Possible burrows of mylagaulids (Rodentia: Aplodontoidea: Mylagaulidae) from the late Miocene (Barstovian) Pawnee Creek Formation of northeastern Colorado

Palaeogeography, Palaeoclimatology, Palaeoecology 237 (2006) 119 – 136 www.elsevier.com/locate/palaeo

Possible burrows of mylagaulids (Rodentia: Aplodontoidea: Mylagaulidae) from the late Miocene (Barstovian) Pawnee Creek Formation of northeastern Colorado Katrina E. Gobetz Biology Department, James Madison University, MSC 7801, Harrisonburg, VA 22801, USA Received 24 November 2004; received in revised form 1 September 2005; accepted 13 September 2005

Abstract The massive siltstone beds of the late Miocene (Barstovian) Pawnee Creek Formation in Logan County, northeastern Colorado, contain abundant terrestrial vertebrate burrows. Among these are distinctive claw-marked burrows attributable to the extinct mylagaulid rodent Pterogaulus [= Mylagaulus] laevis Korth, 2000. These burrows differ in several respects from the spiraling Daemonelix burrows of fossorial beavers. The Pawnee Creek burrows are slightly ovate in cross-section, tubular, and sinuous rather than helical. They consist of primary tunnels of up to at least 7 m long and range from 11 to 18 cm in diameter. They occasionally branch into secondary tunnels. Burrow walls and terminations show prominent, parallel ridges in sets of two to three, up to 9.3 mm deep and 3.7 mm wide. The burrows are too large to have been made by anything but a vertebrate, and their considerable length and complexity implies a mammalian excavator. The meandering, branching morphology resembles the burrow systems of extant fossorial rodents, such as pocket gophers (Geomys) and mole rats (Spalax, Myospalax). Skeletal remains of P. laevis are abundant at sites where the burrows occur, with at least eight individuals represented by humeri in a collected sample. Although mylagaulid remains are not unequivocally associated with fossil burrows, the curved, laterally compressed claws and phalangeal arrangement of mylagaulids produce marks very similar in size and morphology to the ridges on the burrows. Position and pattern of the ridges indicate downward-and-backward motions of the mani, possibly supplemented by motions of the pedes to kick loosened soil backward. A new ichnogenus, Alezichnos, and new ichnospecies, chelecharatos, are erected to describe these burrows. D 2006 Elsevier B.V. All rights reserved. Keywords: Paleontology; Ichnology; Vertebrate burrows; Mylagaulid rodents

1. Introduction This study describes a new fossil burrow complex from the upper Miocene (Barstovian) Pawnee Creek Formation in Logan County, northeastern Colorado

E-mail address: [email protected]. 0031-0182/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.09.004

(Fig. 1). The architectural morphology of these burrows is compared with that of modern fossorial and subterranean mammals and reptiles to determine the most likely excavator. Similar comparative analysis of the surficial morphology of the burrows, which includes digging traces, aids in identifying the excavator and reconstructing its movements (Hasiotis and Mitchell, 1993; Hasiotis et al., 1993, 2004; Hasiotis, 2002).

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Fig. 1. Map showing location of field site in Logan County, northeastern Colorado, where vertebrate burrows occur within the Miocene (late Barstovian) Pawnee Creek Formation.

The vertebrate fossil fauna of the Pawnee Creek Formation has been collected and described, whereas the ichnofossils, which are abundant at some localities, have remained unknown. W. D. Matthew and others made extensive collections of fossil skeletons from this area in the late 19th and early 20th centuries. More recently, E. C. Galbreath worked the region from about 1951 to 1985, collecting and describing its mammalian fauna and revising the stratigraphy. Although he never published any observations of ichnofossils in the Pawnee Creek Formation, he did collect one example of the many tunnels that had eroded out of these beds (KUVP 131597, described in this paper). Galbreath (1953), in his descriptions of stratigraphic sections in Logan County, reported a layer in the Pawnee Creek Formation bearing calcareous nodules. Many of these nodules are actually fossil vertebrate burrows. This large concentration of ichnofossils provides an unusual opportunity to study the behavior of ancient organisms. These burrows of the Pawnee Creek Formation also represent the remains of a fossorial vertebrate community that can be compared to modern fossorial communities in similar ecosystems. 1.1. Geological setting The locality from which the burrows in this study were collected is KU-CO-52, SW1/4 sec. 26, T 12 N, R 55 W, an exposure in the bluffs near the town of Peetz in Logan County, Colorado. The burrows occur on the southwest side of a hogback ridge at about 40858VN, 103829VW. The burrow layer is a natural ledge along a contour line, at an elevation of about 14 m. At this site, sandstones and siltstones up to 14.9 m thick comprises

the Kennesaw local fauna stratal unit of the Pawnee Creek Formation. These siltstones and sandstones contain local conglomeritic channel deposits (Matthew, 1901; Galbreath, 1953), and probably represent a series of cut-and-fill episodes (Galbreath, 1953). The Kennesaw unit is bordered above and below by disconformities and is late Barstovian in age (13 Ma; Tedford et al., 1987). Galbreath (1953) provides a list of fossil vertebrate species from the Kennesaw unit. He describes the site on which this study is based as the bmost productive fossil zone,Q and mentions that calcareous nodules are common in the siltstones, occasionally occurring in layers. Otherwise, there is no mention of anything resembling an ichnofossil. Burrows occur in the upper beds of the formation where the ground surface is littered with tubular burrows and irregular, calcareous nodules of varying size (Fig. 2). The nodules are more abundant than the burrows but lack the burrows’ distinctive morphology, ramifications, and digging marks. The siltstone surrounding the burrows is pale orange (Munsell color 10YR 8/2) when dry and grayish orange (10YR 7/4) when wet. It is easy to excavate with a brush due to being unconsolidated. The burrows themselves are much more resistant than the ambient sediments and are the same color as the siltstone, although their weathered outer surfaces frequently show growths of red and orange lichens. Fossil bone occurring in the burrow beds tends to fragment upon exposure, evidently having lost much of its integrity due to calcium loss, possibly from calcification of roots. Fossilized roots, ranging in diameter from a few millimeters to 4.0 cm or greater, are abundant throughout the burrow layer and within the bur-

