootlets are progressively more abundant upward within the bed, ending abruptly at the base of eggs and
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eggshell debris.
Fractures in the quarry wall often contain white fibrous gypsum crystals and display drab
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blue-gray “haloes”. In cross section, the haloes also surround many eggs and extend into the red mudstone filling the egg interior (Fig. 2E). The sediment size and texture of these reduced zones are identical to the surrounding red mudstone. Some slickensides cut and displace portions of eggs by approximately 3–5 cm along sub-horizontal and sub-vertical surfaces (Fig. 2F, G). However, other slickensides circumvent eggs and exhibit preferentially oriented, blue-gray clay particles; the tuberculate ornamentation of eggs associated with these slickensides is often sheared in the same direction as the clay particles. In rare cases, weathered eggs display slickensides within the mudstone that fills the egg interior. Whereas some eggs are relatively intact and spherical, most are moderately to highly compressed, with the latter displaying substantial distortion in shape (Figs. 2B, D; 3A, B). Although eggshells often remain intact, the upper surfaces (relative to the bedding plane) of some highly compressed eggs are flat or 8
ACCEPTED MANUSCRIPT concave-in (Fig. 2 C, D), whereas others eggs are elongate and ellipsoid in shape (Figs. 3B). These highly modified eggs often occur adjacent to eggs that are significantly less distorted (Fig.
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3B).
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Microscopic examination of radial thin sections of eggshell often reveals highly abraded surface ornamentation and diagonal fractures that cut across accretion lines with minimal offset
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of these features. Other eggshell fragments show more complex and irregular fracturing (Fig.
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3C). The inner portion of the eggshell exhibits substantial dissolution, with secondary calcite precipitated between nuclei located on the outer surface of the permineralized membrane.
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Large egg clusters in some areas of the quarry contain more than 50 eggs (Fig. 4). Depth measurements relative to the datum show that these and other areas of the quarry possess
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significant topographic relief (Fig. 4B, C). Eggs containing embryonic bones in these clusters are sometimes adjacent to eggs lacking such remains (Fig. 4A). In some areas of the quarry
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concave-up, low-angle curved planes intersect at opposite angles (Figs. 5A). The resulting pattern of microhighs and microlows (gilgai), are approximately 1.0 to 1.5 meters wide (Figs. 3A, 5A). Microlows form bowl-shaped depressions, surrounded by slickensided surfaces; these depressions often contain small clusters of eggs (Fig. 3B). The maximum height of the microhigh is about 45 cm above the microlow (Figs. 3A, 5A). With some exceptions (e.g., two eggs on left in Fig. 3A), eggs located on the microhighs typically display extensive compaction, often exhibiting a total thickness of only 1–3 cm (Fig. 5B).
5. Discussion
5.1 Modern Vertisol development 9
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Modern Vertisols typically develop under dry to moist soil conditions spanning a broad
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range of mean annual soil temperatures (4°–24°C). An abundance of expandable layer clay
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minerals and wet-dry seasonality strongly control their distribution (Brady and Weil, 2002, p. 67). Under some conditions, a few hundred years provides adequate time for Vertisol
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development (Coulombe et al., 1996). The high shrink-swell susceptibility, driven by the
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presence of expandable smectitic clay minerals (mixed-layer illite/montmorillonite), serves as a major control on the high degree of pedoturbation characterizing modern Vertisols (Nordt et al.,
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2004). Unlike bioturbation by plant roots and burrowing vertebrates and invertebrates, Vertisol pedoturbation is capable of producing displacement of material over distances ranging from 10
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cm to as much as 1 m (Brady and Weil, 2002). In addition, development of slickenside-bounded blocks of soil material is facilitated by dilation as the dominant strain force (Nordt et al., 2004).
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Under dry conditions, loss of interlayer water in expandable layer smectitic clays leads to tension and development of open fractures propagating downward from the surface, resulting in topographic microlows at the land surface (Brady and Weil, 2010, p. 76–77). These open fractures collect material (soil debris, sediment, organic matter) that falls downward to lower depths within the developing soil. During wet periods, smectitic clays absorb water into interlayer sites, swelling in the process and facilitating upward movement of material between fractures and shear forces producing slickensides with striated surfaces (slickenlines). Upward movement of material between vertical fractures results in topographic microhighs at the land surface. The relatively large magnitude of vertical and lateral material movement in Vertisols through clay mineral shrinking and swelling makes them problematic for construction and agricultural activities. 10
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5.2 Auca Mahuevo paleo-Vertisols
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Vertisols likely characterized many alluvial plains in ancient retro-arc foreland basin settings, where rain-shadow effects and episodic explosive volcanism (e.g. Ver Straeten, 2004)
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provided both the environmental conditions (seasonally wet-dry) (e.g. Fricke et al., 2010) and
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expandable-layer clay minerals necessary to facilitate their development (e.g. Yerima et al., 1987). As recorded in the Anacleto Formation, sauropods extensively used this type of
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floodplain setting for nest construction and egg laying. Infrequent flooding of overbank areas inundated the Auca Mahuevo titanosaur nesting grounds, leading to suspension settling of
2004; Dingus et al., 2009).
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muddy sediment. This in turn facilitated egg and nest burial and preservation (Chiappe et al.,
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Vertisol development that occurred in fine-grained floodplain sediments containing sauropod egg clutches had great potential to affect both the morphology of individual eggs and their distribution within the soil profile during pedogenesis. In simple terms, Vertisol processes at Auca Mahuevo disrupted or over-printed the biological pattern produced by the egg-laying titanosaurs. Mapping of eggs using x, y and z coordinates from a fixed datum reveals substantial topographic relief in the egg bed 3 quarry that is not apparent in conventional two dimensional maps (Fig. 4). Pedogenic processes associated with Vertisol development affected changes in both individual egg condition and the three-dimensional distribution of eggs (Fig. 6). These changes resulted from two physical processes that characterize Vertisols generally and effected the overbank deposits of the Anacleto Formation in particular: 1) lateral movement of material (eggs and sediment) as a result of clay mineral expansion and contraction over time, and 2) 11
ACCEPTED MANUSCRIPT vertical mixing of those materials due to development of tension fractures and fracture wall collapse during phases of wetting and drying.
