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New ankylosaur (Thyreophora, ornithischia) remains from the Upper Cretaceous of Patagonia
Journal of South American Earth Sciences 96 (2019) 102320
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New ankylosaur (Thyreophora, ornithischia) remains from the Upper Cretaceous of Patagonia Alejandro Murraya, Facundo Riguettia,b, Sebastián Rozadillac,b,
T
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Fundación Azara, Hidalgo 775, 7th floor, 1405, Buenos Aires, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina c Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina a
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A R T I C LE I N FO
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Keywords: Ankylosauria Ornithischia Upper cretaceous Gondwana Patagonia
Fossil evidences of the presence of ankylosaurian dinosaurs in Gondwana are scarce but consistent, being found in Antarctica, Oceania and South America. In spite that there are no nominated species in South America, the ankylosaur fossil record has increased in the last years. Indeterminate nodosaurid specimens, some isolated osteoderms and many trackways are known from the Upper Cretaceous of South America. The aim of the present contribution is to report new ankylosaurian remains from the Allen Formation (Campanian-Maastrichtian) at the Salitral Moreno locality, Northern Patagonia, Argentina. These osteoderms are small and conical, and includes thoracic, sacral and caudal scutes. The thoracic and sacral pieces are similar to those belonging to nodosaurids. The caudal osteoderm is a new element for the record of South American ankylosaurs. It resembles the caudal plates of Kunbarrasaurus and some ankylosaurids. The scutes show a mixture of characters so it is not possible to assign these pieces to a nodosaurid-like or ankylosaurid ankylosaur. These elements are consistent with the previously known ankylosaur fossil record of the Upper Cretaceous of Argentina, being a new sample of the diversity of the latest Cretaceous from South America.
1. Introduction The fossil record of dinosaurs from the Late Cretaceous of South America is characterized by a high diversity and abundance of Theropoda and Sauropoda. In Argentina, many species identified as members of these clades have been described, however, ornithischian remains are scarce when compared with saurischian diversity (Coria, 2016). In the South American Senonian (Campanian-Maastrichtian), the ornithischians are mainly represented by hadrosaurids, counting with an abundant record of specimens, nested in a few species of Hadrosauridae (Brett-Surman, 1979; Bonaparte et al., 1984; Juárez Valieri et al., 2010; Coria et al., 2012; Cruzado-Caballero and Powell, 2017). Nonetheless an unidentified species of Ankylosauria have been described by Salgado and Coria (1996) being the first undoubted record for this clade in South America. The ankylosaur remains have been collected in the Salitral Moreno locality, at Río Negro Province, from beds corresponding to the Maastrichtian Allen Formation. A diverse Latest Cretaceous tetrapod assemblage occurs in this formation (Leanza et al., 2004), that shows
some faunistic similarities with the Northern Hemisphere; having coetaneous hadrosaurian and ankylosaurian remains. The ornithischians from Allen Formation are represented by the hadrosaurs Willinakaqe salitralensis (Juárez Valieri et al., 2010; Cruzado-Caballero and Coria, 2016), Lapampasaurus cholinoi (González Riga and Casadío, 2000; Coria et al., 2012), Bonapartesaurus rionegrensis (Cruzado-Caballero and Powell, 2017), and ankylosaurian materials referred to Nodosauridae indet. (Coria and Salgado, 2001). The Allen Formation has also yielded remains of several taxa assigned to Titanosauria (Salgado and Coria, 1993; Salgado and Azpilicueta, 2000; Martinelli and Forasiepi, 2004; Garcia and Salgado, 2003). Bonaparte (1986) suggested a faunistic interchange with North America during the Late Cretaceous, in which hadrosaurs and ankylosaurs immigrated to South America, whilst the South American titanosaurs emigrated to North America. Therefore, the Allen Formation reflects a complex community, consisting of a mixed association of local and immigrant taxa. Since the description of ankylosaur remains by Salgado and Coria (1996), the knowledge of gondwanan ankylosaurs has been gradually increasing with the description of Antarctopelta oliveroi (Salgado and
⁎ Corresponding author. Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina. E-mail addresses: [email protected] (A. Murray), [email protected] (F. Riguetti), [email protected] (S. Rozadilla).
https://doi.org/10.1016/j.jsames.2019.102320 Received 4 June 2019; Received in revised form 15 August 2019; Accepted 15 August 2019 Available online 19 August 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.
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sharp from the apex to the base. The base is deeply concave and their edges are not complete. Dorsally its surface is ornamented by grooves and foramina. The remaining osteoderms are putatively considered as thoracic scutes. In dorsal view the elements are oval or oblong (Fig. 2C). The keel is high, posterodorsally oriented and laterally displaced. One piece is conical (Fig. 2A), resembling a spine but without exceeding in height the scute diameter (the term spine sensu Blows, 2001 represents an osteoderm higher than the longest diameter of the base). This “spine” presents a gradual dorsoventral curvature (Fig. 2A1). The anterior edge of the “spine” is acute from the apex to the base in anterior view, although the posterior edge is rounded. In lateral view the posterior margin is steeper, almost vertical (Fig. 2A2). The base is slightly concave (Fig. 2A3). The ventral surface and edges of this scute are rugose, whereas its dorsal surface is ornamented by grooves and foramina. It is similar in aspect to the spines of MPC 100/1305 from the Upper Cretaceous of North America (Carpenter et al., 2011; Arbour and Currie, 2013) and the problematic spike of Struthiosaurus austriacus (PIUW 2349/15) (Pereda-Suberbiola and Galton, 2001; Ösi, 2015; Ösi and Pereda-Suberbiola, 2017). Other two scutes (Fig. 2B and C) possess a straight and pointed dorsal keel, which in lateral view shows its anterior edge that rises gradually above the base, whereas the posterior edge descends almost vertically. One of these scutes (Fig. 2C) lacks its apex and the posterior border of its base. The anterior border of this piece is sharp in anterior view. The more complete scute displays a vertical groove on its posterior edge (Fig. 2B1). The base of this scute displays a sub-pentagonal outline in ventral view. The edges of its base are slightly expanded and ventrally smooth, with few small foramina. The base of this scute is deeply concave. Dorsally, both scutes are ornamented with foramina interconnected by small shallow radial grooves.