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Fig. 2. Photograph of fossil burrow locality in Pawnee Creek Formation, with nodules of varying size lying about on ground surface of burrow layer, and in situ, indicated by arrows. Hammer for scale = 28 cm.

rows. They are more abundant than fossil bone and resemble bone from a distance, as they are almost the same chalky white color. 2. Results 2.1. Description of burrows This study results from fieldwork conducted in the fall of 2000 and summer of 2001. Specific locations of burrows were determined with a hand-held Garmin GPS 38. Measurements of the architectural and surficial burrow morphologies were made using a measuring box and dial calipers. Institutional abbreviations are as follows: KUVP= University of Kansas Natural History Museum and Biodiversity Research Center, Division of Vertebrate Paleontology; AMNH = American Museum of Natural History; UNSM = University of Nebraska State Museum. In the descriptions of the burrows, brightQ and bleftQ sides are the sides to the right or left of the terminus, respectively, as the tunnel is viewed end-on with the remainder of the passage directed away from the viewer. 2.1.1. Architectural morphology and fill pattern As seen in situ, the fossil burrows have sinuous architectural morphology, undulating slightly within the horizontal plane (Fig. 3A). Some shorter tunnels and shafts, however, may be oriented 308 to 608 from the horizontal (Fig. 3B). At least one in situ shaft,

which was not collected, is loosely helical. The burrows cut across bedding planes and have varying directionality. In map view, they form a network of branching passages, interrupted by areas where portions of burrows lie beneath the ground surface or are weathered away (Fig. 4). Several burrows branch into tunnels at bipartite or multipartite divisions (Fig. 5). Not all of the mapped burrows show branches (e.g., burrow numbered 14 in Fig. 4), but most of them are not preserved in entirety, and originally may have branched in places at which they are now interrupted. Some burrow shafts overlap one another at higher angles (ca. 508) (Fig. 5B) than most tunnels of greater length, which are mainly horizontal. Burrow diameter measures 13.6 cm from ceiling to floor, and 16.7 cm in cross-section, on average (Table 1). Transverse diameter is generally greater, so that the burrows are slightly dorso-ventrally compressed, and ovate in cross-section. Other fossil burrows occurring at the same locality appear to fall into two size classes of 36 cm and 6.0 cm diameter. One collected specimen of the smaller size class shows parallel ridges. These smaller burrows are not included in this study. A comparison of burrow diameters with diameters for extinct fossorial (burrowing) beavers, modern fossorial rodents, and the modern gopher tortoise Gopherus polyphemus (Fig. 6) shows three approximate burrow size classes. Modern fossorial rodents, such as mole rats, form the smallest size class, whereas

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Fig. 3. (A) Tunnel on ground surface at holotype locality, showing typical sinuous form. Hammer for scale = 28 cm. (B) Tunnel projecting from the ground at an angle of about 308. Hammer for scale. Photograph in B taken by E. C. Galbreath.

the tortoise burrows are largest. The fossil burrows fall into a middle-sized class, along with the burrows of an extinct beaver, Palaeocastor fossor and the modern prairie dog, Cynomys ludovicianus. The burrows contain two layers of sediment fill. The outer fill is comprised of relatively consolidated sediment, which forms a concentric layer around the burrow wall, about 4.0 cm thick (Table 2 and Fig. 7). This outer layer appears to consist of the siltstone in which

Fig. 5. (A) Portion of fossilized tunnel in situ, showing Y-shaped, bipartite division and prominent ridges (indicated by arrows). Scale = 28 cm. (B) Overlapping tunnel segments projecting upward from ground surface at about 608. Hammer for scale = 28 cm.

the burrows occur. It is asymmetric, being wider on the lateral walls (up to 7.4 cm) than on the floor and ceiling, where it is disrupted by irregular, upward and downward channel-like openings (Fig. 7A). The outer walls of the tunnel are cemented into a hard, calcareous crust, formed by a network of fossilized roots, whitish in color. Fossilized roots of 1.0 to 4.0 cm diameter

Fig. 4. Portion of field map showing fossil burrows on an exposed ledge. Dashed lines represent possible outlines of tunnels in areas where portions were underneath the ground surface or weathered away. Each square in the grid is 5 m2. KN = knobs projecting from ceilings of burrows (see text, Section 2.1.2). The longest outline, number 18, is a relatively complete burrow.

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Table 1 Measurements of fossil burrows from the Pawnee Creek Formation, Colorado, and surficial morphology

Specimen 131601

Length (cm)

Ceiling–floor diameter (cm)

Transverse diameter (cm)

Average length of ridge (cm)

Width of ridges (cm)

Distance between ridges (cm)

100

13.4

13.9

5.51 (n = 39) S.D. = 23.7; r = 22.3–120.0

0.378 (n = 40) S.D. = 0.543; r = 3.0–5.1

0.638 (n = 24) S.D. = 1.25; r = 4.2–8.6

6.17 (n = 7) S.D. = 1.37; r = 4.51–8.34 7.66 (n = 9) S.D. = 2.02; r = 3.7–15.7 6.27 (n = 12) S.D. = 1.33; r = 3.43–8.45 4.54 (n = 17) S.D. = 19.0; r = 18.5–85.0