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At the scale of individual sauropod eggs, pedogenic sediment movement resulted in egg
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and eggshell fracture and displacement, mechanical abrasion of egg ornamentation, alteration of egg size and shape, and the generally poor quality of preservation (Figs. 2, 3, 5B). These changes
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in morphology can have important implications for fossil egg parataxonomic assignments and
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inferences of sauropod reproductive biology. For example, eggshell thickness, and egg size, shape, and ornamentation are used to assign fossil eggs to parataxonomic categories; therefore,
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pedogenic processes that alter these attributes may lead to incorrect interpretations and proliferation of new oospecies. In addition, changes in egg size and shape, and eggshell thickness
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(due to compaction, dissolution, or abrasion resulting from sheer stress; Fig. 3) also impact estimates of egg volume, surface area, and pore length used in calculations of water vapor
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conductance rates and interpretation of egg incubation strategies (Jackson et al., 2008). Movement of either individual eggs or subsets of eggs along slickensides also affected the biological pattern of sauropod egg and clutch distribution at the Auca Mahuevo locality. This movement, characterized by both lateral and vertical components of displacement, (1) modified the number and relative position of eggs within individual clutches, (2) combined eggs of one or more clutches, and (3) combined eggs from different nesting horizons, producing a timeaveraged assemblage (Figs. 4, 6). For this reason, we define an egg clutch as an accumulation of eggs produced by the female dinosaur, distinguishing these from egg clusters that result from pedogenic modification of the original egg pattern.
5.3 Impact on behavioral inferences 12
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Alterations to the biological pattern produced by the adult sauropod during nest
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construction and egg laying contribute to the difficulty of interpreting reproductive behavior. For
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example, large well-defined clutches (~ 30 eggs) at the Auca Mahuevo locality occur within a 70 cm-thick stratum, suggesting a single nesting event (Chiappe et al., 1999). However, some egg
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clusters in the quarry contain more than 50 eggs (Fig. 4). Eggs containing embryonic remains in
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these large clusters are sometimes adjacent to those lacking fossil bones (Fig. 4A). In addition, some embryos are clearly larger than others, by as much as 25% (Chiappe et al., 2005). Two
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possible explanations include differential preservation of embryonic remains and individual variation among eggs of the same or different clutches; however, a time-averaged fossil
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assemblage represents a third possibility.
Trace fossil nests provide important information about the reproductive behavior of
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extinct animals, yet are rarely reported from overbank facies (Varricchio et al., 1999; Chiappe et al., 2004). With the exception of six nesting traces preserved in sandstone (channel and crevasse splay deposits) in egg bed 4, thousands of eggs at Auca Mahuevo occur in mudstone and show no discernible evidence of nest structure (Chiappe et al., 2004). Significant lateral and vertical displacement of sediment and eggs within the Vertisol horizon over hundreds to thousands of years likely accounts for the absence of trace fossil nests in the mudstone facies. In egg bed 3 these pedoturbation processes destroyed nearly all primary sedimentary structures, producing a homogenous substrate. However, the presence of relic bedding and root horizons that terminate immediately below eggs in an otherwise homogenous mudrock may assist in identification of a single nesting event (Fig. 2C, D).
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ACCEPTED MANUSCRIPT In the absence of a well-preserved trace fossil nest structure, inferences of nest construction and mode of egg incubation often rely on less reliable evidence. For example, clutch
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geometry is used to infer a bowl-shaped depression (Erben et al., 1979; Kérourio, 1981; Cousin
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et al., 1994; Cousin and Breton, 1999; Sander et al., 2008; Vila et al., 2010a,b). However, bowlshaped microlows are characteristic of modern Vertisols (Brady and Weil, 2002, p. 77). At Auca
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Mahuevo these microlows sometimes contain one or more relatively intact eggs (Fig. 3B),
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whereas eggs on the microhighs typically show greater compaction and eggshell fragmentation (Fig. 5B). Weathering and loss of these eggs on the microhighs, in conjunction with differential
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preservation of more intact eggs in the bowl-shaped microlow depressions increases their resemblance to a fossil nest containing eggs. The number of eggs in these pseudo nests, however,
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may be substantially fewer than originally produced by the female sauropod. For example, the cluster in Figure 3B contains 9–10 eggs, whereas well-delineated clutches in the mudstone facies
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and trace fossil nests preserved in sandstone contain 30–35. This underestimation of clutch size in turn may lead to incorrect estimates of sauropod reproductive output that compare clutch size to adult body size. Similarly, egg arrangements (circular/conical, arcs, and multiple or single egg levels) are often interpreted as having biological significance (Moratalla and Powell 1990 and references therein); however, Vila et al. (2010b) note that erosion may also account for these differences in megaloolithid egg distribution patterns.
6. Summary and conclusions
Stratigraphic units containing fossil eggs provide rare opportunities to enhance our understanding of dinosaur reproductive biology, including adult nesting strategy, mode of egg 14
ACCEPTED MANUSCRIPT incubation, parental care of young, and other behaviors. However, shrinking and swelling of expandable layer clay minerals during alternating wet and dry seasons at the Auca Mahuevo
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sauropod nesting site facilitated pedogenic movement within the egg-bearing horizons. This
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pedoturbation modified or combined eggs from different clutches and different temporal intervals, resulting in time-averaging of nests or nesting horizons. The resulting pedogenically-
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formed egg clusters may not accurately portray the egg size and shape, number of eggs per
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clutch, clutch morphology or egg distribution produced by individual female sauropods. For more accurate inferences concerning dinosaur reproductive biology, it is imperative
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to ascertain the degree to which post-burial pedogenic modification has altered the distribution and condition of eggs preserved in fine-grained sedimentary sequences. Investigation should
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include assessment of features that characterize undisturbed, in situ egg clutches and those altered by subsequent pedoturbation. Most dinosaur nests were likely constructed in floodplain
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environments where sedimentation favors deposition by vertical accretion of silt and clay with minimal textural contrast, making nest structure recognition difficult. In addition, the persistence and widespread distribution of paleo-Vertisols in the sedimentary rock record (see Nordt et al., 2004) suggests that many egg-bearing units may have been affected by the physical processes characterizing Vertisol development. With this in mind, we suggest that all investigations of dinosaur egg-bearing strata include the following: (1) strike and dip of beds (2) detailed three dimensional mapping of egg distribution during excavation (3) identification and mapping of all planar features, including both structural and pedogenic surfaces of displacement and(or) mineral growth
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ACCEPTED MANUSCRIPT (4) careful examination of both individual and groups of eggs to assess potential relations between deformation features and physical condition of the eggs
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(5) documentation of all pedogenic features
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(6) careful attention to the relations between fossil eggs and pedogenic features (e.g., roots and burrows, gilgai microtopography, slickensides)
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(7) detailed facies analysis aimed at deciphering the specific depositional processes
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associated with floodplain sedimentation (i.e. suspension settling of mud, traction transport of sand in crevasse-splay lobes, eolian traction transport).
clay minerals when appropriate.
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(8) x-ray diffraction analysis of mudrock to ascertain the presence of expandable layer
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(9) thin section analysis to establish whether sepic-plasmic (bright, oriented clay) microfabrics are evident in the mudrock and for assessment of eggshell fragmentation.
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We feel that the use of this methodological approach in the study of egg-bearing strata will produce more accurate inferences of adult reproductive behavior.