0.35 (n = 6) S.D. = 0.0716; r = 0.24–0.44 0.37 (n = 9) S.D. = 0.058; r = 0.25–0.42 0.35 (n = 12) S.D. = 0.035; r = 0.29–0.40 0.36 (n = 17) S.D. = 0.567; r = 3.0–5.1

0.64 (n = 4) S.D. = 0.185; r = 0.41–0.86 0.97 (n = 5) S.D. = 0.182; r = 0.79–1.20 0.81 (n = 10) S.D. = 0.214; r = 0.60–1.21 0.81 (n = 10) S.D. = 1.73; r = 5.5–11.0

5.88 (n = 5)

0.36 (n = 5)

0.77 (n = 5)

Bilobate end Each of double ends 131599

20.2 12.4

16.5

13.2, 22.0

13.0

15.0

131598

38.0

12.6

15.0

131022*

22.2

9.4

11.9

131597*

34.9

17.2

18.0

60.5 63.0 105 73.5 170 51.5 125 72.1 (n = 12)

n/a 14.0 11.0 15.6 13.0 19.0 11.6 13.6 (n = 13)

18.0 22.0 19.5 16.4 15.4 18.3 17.4 16.7 (n = 13)

Map number 1 2 3 4 5 6 10942 Average

bnQ refers to number of places at which the diameter was measured on a burrow, or number of individual ridges measured; brQ and bS.D.Q refer, respectively, to the range and standard deviation of each measurement where applicable. * indicates specimen was not collected in situ and has no orientation.

occur throughout this fill layer (Fig. 7B). There are no distinguishable meniscate fill patterns in the cross-sectioned burrows. The innermost core of the tunnels is about 4.0 cm wide (Table 2), and contains an inner fill of moderate orange (5YR 8/4) siltstone. This siltstone is much less consolidated than the outer layer. It is easily scraped out of the tunnel with a dental pick, revealing a cavity (Fig. 7A). One tunnel terminus was sagittally sectioned to show these fill patterns in longitudinal view. In this specimen, the relatively resistant outer siltstone layer is packed into the end wall to a thickness of up to 8.61 cm. Fossilized roots are also prominent, running in all directions within the burrow fill. The inner siltstone is eroded out, leaving a cavity with an irregular border, as seen in the cross-sections. The inner siltstone fill contains numerous hackberry (Celtis sp.) seed husks, which are white, thin, and easily broken. There are also dark brown particles of organic matter, and occasional quartz and feldspar pebbles. The inner fill is similar in color and consistency to the siltstone beds that overlie the burrow layer at the type locality. Galbreath (1953) considered this siltstone part

of the upper Miocene Ogallala Formation; however, its identification remains uncertain. 2.1.2. Surficial morphology The walls of the tunnels represent the inner surfaces of hollow tunnel spaces. These surfaces are covered with a fine network of fossilized roots, which are a few mm or less in diameter. Every burrow examined had this covering of fossilized roots on the outermost surface. Only weathered surfaces lack these features. A separate feature commonly occurring on the burrow walls is prominent, parallel ridges. These ridges were observed on about 13% of the 47 burrows that were mapped over an area of about 50 m. The ridges are overlain by the fossilized roots (Fig. 8). As only exposed surfaces were mapped, it is possible that more of these burrows had ridges prior to weathering. On one burrow, individual ridges average 5.79 cm long and 0.38 cm wide (Table 1). The ridges are raised as much as 0.7 1.0 cm from tunnel walls and occur in sets of two to three, about 0.65 cm apart, on end and side walls (Table 1; Figs. 9 and 10). Each ridge has a shallower slope on the side toward the ceiling, whereas

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Fig. 6. Scatterplot comparing diameter of fossil burrows from the Pawnee Creek Formation with other fossil and modern burrow diameters. Modern species include pocket gopher Geomys bursarius (Andersen, 1986), G. b. bursarius Heaney and Timm (1983) [= majusculus] (Smith, 1948); white tailed prairie dog Cynomys ludovicianus (Peterson, 1905; Wood and Wood, 1933); corruro Spalacopus cyanus (Begall and Gallardo, 2000); bamboo rats Rhizomys pruinosus and Rhizomys sinensis (He, 1984; Flynn, 1990); Ukrainian blind mole rat Spalax sp. (Montagu, 1924; Savicˇ and Nevo, 1999), African mole rat Tachyorctes splendens (Jarvis and Sale, 1971), common mole rat Cryptomys hottentottus (Davies and Jarvis, 1986); giant mole rat Cryptomys mechowii (Scharff et al., 2001), Cape dune mole rat Bathyergus suillus (Davies and Jarvis, 1986), golden mole Amblysomus hottentottus (Kuyper, 1985), and modern gopher tortoise Gopherus polyphemus (Auffenberg and Weaver, 1969). Burrow diameters from extinct diggers include fossorial beaver Palaeocastor fossor, pocket mouse Perognathus sensu lato (Voorhies, 1974), and kangaroo rat Eodipodomys celtiservator (Voorhies, 1975b). As most modern burrow measurements include only one dimension, these data points may not plot along an exactly linear line, and should be considered approximate.

the floor side forms an abrupt shelf. This sediment displacement pattern appears to be consistent among the ridges on all examined, oriented burrows. The majority of ridge sets curves upward from tunnel floors, away from the direction of the terminus, at an angle of about 258 (Fig. 10A, E). These sets are positioned about mid-way between the floor and ceiling. Occasionally, sets are angled in the opposite direction (e.g., labeled marks in Fig. 10E). A few sets of two ridges each, positioned relatively close to the ceiling, form nearly vertical arcs (about 578) (Fig. 10E) that cut

across the more frequent 258 marks. Ridges on the terminus curve downward and back, diverging from the midline and wrapping around the sidewalls (Figs. 9C–D, 10C–D, and 11). Ridges on the floor near end walls form continuous lines, without clear distinctions between where one ridge ends and another begins (Fig. 11A). These continuous ridges are also found on other end-segments that were collected in situ. The tunnel floor further away from the end wall, however, does not show this longitudinal pattern. In addition to ridges, irregular knobs occur on and near the ceilings of some tunnels (Figs. 10A–B and 12). Ridges are interrupted in places by these knobs. In cross-section, the knobs are featureless, consisting of burrow sediment and showing no internal structure that might identify them as fossilized roots cross-cutting the tunnel. Other fossilized roots from these beds have clear vascular bundle structures. Some of the knobs are covered by ridges on the outer surface, with the ridges radiating out from the approximate center of the knob (Fig. 12B–C). Knobs with and without ridges are approximately the same size (Table 2). 3. Discussion 3.1. Paleoecological implications High density of roots and burrows at the study site is diagnostic of the topmost horizon of a fossil soil, situated above the water table (Retallack, 1991, 2001). In addition, the abundant fossilized roots throughout the burrow layer and all over burrow walls indicates substantial vegetative cover and rich soil, supporting Galbreath’s (1953) description of the beds as part of a floodplain. The depositional environment may have been similar to lower Pliocene floodplain deposits in which burrows of the pocket mouse Perognathus sp. occur (Voorhies, 1974). Fossilized roots with diameter of 3.0 cm or greater (Table 2) are probably from shrubs or trees. Growth of small roots and rootlets on burrow walls might have resulted from roots seeking the open space within the burrow, as Martin and Bennett (1977) hypothesised for fossil beaver burrows. Constant high humidity, a typical condition in modern tunnels (Nevo, 1999; Lacey et al., 2000), might have encouraged root growth as well. The resistant outer surface of the burrows may be a product of the decay of this covering of roots, with the decay of plant matter producing a calcareous solution re-deposited as CaCO3 (cf. Glennie and Evamy, 1968), which replaced the roots’ internal structure.

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Table 2 Measurements of fossilized roots, fill layers, and knobs on fossil burrows Specimen

Fossil root diameter (cm)

Outer fill layer thickness (cm)

Inner fill layer thickness (cm)

Knob average diameter (cm)

131601 131599 131598 131597*

5.2 (n = 3) 0.33, 0.37 n/a 2.1

3.91 5.44 4.60 2.19

5.22 4.25 4.77 7.0

2.98  3.42 (n = 4) n/a n/a 4.06  5.59 (n = 3)

All specimens from KU. * indicates specimen was not collected in situ and has no orientation.

Abundant hackberry (Celtis sp.) seed husks in the inner fill of the burrows may have originated from the infilling sediment, or from food caches of the latest burrow inhabitants. Voorhies (1975a) reports a cache of hackberry seeds in a fossil burrow from the lower Pliocene ground squirrel Citellus [= Spermophilus] sp. in Nebraska. As with fossil pocket mouse burrows described by Voorhies (1974), the presence of hackberry seeds indicates a habitat with abundant trees. The occurrence of an inner fill in the center of the tunnels suggests that the tunnels were originally filled com-

pletely with the first (outer) siltstone, and that an inner cavity was formed subsequently and then filled with the second siltstone. The inner siltstone may thus represent a tunnel that was excavated inside a filled one. 3.2. Interpretation of tracemaker and ethology Possible excavators of the burrows, based on the fauna known from the Kennesaw unit at the type locality (Galbreath, 1953), include the gopher tortoise Gopherus sp., carnivorous digging mammals such as early badgers, and the mylagaulid rodent Pterogaulus [= Mylagaulus] laevis Korth, 2000. General evaluation of the excavator is based on body size, burrows excavated by modern relatives (if applicable), and morphology and position of traces, as has been done for other types of fossil vertebrate burrows (Quintan˜a, 1992; Groenewald et al., 2001; Miller et al., 2001; Hasiotis et al., 2004). 3.3. Evidence from burrow architecture The diameter of any tunnel very closely matches the body diameter of its excavator (Martin and Bennett, 1977; Andersen, 1982; Heth, 1989; Hickman, 1990; Nevo, 1999). Given this assumption, diameter suggests that the excavator of the burrows was the size of a

Fig. 7. Photographs of cross-sectioned fossil burrows, showing fossilized roots and outline of inner sediment fill: (A) KUVP 131061; (B) KUVP 131598.

Fig. 8. Photograph of outer surface of KUVP 131061, showing a set of ridges overlain by a fine network of rootlet casts.

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Fig. 9. (A) Photograph of lateral view of KUVP 131598, single rounded terminus (broken) to right, with set of three ridges; (B) drawing of KUVP 133022 showing similar patterns of ridges to those in A; (C) end-on view of single rounded terminus of KUVP 133022, showing ridges on side toward floor, indicated by arrows; (D) drawing of C. Grey outline indicates area in which ridges are not preserved.

relatively large (prairie-dog-sized) fossorial rodent. The nearly spherical shape of the burrows also suggests a mammalian excavator. Gopher tortoise burrows are typically hemispherical in cross-section, reflecting the shape of the carapace (Auffenberg and Weaver, 1969), whereas mammalian tunnels, such as those of the modern golden mole (Kingdon, 1974), are more cylindrical. The long, branching tunnels of the fossil burrows are similar to the distinctive primary and secondary branches of mammal burrows (Hickman, 1990; Nevo, 1999). Nearly all tunnels of subterranean rodents have primary shafts with side branches, termed blateralsQ (Vleck, 1981). Laterals are about 0.3 3 m long for some pocket gophers, and are inclined at angles of 458 508 (Hickman, 1977; Andersen, 1988). In contrast, the burrows of reptiles, such as snakes and tortoises, are shallow, straight, and without branches (Hallinan, 1923; Retallack, 2001). Exceptions include the communal hibernacula of tortoises, which may be longer and more anastamosing than most tortoise tunnels (Auffenberg and Weaver, 1969). Carnivorous mammals also dig relatively short, straight tunnels (Voorhies, 1975a; Hunt et al., 1983).