Acknowledgements
We thank L. Chiappe, R. Coria, and crew members of the 1999 and 2000 LACM/MCF joint expeditions. Field and laboratory research for this project was supported by the Ann and Gordon Getty Foundation, Charlotte and Walter Kohler Charitable Trust, Dirección General de Cultura de Neuquén, Fundación Antorchas, Infoquest Foundation, Municipalidad de Plaza Huincul, and National Geographic Society. Finally, we thank F. Surlyk, S. Driese and B.Vila for their helpful comments that improved the manuscript. 16
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Garcia, R.A., 2007. An “egg-tooth”-like structure in titanosaurian sauropod embryos. Journal of Vertebrate Paleontology 27, 247–252. Garcia, R.A., Cerda, I.A., 2010. Dentition and histology in titanosaurian dinosaur embryos from Upper Cretaceous of Patagonia, Argentina. Palaeontology 53, 335–346. Garrido, A., 2010. Paleoenvironment of the Auca Mahuevo and Los Barreales sauropod nesting-sites (Late Cretaceous, Neuquén Province, Argentina). Ameghiniana 47, 99– 106. Grellet-Tinner, G., Chiappe, L.M., Coria, R., 2004. Eggs of titanosaur sauropods from the Upper Cretaceous of Auca Mahuevo (Argentina). Canadian Journal of Earth Sciences 41, 949–960.
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ACCEPTED MANUSCRIPT Grellet-Tinner G., Chiappe, L.M., Norell, M., Bottjer, D., 2006. Dinosaur eggs and nesting ecology: a paleobiological investigation. Paleogeography, Paleoclimatology,
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Jackson, F.D., 2007. Titanosaur reproductive biology: comparison of the Auca Mahuevo titanosaur nesting locality (Argentina), to the Pinyes Megaloolithus nesting locality (Spain). PhD dissertation, Montana State University, Bozeman, Montana. 166 pp. Jackson, F., Varricchio, D., Jackson, R., Vila, B., Chiappe, L., 2008. Comparison of water vapor conductance of a titanosaur egg from, Argentina, with a Megaloolithus siruguei egg from Spain. Paleobiology 34, 229–246. Kérourio, P., 1981. La distribution des “Coquilles d’oeufs de Dinosauriens multistratifiees” dans le Maestrichtien continental du Sud de la France. Geobios 14, 533–536. Kraus, M., 1987. Integration of channel and floodplain suites, II. Vertical relations of alluvial paleosols. Journal of Sedimentary Petrology 57, 602–612.
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ACCEPTED MANUSCRIPT Kraus, M., 1999. Paleosols in clastic sedimentary rocks: their geologic applications. EarthScience Reviews 47, 41–70.
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Loope, D., Schmitt, J., Jackson, F., 2000. Thunder platters: thin discontinuous limestones in Late
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Cretaceous continental mudstones of Argentina may be chemically infilled sauropod tracks. Geological Society of America, Abstracts with Programs: A450.
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Lopez-Martinez, N., 1999. Eggshell sites from the Cretaceous-Tertiary transition in south
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central Pyrenees (Spain). In: Bravo, A.M., Reyes, T. (Eds.). First International Symposium on Dinosaur Eggs and Babies/ Extended Abstracts, pp. 95–115.
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MATHLAB, 2011. MathWorks. http://www.mathworks.com/products/matlab/ Moratalla, J. J., Powell, J. E., 1990. Dinosaur nesting patterns. In: Carpenter, K., Hirsch, K.F.,
pp. 37–46.
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Horner, J.R., (Eds.), Dinosaur Eggs and Babies. Cambridge University Press, Cambridge,
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Nordt, L.C., Wilding, L.P., Lynn, W.C., Crawford, C.C., 2004. Vertisol genesis in a humid climate of the coastal plain of Texas, U.S.A. Geoderma 122, 83–102. Pizzuto, J.E., Moody, J.A., Meade, R.H., 2008. Anatomy and dynamics of a floodplain, Powder River, Montana, U.S.A. Journal of Sedimentary Research 78, 16–28. Retallack, G.J., 1990. Soils of the Past. Unwin Hyman, Boston. 520 pp. Salgado, L., Coria, R., Chiappe, L., 2005. Osteology of the sauropod embryos from the Upper Cretaceous of Patagonia. Acta Palaeontologica Polonica 50, 79–92. Sander, P.M., Peitz, C., Gallemí, J., Cousin, R., 1998. Dinosaurs nesting on a red beach? C. R. Academy of Science Paris, Sciences de la Terre er des Planètes 327, 67–74.
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ACCEPTED MANUSCRIPT Sander, P.M., Peitz, C., Jackson, F.D., Chiappe, L.M., 2008. Upper Cretaceous titanosaur nesting sites and their implications for sauropod dinosaur reproductive biology.
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Palaeontographica A, 284, 69–107.
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Sanz, J.L., Moratalla, J.J., Díaz-Molina, M., López-Martínez, N., Kälin, O., Vianey-Liaud, M., 1995. Dinosaur nests at the sea shore. Nature 376, 731–732.
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Schweitzer, M.H., Chiappe, L., Garrido, A.C., Lowenstein, J.M., Pincus S.H., 2005. Molecular
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preservation in Late Cretaceous sauropod dinosaur eggshells. Proceedings of the Royal Society B 272, 775–784.
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Storrs, G.W., Oser, S.E., Aull, M., in press. Three-dimensional computed reconstruction of a Late Jurassic dinosaur bone-bed. Earth and Environmental Science Transactions of the
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Royal Society of Edinburgh, Wann Langston Jr Festschrift Special Issue. Therrien, F., 2005. Palaeoenvironments of the latest Cretaceous (Maastrichtian) dinosaurs of
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Romania: insights from fluvial deposits and paleosols of the Transylvanian and Hateg basins. Palaeogeography, Palaeoclimatology, Palaeoecology 218, 15–56. Varricchio, D.J., Jackson, F., Truman, C., 1999. A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus. Journal of Vertebrate Paleontology 19, 91–100. Ver Straeten, C.A., 2004, K-bentonites, volcanic ash preservation, and implications for Early to Middle Devonian volcanism in the Acadian orogen, eastern North America. Geological Society of America Bulletin 116, 474–489. Vila, B., Jackson, F., Galobart, A., 2010a. First data on dinosaur eggs and clutches from Pinyes locality (Upper Cretaceous, Southern Pyrenees). Ameghiniana 47, 79–87.
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ACCEPTED MANUSCRIPT Vila, B., Galobart, A., Oms, O., Poza, B., Bravo, A.M., 2010b. Assessing the nesting strategies of Late Cretaceous titanosaurs: 3D-clutch geometry from a new megaloolithid eggsite.
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Lethaia 43: 197–208.