Modern rodent tunnels reach lengths of 30 68 m (Smith, 1948; Flynn, 1990; Zuri and Terkel, 1996); some even up to 400 m (Davies and Jarvis, 1986) or 600 m (Begall and Gallardo, 2000). Length, however, is not a reliable means of distinguishing mammalian from reptilian fossil burrows. Tunnels of modern tortoises Gopherus polyphemus and Gopherus agassizi reach lengths of about 6 10 m (Hallinan, 1923; Voorhies, 1975a), and hibernacula may be much longer (Auffenberg and Weaver, 1969). Complexity of burrows (e.g., branching pattern) and size and morphology of digging traces may be more useful indicators. In map view, the burrows resemble the wandering, directionless tunnels of modern fossorial rodents, such as the pocket gopher Geomys bursarius (Smith, 1948; Andersen, 1988), bamboo rat Rhizomys sinensis (He, 1984), or mole rats (Jarvis and Sale, 1971, Figs. 6 and 7; Heth, 1989, Fig. 1). Short (1 2 m long) shafts that angle upward from the horizontal (Fig. 3B) may be analogous to the laterals of modern rodent tunnels. Overlapping shafts (Fig. 5B) may represent parts of complex, inter-weaving passages within a single burrow system, excavated by one individual. Alternatively,

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Fig. 10. (A–D) Photographs of KUVP 131061, with labels denoting surficial morphologic features: (A) lateral view (bilobate terminus to left), with line A–A1 marking location of cross-section; (B) ceiling view showing comparatively few surficial features; (C) end-on view of bilobate terminus; (D) drawing of C; (E) close-up of lateral view in A, showing prominent ridges in sets of two to three. Most ridges angle upward at about 258, with nearly vertical arcs cutting across at about 578.

they may have been part of two or more systems that by-passed one another. Bilobate termini seen in these fossil burrows (Figs. 10C–D) are similar to clawed-out areas in other fossil burrows attributable to rodents (Gobetz and Martin, 2006), or the bbootQ at the end of other possible rodent burrows (Morgan and Lucas, 2000). A bilobate terminus may represent a chamber or, in the case of some short, Y-shaped partitions seen at the burrow site (Fig. 5A), an abandoned digging attempt. In either case, similar structures are found in the

tunnels of modern burrowing rodents, as suggested by illustrated burrows of the mole rat Heterocephalus glaber (Nevo, 1999, Fig. 8.1). 3.4. Evidence from surficial morphology If the burrows described herein are those of rodents, the traces on them would most likely be from the rodents’ incisors or fore claws, reflecting the most common digging strategies among modern rodents

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Fig. 11. Drawings of fossil tunnel segment KU 133022, collected in situ, showing divergent ridges on the sides and ceiling and linear ridges along the floor. The end wall is to the right in each drawing: (A) floor view, showing ridges running longitudinally from the single, rounded terminus (except for ridges on the right side, which curve downward from side wall); (B) ceiling view, showing some longitudinal ridges in the middle of the ceiling and others sweeping down and back from ceiling. Scale bar in A for both drawings.

(Nevo, 1999; Stein, 2000). Incisor mark morphology is distinguishable from that of claw marks, aiding in the identification of traces on fossil burrows. Incisor grooves are always paired and flat-edged, in contrast to the single, more knife-edged marks left by claws. Paired incisor grooves have been observed on Daemonelix burrows of fossorial beavers (Martin and Bennett, 1977), as well as prairie dog burrows (Schulz, 1942; Burns et al., 1989). Modern rodents also gnaw on limestone and chalk in the wild (Cuffey and Hattin, 1965; Gobetz and Hattin, 2002), leaving well-preserved examples of their tooth marks on rock. In contrast, claw marks tend to occur in sets that represent several digits on the same manus. For example, in an analysis of claw marks on a cave wall in Germany, Bachofen-Echt (1931) used distance between each claw mark in a set to identify the maker as a cave bear. The narrow width of the ridges on the burrows, along with their occurrence in sets of two to three, indicates excavation by claws rather than paired incisors. The relief of the claw marks on the burrows is probably less distinct and sharp than the claws that produced them, because the entire inside surface of the damp tunnel walls, including the inner surface of each claw mark, was eventually covered by roots and rootlets. The size and shape of traces on a fossil burrow should match the digging apparatus (teeth, claws) of the digger. Direct comparison is possible only when the excavator is found in its own burrow, as with fossil therapsids (Smith, 1987; Hotton, 1991; Groenewald et al., 2001) and paleocastorid beavers (Martin and Bennett, 1977). In burrows from which the digger is unknown, trace morphology provides an approximate identification. Skeletal remains of mylagaulid rodents were inventoried from the burrow site and sites adjacent to it and within the same unit. Morphometric analysis of these mylagaulids shows only insignificant variation in

dental (premolar) and postcranial parameters (Gobetz, unpublished data). This general similarity suggests one species, probably P. laevis, a dhornlessT mylagaulid described from the burrow site by Fagan (1960). Right humeri represent a minimum number of eight individuals. 3.5. Mylagaulids as scratch-diggers Mylagaulids, which are an extinct rodent family, were powerful scratch-diggers based on their morphology (Scott, 1937; Fagan, 1960), which closely resembles that of mole rats (e.g., Bathyergus suillus) and pocket gophers (e.g., G. bursarius). Scratch-digging is considered the oldest, most common digging strategy in mammals (Hildebrand, 1985; Nevo and Reig, 1990). Most quadrupedal, digitigrade mammals have some ability to dig. Even the platypus (Ornithorynchus anatinus) excavates burrows of about 13 m length in riverbanks (Eisenberg, 1983). Thus, mylagaulid rodents became specialized for a basic mammalian behavior. Based on forelimb morphology, mylagaulids are categorized with all other known scratch-diggers (Hildebrand, 1985). L. Martin (personal communication) suspected that the piece of tunnel collected by Galbreath might be that of a mylagaulid rodent, based on the tunnel’s diameter. In addition, the prone, splayed position and nearly complete articulation of the holotype P. laevis from the Pawnee Creek locality (KUVP 9808) suggests its preservation within a burrow, although the burrow itself was not recognized. Some fossil burrows are at least tentatively attributed to mylagaulids. For instance, the relatively large size of a burrow in the Niobrara Formation of Nebraska, and the strength presumably required to dig it, led Voorhies and Toots (1970) to suggest the horned mylagaulid Ceratogaulus [= Epigaulus] Korth, 2000 as a possible excavator. In the case of mylagaulids, burrows are

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3.6. Claw mark morphology from casts

Fig. 12. Knobs projecting from ceiling of fossil tunnels. These knobs show distinct ridges like those on the tunnel walls: (A) tunnel in situ, with several knobs indicated by arrows; (B) KUVP 131597 with knob in lateral view; (C) knob in A shown in dorsal view.

particularly valuable. Holotypes of some horned mylagaulids, such as Ceratogaulus minor, were deposited in stream channels and were probably transported away from the source area (Hibbard and Phillis, 1945). Thus, they are not associated with burrows. In contrast, P. laevis from the Pawnee Creek Formation is common in the same paleosol in which the fossil burrows occur. I here include a description of these burrows in the context of P. laevis morphology. As the tortoise Gopherus is also known to be a scratch-digger (Hildebrand, 1985), I compare and contrast its claw morphology and digging abilities with those of mylagaulids.

The claw marks on the burrows match the size of P. laevis fore claws, which are about 0.42 cm wide at the base, narrowing to about 0.22 cm at the tip (Fig. 13A). In contrast, the fore claws of a modern gopher tortoise (G. berlandieri, KU 20599) are short and blunt (Fig. 13B), with the tips averaging 0.63 cm across. The formation of marks from P. laevis and G. berlandieri claws was simulated by dragging fossil and modern claws through damp sand, as Martin and Bennett (1977) did for paleocastorin beavers. The sand was then cast in plaster, to simulate the positive relief of claw marks on the burrows. The modern American badger, Taxidea taxus jacksoni (KU 160830, male) was included in this experiment to represent possible claw marks from early badger-like carnivores occurring at the burrow site. Comparison with the badger was particularly important, as badger mani have claws up to 60 mm long and are capable of making fine motions (Quaife, 1978), such as those suggested by the knobs on the burrows. When the laterally compressed claws of P. laevis were experimentally dragged through wet sand, they left deep (up to 1.0 cm), narrow traces similar to the claw marks on the burrows. When held at a slight angle, the claws displaced the soil to one side as in the marks on the burrows. The claws of the badger left marks similar in morphology to the claws of the mylagaulid, but slightly rounder (e.g. not as laterally compressed and blade-like), and about twice the width (6.3 mm from a claw of 5.5 mm width). The shorter, straighter, and blunter claws of G. berlandieri left relatively wide, shallow marks and could not be manipulated to leave deep, recurved marks. Some sets of claw marks on the burrows are in deeply scooped out areas at one end of which the claw marks curve sharply. These marks suggest that the claws had considerable curvature and possibly that the manus was in a flexed position as it was withdrawn from the burrow wall. The morphology of these curving marks matches that of the claws of a mylagaulid (Fig. 13C). The curvature of the claws of mylagaulids would make them more likely to produce sharply curved marks than the claws of a tortoise. Fagan (1960) suggested that the P. laevis manus was permanently flexed and incapable of extension, which would have made it even more likely to leave curving marks. The medial phalanges and claws of mylagaulids have deep insertions for flexor muscles (personal observation), and their mani are sometimes preserved in a fully flexed (closed-fist) position. In contrast, gopher tortoises have relatively inflexible digits (Auffenberg and Weaver, 1969).

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Fig. 13. (A) Left manus of Pterogaulus laevis, KUVP 131699, a mylagaulid rodent that occurs abundantly in the Pawnee Creek Formation and in the burrow beds of the locality in this study. The laterally compressed foreclaws of exaggerated length are analogous to the claws of modern pocket gophers (Geomys bursarius) and mole rats (Bathyergus suillus), and apparently are designed for digging in loose, sandy sediments. (B) Manus of Gopherus polyphemus, KU 20599, a modern gopher tortoise with short, wide, blunt claws. (C) Curving set of ridges on fossil burrow KUVP 131597, showing that the morphology of the ridges and the mylagaulid claws are very similar.