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Walling, D.E., Quine, T.A., He, O., 1992. Investigating contemporary rates of floodplain sedimentation. In: Carling, P.A., Petts, G.E. (Eds.), Lowland Floodplain Rivers:
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Geomorphological Perspectives. Wiley, Chichester, pp. 165–184.
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Yerima, B.P.K., Wilding, L.P., Calhoun, F.G., and Hallmark, C.T., 1987, Volcanic ashinfluenced Vertisols and associated Mollisols of El Salvador: Physical, Chemical, and
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Morphological Properties: Soil Science Society of America Journal 51, 699–708.
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Fig. 1. The Auca Mahuevo locality. A) Composite stratigraphic section showing four titanosaur egg beds and osteological remains. Column width represents weathering profile. B) Map of
al. (2004).
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Neuquén Province (Argentina) indicating location of Auca Mahuevo. Modified from Chiappe et
Fig, 2. Titanosaur eggs from egg-bed 3. A) Partial egg on polished slickenside indicated by arrow. B) Titanosaur eggs draping a microhigh and additional highly compressed eggs in foreground. White arrow points to blue-gray reduction spot. Note relatively intact egg in lower right corner of photo; person for scale. C) Faint relic cross-lamination with tubule that ends abruptly beneath titanosaur egg. Note the flattened and concave surface of the overlying egg. D) Crushed egg clutch; arrow points to tubule truncated by egg in C. Black dots mark eggs; tape measure extends 9 m. E) Partial eggs with arrows indicating reduction “haloes” along tectonic fracture in the rock (upper right) and adjacent to broken edge of the egg (lower left). Lens cap for 23
ACCEPTED MANUSCRIPT scale. F) Two partial eggs sheared along slickenside. Lens cap for scale. G) Sheared but otherwise relatively intact egg hemispheres; arrow indicates direction of shear of lower half
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relative to the upper hemisphere. Egg on the left contains tubule traces and branching rootlets.
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Gypsum crystals above arrow are pervasive within fractures in the quarry.
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Fig. 3. Eggs and eggshell. A) Elongate eggs uplifted on microhigh; horizontal arrow indicates
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extremely flattened egg with a total thickness < 2 cm on far side of microlow. Slickenside visible below left vertical arrow. Compare highly flattened egg (right vertical arrow) to two eggs
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on the left. B) Bowl-shaped depression with slickensides (white arrows). Note combination of both extremely flattened eggs and mostly intact, three-dimensional specimens; 15 cm-long
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mechanical pencil for scale. C) Titanosaur eggshell with complex fracturing and abraded outer surface at the top of the image.. Note eggshell dissolution (D) between nuclei at the inner shell
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surface.
Fig. 4. Egg distribution in egg bed 3. A) Field map in plan view. B) Same view as A showing different egg depths by color. C) Cross-section view showing microtopography within the quarry.
Fig. 5. Pedogenic features in egg bed 3. A) concave-up, low-angle curved planes intersecting at opposite angles; arrows indicate concave surface. Note gleying in upper left corner of photograph; 16 cm-long chisel for scale. B) Sheared and compressed eggs; egg between arrows measures about 4 cm thick.
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ACCEPTED MANUSCRIPT Fig. 6. Conceptual model showing influence of Vertisol development on titanosaur egg distribution. A) Eggs at time 1, overlain by second clutch at time 2. Eggs at time 1 show
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evidence of displacement and shearing along slickensides. B) Both clutches at deeper burial and
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subjected to uplift along microhigh; note eggs of both clutches sheared along sickensides. C) Time-averaged clutches exposed to erosion. Illustrations modified from Brady and Weil (2002:
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ACCEPTED MANUSCRIPT Highlights: We examined the influence of vertisol development on titanosaur eggs.
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Sediment and egg movement resulted from clay mineral expansion/contraction.
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Soil movement modified the condition, number and position of eggs in clutches.
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Vertisols adversely impact interpretations of dinosaur reproductive biology.
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S0031-0182(13)00273-3 doi: 10.1016/j.palaeo.2013.05.031 PALAEO 6520
To appear in:
Palaeogeography, Palaeoclimatology, Palaeoecology
Received date: Revised date: Accepted date:
20 February 2013 21 May 2013 23 May 2013
Please cite this article as: Jackson, Frankie D., Schmitt, James G., Oser, Sara E., Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.05.031
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ACCEPTED MANUSCRIPT Influence of Vertisol development on sauropod egg taphonomy and distribution at the Auca Mahuevo locality, Patagonia Argentina
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Frankie D. Jackson*, James G. Schmitt, Sara E. Oser
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Department of Earth Sciences, Montana State University, 226 Traphagen Hall, Bozeman,
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Montana, 59717, USA.
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*Corresponding author: Tel.: +1 406 994 6642; fax: +1 406 994 6923. E-mail addresses: [email protected] (F. Jackson); [email protected] (J. Schmitt); [email protected] (S. Oser)
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ACCEPTED MANUSCRIPT ABSTRACT At the Auca Mahuevo locality in the Upper Cretaceous Anacleto Formation in Patagonia,
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Argentina pedogenic processes associated with Vertisol development affected changes in both
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individual titanosaur egg morphology and three-dimensional egg distribution. These changes resulted primarily from vertical and lateral movement within fluvial overbank sediments due to
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clay mineral expansion and contraction in alternating wet and dry seasonal conditions. At the
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scale of individual sauropod eggs, pedogenic sediment movement produced egg shearing, eggshell fracture and displacement, mechanical abrasion of egg ornamentation, and alteration of
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egg size and shape. Movement of either individual eggs or subsets of eggs along slickensided surfaces (1) modified the number and relative position of eggs within individual clutches, (2)
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combined eggs of one of more clutches produced by different females, and (3) combined eggs from one or more nesting horizons, producing a time-averaged fossil assemblage. These
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mechanisms of egg rearrangement suggest that accurate interpretation of dinosaur reproductive behavior using fossil egg assemblages preserved in fine-grained fluvial overbank deposits require careful assessment of pedogenic processes.
Keywords: sauropod eggs, Auca Mahuevo, paleo-Vertisols
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ACCEPTED MANUSCRIPT 1. Introduction
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In alluvial basins episodic flood events produce thick fine-grained overbank sedimentary
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sequences that comprise a volumetrically dominant and important component of many fluvial systems (Kraus, 1987). Only major flood events typically deposit more than a few centimeters of
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sediment and therefore sediment accumulation rates on the floodplain are commonly low and
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result primarily from vertical accretion through suspension settling of fine grained materials (e.g. Walling et al., 1992; Aalto et al., 2003; Pizzuto et al., 2008). As such, these fluvial overbank
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sequences often provide an excellent record of post-depositional pedogenesis, with paleosols better preserved during periods of slower subsidence or at greater distances from active channels
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(Kraus, 1999). Furthermore, floodplain environments are subject to relatively low rates of erosion beyond that caused by laterally migrating or incising fluvial channels and, therefore, the
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overprint of pedogenic processes is recorded with high resolution (Kraus, 1999). The resulting paleosols provide important information relative to rates of basin subsidence, paleoclimate, and paleoecology of floodplain ecosystems (Retallack, 1990) Many dinosaur eggs are preserved in pedogenically modified fine-grained fluvial floodplain deposits that record episodic flooding in overbank environments (Varricchio et al., 1999; Chiappe et al., 1999; Sander et al., 1998; 2008, Lopez-Martinez, 1999; Bravo et al., 2000; Cojan et al., 2002; Therrien, 2005; Jackson, 2007; Jackson et al., 2008; Grigorescu and Csiki, 2008; Vila et al., 2010a). In addition, in situ embryos are occasionally preserved within eggs when water and fine-grained sediment fill the pores that traverse the eggshell, prohibiting gas exchange between the embryo and atmosphere. Inundation of these nesting grounds by floodwaters also leads to further interment of eggs and clutches by mud settling from suspension 3
ACCEPTED MANUSCRIPT (e.g. Chiappe et al., 2004). These deposits provide new surfaces for colonization by plants and animals that contribute to subsequent soil development. However, taphonomic studies of
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stratigraphic intervals containing dinosaur eggs typically focus on egg and clutch distribution,
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mode of incubation, and nest structure, while often disregarding pedogenic processes that may influence site taphonomy. As a consequence, the taphonomic role of pedogenesis in post-
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depositional modification of egg-bearing strata remains poorly documented.
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The Upper Cretaceous (Campanian) Anacleto Formation of the Neuquén basin, Argentina represents one of the best preserved and extensive dinosaur egg- and clutch-bearing
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units in the fossil record (Fig. 1). A series of papers describe the Auca Mahuevo locality, including sedimentology and stratigraphy (Dingus et al., 2000, 2009; Garrido, 2010), sauropod
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embryonic remains (Chiappe et al., 1998, 2001; Salgado et al., 2005), eggshell microstructure (Grellet-Tinner et al., 2004, 2006), excavated nests preserved as trace fossils (Chiappe et al.,
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2004), reproductive behavior (Chiappe et al., 1999, 2004, 2005), and other aspects of sauropod biology (Jackson et al., 2004, 2008; Grellet-Tinner, 2005; Grellet-Tinner et al., 2006; Jackson, 2007; Schweitzer et al., 2005; Garcia and Cerda, 2010). These sauropod egg-bearing deposits were subject to Vertisol development, characterized by episodic periods of saturation and drying of the substrate (Chiappe et al., 1998; Loope et al., 2000; Chiappe and Dingus, 2001; Jackson, 2007), leading to extensive vertical and lateral movement of sediments, driven by shrinking and swelling of clay minerals during soil development. The relatively extreme nature of material displacement typical of Vertisols in general, combined with the well-documented distribution of sauropod egg-bearing layers in the Anacleto Formation, provides an opportunity to examine the role of Vertisol development in affecting changes in sauropod egg distribution within a developing soil. Here, we document specifically the role of physical soil processes on individual 4
ACCEPTED MANUSCRIPT eggs and clutches, and the resulting influence of pedogenesis on interpretation of dinosaur
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reproductive biology.
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2. Geology
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The Auca Mahuevo locality lies approximately mid-way between Neuquén and Rincón
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de Los Sauces, just west of Highway 8 in the northeast corner of Neuquén Province, Argentina (Fig. 1). The 86-m thick section exposed at the Auca Mahuevo locality consists of Upper
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Cretaceous terrestrial fluvial deposits of the Anacleto Formation (Dingus et al., 2000; 2009; Garrido, 2010) (Fig. 1A). The contact with the underlying Bajo de la Carpa Formation (the lower
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formation in the Río Colorado Subgroup) is absent at Auca Mahuevo, although these outcrops are exposed about 10 km to the west (Dingus et al., 2009). An erosional contact separates the
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Anacleto Formation from the overlying 10 m-thick, estuarine and shallow marine deposits of Allen Formation (Ardolino and Franchi, 1996). At the Auca Mahuevo study area the Anacleto Formation consists primarily of reddish brown sandstone, siltstone, and mudstone. Four titanosaur egg-bearing stratigraphic intervals (termed “egg beds” in Chiappe et al., 1998, 1999) are present within the finer-grained siltstone and mudstone intervals (Dingus et al., 2000, 2009). Garrido (2010) interpreted these mudrocks and associated sandstones as fine-grained, mixedload, meandering fluvial floodplain and channel deposits, respectively, developed under semiarid climatic conditions with well-differentiated wet and dry seasons. Most egg clutches are preserved in levee and proximal overbank facies. In these areas rare flooding events during the rainy season covered extensive areas with muddy standing water from which silt and clay settled from suspension (Dingus et al., 2000, 2009; Chiappe et al., 1999; Garrido, 2010). 5
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3. Materials and methods
At least four egg-bearing layers (numbered 1–4, from stratigraphically lowest to highest)
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are present at the Auca Mahuevo locality (Fig. 1A). The quarry and adjacent “flats” of egg bed 3
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have yielded abundant fossil eggs containing titanosaur embryonic bone and skin (Chiappe et al.,
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1998, 1999, 2004; Grellet-Tinner et al., 2005; Garcia, 2007; Schweitzer et al., 2005). The study reported here focuses on a quarry excavated in egg bed 3 because the mudrock provides
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abundant evidence of Vertisol development (Loope et al., 2000; Chiappe and Dingus, 2001). Strike and dip of the strata and slickensides, and trend and plunge of slickenlines were measured
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with a Brunton compass. The low stratal dip (3°) requires no correction to the original bed orientation to accurately orient features. To document these paleosol features, the eggs within a
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27 m2 area were mapped using a metric grid and graph paper, with the azimuth of slickenlines recorded on the map. Measurements of the distance and depth of eggs, relative to a datum established adjacent to the quarry, were recorded in order to determine their location stratigraphically and within the grid. These measurements allowed 3-D reconstruction of egg positions, using MATHLABS (Mathworks, 2011; see also Storrs et al., in press). Taphonomic data (e.g., presence of embryonic remains, egg physical attributes, relationship of eggs and clutches to pedogenic features) were also noted and specimens photographed during excavation. X-ray diffraction was used to identify bulk and clay mineral composition in six mudrock matrix samples (three from egg bed 1 and three from egg bed 3). The samples were finely ground into powder using a mortar and pestle, and passed through a 210-mesh (63 µm) sieve. The samples were analyzed using an XGen-4000 x-ray diffractometer at 1 kV with CuKα 6
ACCEPTED MANUSCRIPT radiation (λ= 1.5418 Å) and acceleration voltage of 1 kV. Data were collected at 2θ values ranging from 15°to 75° for bulk mineral identification and 3° to 18° for clay mineral analysis.