Long, laterally compressed claws, such as those of mylagaulids, are best suited for cutting through and loosening soil, as opposed to pushing it, which is the adaptive value of flat, blunt claws. As Gobetz and Martin (2006) mention in the description of possible Gregorymys burrows, claws are best for excavating soft, loose sediments. Incisor-diggers with small claws, such as the blind mole rat Spalax ehrenbergi, are unable to enter the ground when faced with these soils (Reed, 1958). Mylagaulids, as highly adapted scratch-diggers, would have been well-suited to envir-

onments such as that of the Pawnee Creek Formation. Considering this, it is highly unlikely that mylagaulids were not burrowing extensively, mainly with their large, specialized claws, in areas where the fossil burrows occur. Limb morphology suggests that P. laevis, and probably other mylagaulids, were capable of making postero-lateral power strokes (Gobetz, unpublished data), and thus could have produced the patterns of claw marks seen on these burrows. Mylagaulid morphology also suggests an ability to dig with the head (Gobetz and Beatty, 2005); however, no unequivocal

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marks attributable to this behavior were seen on the fossil burrows and only the scratch-digging component of mylagaulid behavior is discussed herein. 3.7. Digging motions from orientation of surficial morphology The law of cross-cutting relationships is apt for determining the order in which structures were added to the burrow walls. The claw marks on the burrows do not cut across the fine network of rootlet traces. Instead, the rootlet traces cover the claw marks (Fig. 8), indicating that the claw marks preceded the growth of roots. The distinct preservation of the claw marks suggests that the sandy soil was compacted enough at the time of excavation that it did not cave in as the excavator’s manus withdrew for the next power stroke. Soil displacement around each claw mark indicates the approximate direction of excavation, similar to the way Brady (1947) determined direction from modern and fossilized scorpion trackways. The consistently shallower displacement of soil on the ceiling side of each mark indicates that the manus was held with the lateral edge angled toward the ceiling. Tunnels with blind end walls were intentionally collected for this study, because the excavator was presumably moving toward these termini before it stopped excavating at the end. Thus, the claw marks on termini may record the direction of excavation. Following this assumption, the claw mark patterns can be interpreted as specific digging motions. Most scratch-digging animals (sensu Hildebrand, 1985; Nevo, 1999; Stein, 2000) excavate soil with alternating motions of the manus. These motions might have produced the offset claw marks on either side of bilobate end walls (Fig. 10C–D), which probably do not represent simultaneous power strokes. Tunnels of Recent rodents (probably the prairie dog, Cynomys sp.) (Schulz, 1942, Fig. 15) show claw marks of comparable length and morphology to those on Alezichnos chelecharatos. Highly specialized sandswimmers, such as golden moles and marsupial moles, move both forelimbs in simultaneous lateral arcs (Gasc et al., 1986). These animals dig through very loose surface sand, however, and do not leave tunnels behind them; the sand collapses in their wake (Kingdon, 1974; Gasc et al., 1986). The excavator of the fossil burrows was not a sand-swimmer, from the similarity of these tunnels to those of other types of diggers, described below. In addition, the sediment filling the tunnels is different from the ambient sediments, suggesting that the tunnels were hollow spaces (actual burrows) that subsequently were filled.

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The pattern of claw marks on the burrows indicates a series of digging motions as follows: 1) the excavator began each power stroke with the claws of one manus touching the ceiling or the upper half of the side wall (Fig. 14A). Touching the claws to the ceiling would not be difficult if the claws were significantly longer than the manus itself. The opposite manus might have been braced against the side of the tunnel, or underneath the animal, at the start of the stroke. 2) With the lateral edge of the manus rotated dorsally, the forelimb swept downward and back to displace the soil (Fig. 14B–C). The excavator may also have left marks when traveling back the opposite way through the tunnel, bracing itself, turning around, or locomoting through the tunnel during everyday activity. This may help explain the occasional marks that trend in the opposite direction of the majority. Many subterranean rodents, such as the scratch-digging mole rats B. suillus and Tachyoryctes splendens, kick excavated soil behind them by abducting the pedes (Jarvis and Sale, 1971; Nevo, 1999). This is a backward, probably upward-angled motion. The pedes work simultaneously in mole rats, unlike the alternating mani (Jarvis and Sale, 1971; Nevo, 1999). In addition, mole rats brace themselves against the side walls of their tunnels with the pedes rotated laterally (Jarvis and Sale, 1971), as do geomyids (Andersen, 1988). Either of these actions might produce marks similar to the continuous longitudinal or upward-angled claw marks along or near the floors of the fossil burrows. Longitudinal marks on the floor near end walls may also represent places where the excavator pushed soil beneath its body with the mani and kicked it backward with the pedes. The excavator probably used the mani to scratch out all of the soil near an end wall, where it would have been facing into the tunnel end, and would not have had the space to use the hind limbs to kick soil. This may explain divergent claw marks on the bottom halves of burrow termini, which sweep downward and back like the marks on the ceiling. The nearly horizontal aspect of the marks on bilobate termini (Fig. 10C–D) may have resulted from the excavator rotating its body 908 to begin a sharp bend (Fig. 14E–F), as Andersen (1988) observed in pocket gophers. The knobs on the ceilings of some burrows that show radiating claw marks were probably dug out by the excavator. These clawed-out knobs indicate fine digging actions, more likely executed by a mammal than a tortoise. To date, modern tortoises are not known to dig out small areas in the ceilings of their tunnels, and it is possible that they would not be able to maneuver themselves within the tunnel to produce these struc-

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Fig. 14. (A–C) Diagram showing how the excavator of the fossil burrows might have excavated its tunnels, based on position and morphology of ridges on tunnel walls; (D) hypothetical ridge patterns on walls after excavation; (E) diagram showing possible position of the excavator at the beginning of a sharp bend in the tunnel, with body rotated about 908 as suggested in Andersen (1988); F) hypothetical ridge patterns after excavation.

tures. The clawed-out knobs may indicate places where the excavator dug around a stone to dislodge it, or dug out a root or tuber projecting down into the tunnel. They resemble the accessory tunnels seen in modern prairie dog burrows (Burns et al., 1989, Fig. 1A). It is not known whether the tunnels of any modern, geophyteeating fossorial rodent show similar knobs. 3.8. Systematic ichnotaxonomy Alezichnos, n. igen. Type ichnospecies—Alezichnos chelecharatos isp. nov. Diagnosis—Natural casts of sinuous, slightly ovate burrows up to at least 7.0 m in length, consisting of

nearly horizontal primary tunnels from which secondary tunnels and shafts arise. The asymmetrical, sinuous shape of these tunnels indicates that they form part of a rambling, labyrinthine system, which designates them as the genus Alezichnos. Asymmetry, varying directionality, and irregular branching patterns distinguish ichnofossils of the Alezichnos type from more distinctively shaped (strongly helical) terrestrial burrows, such as Daemonelix, and from other previously described continental vertebrate burrows. Etymology—This genus name is a combination of the Greek prefix alez-, which means bforeignerQ or bwandererQ; referring to the wandering aspect of these tunnels, and the Greek suffix -ichnos means btraceQ and is traditionally assigned to all ichnotaxa.