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Clay minerals were separated from calcite with glacial acetic acid, and then mounted as an
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oriented aggregate mount so that the incident x-ray beam was directed along the crystallographic z-axis of the minerals. A series of treatments including air drying, glycolation with ethylene
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glycol, and heating to 400°C for overnight were used to ascertain the presence of expandable
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layer clay minerals.
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4. Description
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The reddish to reddish-brown color (10R 5/4) of the siliciclastic mudstone exposed in egg bed 3 is relatively uniform throughout the study area (Fig. 1), and clay mineral assemblages
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identified by x-ray diffraction are consistent with the presence of illite and mixed layer illite/montmorillonite. This mudstone and that of the stratigraphically higher egg bed 4 contain abundant slickensides, whereas these features are absent in underlying egg beds 1 and 2 (Dingus et al., 2009: fig. 7). These polished, randomly oriented slickensides served as surfaces for slickenfibre (mineral) growth, primarily calcite or fibrous gypsum crystals (Fig. 2A). Exposures of the fine-grained massive mudstone in the egg bed 3 quarry occasionally exhibit medium subangular to angular blocky peds from two to several centimeters in diameter. These aggregations of soil particles separate along planes of weakness, giving a hackley appearance to the quarry walls. Compaction and absence of obvious mechanically infiltrated or chemically precipitated cutans around the peds contribute to the difficulty of their identification. Abundant drab, blue-gray, irregular mottling occurs throughout the profile and includes spots (sensu 7
ACCEPTED MANUSCRIPT Retallack, 1990) with sharp or diffuse borders up to 8 cm in diameter (Fig. 2B) and cylindrical tubules (~ 1.0 to 2.5 cm diameter) that sometimes taper downward. A thin ( 5 cm) lens of fine-
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grained reddish-tan sandstone is intercalculated with mudstone in the egg bed 3 quarry. This
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sandstone exhibits faint relic trough cross lamination and moderately abundant root traces,
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including a 1.5 cm cylindrical root cast abruptly truncated by an overlying egg-bearing stratum (Fig. 2 C, D). Although rare, a shallow, v-shaped fissure (1–1.5 cm wide) in-filled by slightly
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coarser, lighter colored reddish brown mudstone is also present within the quarry. A 10–15 cmthick bed occurs at the stratigraphically lowest excavated level within the quarry and includes a
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zone of pale green mudstone containing red, thread-like branching root traces. These rootlets are progressively more abundant upward within the bed, ending abruptly at the base of eggs and
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eggshell debris.
Fractures in the quarry wall often contain white fibrous gypsum crystals and display drab
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blue-gray “haloes”. In cross section, the haloes also surround many eggs and extend into the red mudstone filling the egg interior (Fig. 2E). The sediment size and texture of these reduced zones are identical to the surrounding red mudstone. Some slickensides cut and displace portions of eggs by approximately 3–5 cm along sub-horizontal and sub-vertical surfaces (Fig. 2F, G). However, other slickensides circumvent eggs and exhibit preferentially oriented, blue-gray clay particles; the tuberculate ornamentation of eggs associated with these slickensides is often sheared in the same direction as the clay particles. In rare cases, weathered eggs display slickensides within the mudstone that fills the egg interior. Whereas some eggs are relatively intact and spherical, most are moderately to highly compressed, with the latter displaying substantial distortion in shape (Figs. 2B, D; 3A, B). Although eggshells often remain intact, the upper surfaces (relative to the bedding plane) of some highly compressed eggs are flat or 8
ACCEPTED MANUSCRIPT concave-in (Fig. 2 C, D), whereas others eggs are elongate and ellipsoid in shape (Figs. 3B). These highly modified eggs often occur adjacent to eggs that are significantly less distorted (Fig.
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3B).
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Microscopic examination of radial thin sections of eggshell often reveals highly abraded surface ornamentation and diagonal fractures that cut across accretion lines with minimal offset
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of these features. Other eggshell fragments show more complex and irregular fracturing (Fig.
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3C). The inner portion of the eggshell exhibits substantial dissolution, with secondary calcite precipitated between nuclei located on the outer surface of the permineralized membrane.
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Large egg clusters in some areas of the quarry contain more than 50 eggs (Fig. 4). Depth measurements relative to the datum show that these and other areas of the quarry possess
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significant topographic relief (Fig. 4B, C). Eggs containing embryonic bones in these clusters are sometimes adjacent to eggs lacking such remains (Fig. 4A). In some areas of the quarry
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concave-up, low-angle curved planes intersect at opposite angles (Figs. 5A). The resulting pattern of microhighs and microlows (gilgai), are approximately 1.0 to 1.5 meters wide (Figs. 3A, 5A). Microlows form bowl-shaped depressions, surrounded by slickensided surfaces; these depressions often contain small clusters of eggs (Fig. 3B). The maximum height of the microhigh is about 45 cm above the microlow (Figs. 3A, 5A). With some exceptions (e.g., two eggs on left in Fig. 3A), eggs located on the microhighs typically display extensive compaction, often exhibiting a total thickness of only 1–3 cm (Fig. 5B).
5. Discussion
5.1 Modern Vertisol development 9
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Modern Vertisols typically develop under dry to moist soil conditions spanning a broad
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range of mean annual soil temperatures (4°–24°C). An abundance of expandable layer clay
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minerals and wet-dry seasonality strongly control their distribution (Brady and Weil, 2002, p. 67). Under some conditions, a few hundred years provides adequate time for Vertisol
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development (Coulombe et al., 1996). The high shrink-swell susceptibility, driven by the
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presence of expandable smectitic clay minerals (mixed-layer illite/montmorillonite), serves as a major control on the high degree of pedoturbation characterizing modern Vertisols (Nordt et al.,
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2004). Unlike bioturbation by plant roots and burrowing vertebrates and invertebrates, Vertisol pedoturbation is capable of producing displacement of material over distances ranging from 10
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cm to as much as 1 m (Brady and Weil, 2002). In addition, development of slickenside-bounded blocks of soil material is facilitated by dilation as the dominant strain force (Nordt et al., 2004).
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Under dry conditions, loss of interlayer water in expandable layer smectitic clays leads to tension and development of open fractures propagating downward from the surface, resulting in topographic microlows at the land surface (Brady and Weil, 2010, p. 76–77). These open fractures collect material (soil debris, sediment, organic matter) that falls downward to lower depths within the developing soil. During wet periods, smectitic clays absorb water into interlayer sites, swelling in the process and facilitating upward movement of material between fractures and shear forces producing slickensides with striated surfaces (slickenlines). Upward movement of material between vertical fractures results in topographic microhighs at the land surface. The relatively large magnitude of vertical and lateral material movement in Vertisols through clay mineral shrinking and swelling makes them problematic for construction and agricultural activities. 10
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5.2 Auca Mahuevo paleo-Vertisols
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Vertisols likely characterized many alluvial plains in ancient retro-arc foreland basin settings, where rain-shadow effects and episodic explosive volcanism (e.g. Ver Straeten, 2004)
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provided both the environmental conditions (seasonally wet-dry) (e.g. Fricke et al., 2010) and
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expandable-layer clay minerals necessary to facilitate their development (e.g. Yerima et al., 1987). As recorded in the Anacleto Formation, sauropods extensively used this type of
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floodplain setting for nest construction and egg laying. Infrequent flooding of overbank areas inundated the Auca Mahuevo titanosaur nesting grounds, leading to suspension settling of
2004; Dingus et al., 2009).
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muddy sediment. This in turn facilitated egg and nest burial and preservation (Chiappe et al.,
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Vertisol development that occurred in fine-grained floodplain sediments containing sauropod egg clutches had great potential to affect both the morphology of individual eggs and their distribution within the soil profile during pedogenesis. In simple terms, Vertisol processes at Auca Mahuevo disrupted or over-printed the biological pattern produced by the egg-laying titanosaurs. Mapping of eggs using x, y and z coordinates from a fixed datum reveals substantial topographic relief in the egg bed 3 quarry that is not apparent in conventional two dimensional maps (Fig. 4). Pedogenic processes associated with Vertisol development affected changes in both individual egg condition and the three-dimensional distribution of eggs (Fig. 6). These changes resulted from two physical processes that characterize Vertisols generally and effected the overbank deposits of the Anacleto Formation in particular: 1) lateral movement of material (eggs and sediment) as a result of clay mineral expansion and contraction over time, and 2) 11
ACCEPTED MANUSCRIPT vertical mixing of those materials due to development of tension fractures and fracture wall collapse during phases of wetting and drying.
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At the scale of individual sauropod eggs, pedogenic sediment movement resulted in egg
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and eggshell fracture and displacement, mechanical abrasion of egg ornamentation, alteration of egg size and shape, and the generally poor quality of preservation (Figs. 2, 3, 5B). These changes
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in morphology can have important implications for fossil egg parataxonomic assignments and
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inferences of sauropod reproductive biology. For example, eggshell thickness, and egg size, shape, and ornamentation are used to assign fossil eggs to parataxonomic categories; therefore,
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pedogenic processes that alter these attributes may lead to incorrect interpretations and proliferation of new oospecies. In addition, changes in egg size and shape, and eggshell thickness
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(due to compaction, dissolution, or abrasion resulting from sheer stress; Fig. 3) also impact estimates of egg volume, surface area, and pore length used in calculations of water vapor
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conductance rates and interpretation of egg incubation strategies (Jackson et al., 2008). Movement of either individual eggs or subsets of eggs along slickensides also affected the biological pattern of sauropod egg and clutch distribution at the Auca Mahuevo locality. This movement, characterized by both lateral and vertical components of displacement, (1) modified the number and relative position of eggs within individual clutches, (2) combined eggs of one or more clutches, and (3) combined eggs from different nesting horizons, producing a timeaveraged assemblage (Figs. 4, 6). For this reason, we define an egg clutch as an accumulation of eggs produced by the female dinosaur, distinguishing these from egg clusters that result from pedogenic modification of the original egg pattern.
5.3 Impact on behavioral inferences 12
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Alterations to the biological pattern produced by the adult sauropod during nest
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construction and egg laying contribute to the difficulty of interpreting reproductive behavior. For
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example, large well-defined clutches (~ 30 eggs) at the Auca Mahuevo locality occur within a 70 cm-thick stratum, suggesting a single nesting event (Chiappe et al., 1999). However, some egg
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clusters in the quarry contain more than 50 eggs (Fig. 4). Eggs containing embryonic remains in
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these large clusters are sometimes adjacent to those lacking fossil bones (Fig. 4A). In addition, some embryos are clearly larger than others, by as much as 25% (Chiappe et al., 2005). Two
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possible explanations include differential preservation of embryonic remains and individual variation among eggs of the same or different clutches; however, a time-averaged fossil
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assemblage represents a third possibility.
Trace fossil nests provide important information about the reproductive behavior of
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extinct animals, yet are rarely reported from overbank facies (Varricchio et al., 1999; Chiappe et al., 2004). With the exception of six nesting traces preserved in sandstone (channel and crevasse splay deposits) in egg bed 4, thousands of eggs at Auca Mahuevo occur in mudstone and show no discernible evidence of nest structure (Chiappe et al., 2004). Significant lateral and vertical displacement of sediment and eggs within the Vertisol horizon over hundreds to thousands of years likely accounts for the absence of trace fossil nests in the mudstone facies. In egg bed 3 these pedoturbation processes destroyed nearly all primary sedimentary structures, producing a homogenous substrate. However, the presence of relic bedding and root horizons that terminate immediately below eggs in an otherwise homogenous mudrock may assist in identification of a single nesting event (Fig. 2C, D).
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ACCEPTED MANUSCRIPT In the absence of a well-preserved trace fossil nest structure, inferences of nest construction and mode of egg incubation often rely on less reliable evidence. For example, clutch
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geometry is used to infer a bowl-shaped depression (Erben et al., 1979; Kérourio, 1981; Cousin
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et al., 1994; Cousin and Breton, 1999; Sander et al., 2008; Vila et al., 2010a,b). However, bowlshaped microlows are characteristic of modern Vertisols (Brady and Weil, 2002, p. 77). At Auca
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Mahuevo these microlows sometimes contain one or more relatively intact eggs (Fig. 3B),
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whereas eggs on the microhighs typically show greater compaction and eggshell fragmentation (Fig. 5B). Weathering and loss of these eggs on the microhighs, in conjunction with differential
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preservation of more intact eggs in the bowl-shaped microlow depressions increases their resemblance to a fossil nest containing eggs. The number of eggs in these pseudo nests, however,
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may be substantially fewer than originally produced by the female sauropod. For example, the cluster in Figure 3B contains 9–10 eggs, whereas well-delineated clutches in the mudstone facies
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and trace fossil nests preserved in sandstone contain 30–35. This underestimation of clutch size in turn may lead to incorrect estimates of sauropod reproductive output that compare clutch size to adult body size. Similarly, egg arrangements (circular/conical, arcs, and multiple or single egg levels) are often interpreted as having biological significance (Moratalla and Powell 1990 and references therein); however, Vila et al. (2010b) note that erosion may also account for these differences in megaloolithid egg distribution patterns.