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chelecharatos, isp. nov. Diagnosis—Surficial morphology includes longitudinal and curving ridges, occurring most commonly on the sidewalls and floor. Termini of burrows may show ridges on the ceiling, and diverging away from the midline of the terminus. These ridges measure 5.88 cm in length, 0.36 cm in width (Table 1) and 0.93 cm depth on average. They frequently occur in sets of two or three, with about 0.77 cm between each ridge (Table 1). They are interpreted as claw marks. Etymology—The Greek prefix chele- translates to bnailQ or bclaw;Q the suffix -charatos means bengraved.Q Q This species name refers to the abundant ridges on the burrow walls. Holotype—KUVP 131601. Tunnel segment of about 100 cm length, 13.4 cm ceiling–floor diameter, and 13.9 cm transverse diameter (Table 1), with one bilobate terminus; the other end of the tunnel is broken (Fig. 10). This specimen was collected in situ, with the terminus oriented northward and angled dorsally across the strata at 308. Claw marks occur on the side and end walls, floor, and (less commonly) on the ceiling. Projecting knobs averaging 2.98  3.42 cm width occur sporadically on the walls and ceiling (Figs. 10A and 12 (paratype)). Many of these knobs have ridges radiating from their centers. Paratypes—Listed in Table 2. Type locality—KU-CO-55, SW1/4 sec. 26, T. 12N, R. 5 SW, near the town of Peetz, Logan County, Colorado, USA.

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Remarks—The branching pattern of A. chelecharatos resembles that of modern mammalian burrow systems, particularly the foraging tunnels of subterranean rodents. Burrow termini in A. chelecharatos may be either bilobate, as in the holotype, or single and rounded, as in the paratypes. The deep, narrow ridges were presumably made by mani with long, laterally compressed claws. 4. Conclusions New terrestrial vertebrate burrows from the Pawnee Creek Formation, termed A. chelecharatos n. igen. et isp., represent part of a terrestrial ichnofauna from a floodplain ecosystem in the late Miocene (Barstovian) of Colorado. Diameters of about 11 cm suggest a vertebrate excavator. The sinuous shape, random orientation, complex branching pattern, and considerable lengths of these burrows are most similar to mammalian excavations. Claw mark patterns on side and end walls are interpreted as the work of a scratch-digging excavator that dug with alternate motions of the mani while bracing itself and kicking soil backward with the pedes. The mani may have produced the downward-angled claw marks on the upper half of the burrow walls, and the divergent marks on the ceiling and end walls. The pedes may have produced the upward-angled marks on the bottom half of the burrows. Combining data from burrow morphology, traces on the burrows, and functional morphology of potential

Fig. 15. Schematic representation of how information from continental vertebrate burrows provides clues for a possible excavator. In this case, the summary of evidence from architectural and surficial morphology suggests the excavator was a scratch-digging mammal, probably the extinct mylagaulid rodent Pterogaulus laevis.

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excavators suggests that the most likely candidate was the mylagaulid rodent P. laevis (Fig. 15), an abundant component of the fossil fauna. Mylagaulids are traditionally pictured in or near burrows (for example, in Scott, 1937). In the famous mural by J. H. Matternes in the Smithsonian Museum of Natural History, a mylagaulid is depicted with badger-like pelage, sitting at a prairie-dog-like burrow mound. According to this reconstruction, mylagaulids would have excavated open burrows with entrance mounds, regularly come above ground, and perhaps lived in social groups like prairie dogs or paleocastorin beavers. Mammals that dig directionless tunnels (as opposed to vertical or vertically helical ones) are usually solitary, and spend nearly their entire lives underground. If these burrows are from P. laevis, they imply that this mylagaulid was not as similar to a prairie dog as traditional reconstructions suggest. The gopher-like body shape of mylagaulids, in addition to heavily muscled forelimbs with enormous claws, is also evidence that these animals may seldom have ventured above ground. The laterally compressed claws of P. laevis match the shape and size of the claw marks on the burrows. As such, these new burrows represent the first known evidence of mylagaulid digging behavior and ecology. Mylagaulids are curiosities among extinct organisms; they are featured in popular texts as the only horned rodents ever to evolve. The study of their possible behavioral artifacts sheds new light on the paleobiology and ecology of this extinct rodent family, which may have been an important component of the Miocene plains ecosystem. Acknowledgements L. Martin provided the insight for this project, and he and L. Krishtalka, R. Timm, W. Johnson, and D. Miao critically read the manuscript. S. Hasiotis, T. Holmes, and A. Lerner provided additional editorial comments. Thanks also to the Ross family, particularly D. Ross and his son Elliott, for their hospitality and guidance in the field. J. Chorn photographed the burrow specimens, and A. Maltese and J. Kozisek assisted with fieldwork. This research was completed as part of a Ph.D. dissertation at the University of Kansas, and was funded by a Geological Society of America Student Research Grant and by the Panorama Society of the University of Kansas Natural History Museum. References Andersen, D.C., 1982. Below-ground herbivory: the adaptive value of geomyid burrows. American Naturalist 119, 18 – 28.

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