6. Summary and conclusions
Stratigraphic units containing fossil eggs provide rare opportunities to enhance our understanding of dinosaur reproductive biology, including adult nesting strategy, mode of egg 14
ACCEPTED MANUSCRIPT incubation, parental care of young, and other behaviors. However, shrinking and swelling of expandable layer clay minerals during alternating wet and dry seasons at the Auca Mahuevo
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sauropod nesting site facilitated pedogenic movement within the egg-bearing horizons. This
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pedoturbation modified or combined eggs from different clutches and different temporal intervals, resulting in time-averaging of nests or nesting horizons. The resulting pedogenically-
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formed egg clusters may not accurately portray the egg size and shape, number of eggs per
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clutch, clutch morphology or egg distribution produced by individual female sauropods. For more accurate inferences concerning dinosaur reproductive biology, it is imperative
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to ascertain the degree to which post-burial pedogenic modification has altered the distribution and condition of eggs preserved in fine-grained sedimentary sequences. Investigation should
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include assessment of features that characterize undisturbed, in situ egg clutches and those altered by subsequent pedoturbation. Most dinosaur nests were likely constructed in floodplain
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environments where sedimentation favors deposition by vertical accretion of silt and clay with minimal textural contrast, making nest structure recognition difficult. In addition, the persistence and widespread distribution of paleo-Vertisols in the sedimentary rock record (see Nordt et al., 2004) suggests that many egg-bearing units may have been affected by the physical processes characterizing Vertisol development. With this in mind, we suggest that all investigations of dinosaur egg-bearing strata include the following: (1) strike and dip of beds (2) detailed three dimensional mapping of egg distribution during excavation (3) identification and mapping of all planar features, including both structural and pedogenic surfaces of displacement and(or) mineral growth
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ACCEPTED MANUSCRIPT (4) careful examination of both individual and groups of eggs to assess potential relations between deformation features and physical condition of the eggs
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(5) documentation of all pedogenic features
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(6) careful attention to the relations between fossil eggs and pedogenic features (e.g., roots and burrows, gilgai microtopography, slickensides)
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(7) detailed facies analysis aimed at deciphering the specific depositional processes
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associated with floodplain sedimentation (i.e. suspension settling of mud, traction transport of sand in crevasse-splay lobes, eolian traction transport).
clay minerals when appropriate.
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(8) x-ray diffraction analysis of mudrock to ascertain the presence of expandable layer
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(9) thin section analysis to establish whether sepic-plasmic (bright, oriented clay) microfabrics are evident in the mudrock and for assessment of eggshell fragmentation.
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We feel that the use of this methodological approach in the study of egg-bearing strata will produce more accurate inferences of adult reproductive behavior.
Acknowledgements
We thank L. Chiappe, R. Coria, and crew members of the 1999 and 2000 LACM/MCF joint expeditions. Field and laboratory research for this project was supported by the Ann and Gordon Getty Foundation, Charlotte and Walter Kohler Charitable Trust, Dirección General de Cultura de Neuquén, Fundación Antorchas, Infoquest Foundation, Municipalidad de Plaza Huincul, and National Geographic Society. Finally, we thank F. Surlyk, S. Driese and B.Vila for their helpful comments that improved the manuscript. 16
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ACCEPTED MANUSCRIPT Vila, B., Galobart, A., Oms, O., Poza, B., Bravo, A.M., 2010b. Assessing the nesting strategies of Late Cretaceous titanosaurs: 3D-clutch geometry from a new megaloolithid eggsite.
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Fig. 1. The Auca Mahuevo locality. A) Composite stratigraphic section showing four titanosaur egg beds and osteological remains. Column width represents weathering profile. B) Map of
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Neuquén Province (Argentina) indicating location of Auca Mahuevo. Modified from Chiappe et
Fig, 2. Titanosaur eggs from egg-bed 3. A) Partial egg on polished slickenside indicated by arrow. B) Titanosaur eggs draping a microhigh and additional highly compressed eggs in foreground. White arrow points to blue-gray reduction spot. Note relatively intact egg in lower right corner of photo; person for scale. C) Faint relic cross-lamination with tubule that ends abruptly beneath titanosaur egg. Note the flattened and concave surface of the overlying egg. D) Crushed egg clutch; arrow points to tubule truncated by egg in C. Black dots mark eggs; tape measure extends 9 m. E) Partial eggs with arrows indicating reduction “haloes” along tectonic fracture in the rock (upper right) and adjacent to broken edge of the egg (lower left). Lens cap for 23
ACCEPTED MANUSCRIPT scale. F) Two partial eggs sheared along slickenside. Lens cap for scale. G) Sheared but otherwise relatively intact egg hemispheres; arrow indicates direction of shear of lower half
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relative to the upper hemisphere. Egg on the left contains tubule traces and branching rootlets.
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Gypsum crystals above arrow are pervasive within fractures in the quarry.
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Fig. 3. Eggs and eggshell. A) Elongate eggs uplifted on microhigh; horizontal arrow indicates
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extremely flattened egg with a total thickness < 2 cm on far side of microlow. Slickenside visible below left vertical arrow. Compare highly flattened egg (right vertical arrow) to two eggs
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on the left. B) Bowl-shaped depression with slickensides (white arrows). Note combination of both extremely flattened eggs and mostly intact, three-dimensional specimens; 15 cm-long
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mechanical pencil for scale. C) Titanosaur eggshell with complex fracturing and abraded outer surface at the top of the image.. Note eggshell dissolution (D) between nuclei at the inner shell
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Fig. 4. Egg distribution in egg bed 3. A) Field map in plan view. B) Same view as A showing different egg depths by color. C) Cross-section view showing microtopography within the quarry.
Fig. 5. Pedogenic features in egg bed 3. A) concave-up, low-angle curved planes intersecting at opposite angles; arrows indicate concave surface. Note gleying in upper left corner of photograph; 16 cm-long chisel for scale. B) Sheared and compressed eggs; egg between arrows measures about 4 cm thick.
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ACCEPTED MANUSCRIPT Fig. 6. Conceptual model showing influence of Vertisol development on titanosaur egg distribution. A) Eggs at time 1, overlain by second clutch at time 2. Eggs at time 1 show
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evidence of displacement and shearing along slickensides. B) Both clutches at deeper burial and
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subjected to uplift along microhigh; note eggs of both clutches sheared along sickensides. C) Time-averaged clutches exposed to erosion. Illustrations modified from Brady and Weil (2002:
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ACCEPTED MANUSCRIPT Highlights: We examined the influence of vertisol development on titanosaur eggs.
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Sediment and egg movement resulted from clay mineral expansion/contraction.
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Soil movement modified the condition, number and position of eggs in clutches.
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Vertisols adversely impact interpretations of dinosaur reproductive biology.
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