Gasparini, 2006) from the Campanian of Antarctica, and Kunbarrasaurus ieversi (Leahey et al., 2015) from the lower Cretaceous of Australia. These taxa, together with the previously described Minmi paravertebra (Molnar, 1980) from the Lower Cretaceous of Australia, exhibit a mix of features that make them focus of hard phylogenetic debate that ends in an unstable phylogenetic position for these dinosaurs (Carpenter, 2001; Salgado and Coria, 1996; Arbour and Currie, 2015; Arbour et al., 2016). Nevertheless, since the work of Salgado and Coria (1996) the osteological record of South American ankylosaurs has barely increased, only summing up a couple of isolated osteoderms, vertebrae and a tooth (Coria and Salgado, 2001; De Valais et al., 2003; Gasparini et al., 2015). We here describe new ankylosaur materials from the Allen Formation (Campanian-Maastrichtian) of the Rio Negro province, Argentina, and discuss both its phylogenetic affinities and palaeobiogeographic implications. Institutional abbreviations: MACN Pv, Colección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina. 2. Systematic palaeontology DINOSAURIA Owen (1842). ORNITHISCHIA Seeley (1887). THYREOPHORA Nopcsa (1915). ANKYLOSAURIA Osborn (1923). gen. et sp. indet Referred material: MACN Pv RN1149, five isolated osteoderms. 3. Locality and horizon
5. Discussion and conclusions
These materials come from strata belonging to the Allen Formation (late Campanian – early Maastrichtian; Hugo y Leanza, 2001) which are exposed in the Eastern region of the Salitral Moreno, about 50 km South of the city of Gral. Roca, Río Negro Province, Argentina. These materials were collected from the Lower Member of the Allen Formation, conformed by light grey to yellowish mid to coarse-grained sandstones, well to moderately selected, interpreted as a fluvial palaeoenvironment with moderate energy (Hugo and Leanza, 2001; Simon, 2006). The ankylosaur bone plates were recovered together with hadrosaur and titanosaur materials, and these lack a clear association each other. In this regard, is hard to tell if the ankylosaur materials belong to a unique or different individuals.
The isolated and fragmentary nature of the specimens here described precludes an assignation to the genus level. The material is confidently identified as an ankylosaur. Conical scutes of several heights with a concave base of different depths and extended basal borders are characteristic of ankylosaurian osteoderms (see Ford, 2000). Although there have been many studies that tried to develop a comprehensive ankylosaur osteoderm systematics (Ford, 2000; Scheyer and Sander, 2004; Burns, 2008; Hayashi et al., 2010; Arbour et al., 2011; Burns and Currie, 2014; Ösi and Pereda-Suberbiola, 2017), we are cautious to assign these elements to some of the major ankylosaur subclades (e.g. Nodosauridae or Ankylosauridae). Excepting the caudal osteoderm, the other pieces are similar to the scutes described as “first type” by Coria and Salgado (2001) from Salitral Moreno. The depressed scute with projected borders is similar to those from the sacral region of the Nodosauridae Stegopelta (Moodie, 1910: pl. LV), Gastonia (Kirkland, 1998; Kinneer et al., 2016) and Edmontonia (Gilmore, 1930), and to the dorsosacral scutes of Sauropelta (Ostrom, 1970) and Borealopelta (Brown et al., 2017). On the other hand, by comparison with the above mentioned taxa and Asian Ankylosauridae such as Dyoplosaurus (Maryanska, 1970) or Saichania (Maryanska, 1977), the conical scutes could pertain to in the thoracic region, and the curved spine in the cervico-pectoral region (following the regionalization proposed by Ford, 2000). Finally, the caudal osteoderm is similar to the lateral caudal plates of Ankylosaurus (Carpenter, 2004), Kunbarrasaurus (Molnar, 2001), Saichania (Carpenter et al., 2011) and Tarchia (Arbour et al., 2013). Up to date, caudal armour was still unknown in South American ankylosaurs. The referral of available material to a subclade within Ankylosauria is complex, since present osteoderms display characteristics shared with both nodosaurids and ankylosaurids. A similar combination of nodosaurid and ankylosaurid characters is shared with southern ankylosaurs, including Antarctopelta oliveroi (Salgado and Gasparini, 2006)
4. Description The recovered osteoderms can be grouped in three different morphotypes. The first morphotype (Fig. 1A0) resembles a ridged scute (see Ford, 2000: Fig. 2B, and Scheyer and Sander, 2004: Fig. 2C). It presents an elliptical contour in dorsal view (Fig. 1A1). There is a domed, low central keel, which runs along the major axis of the scute. The height of the central keel gradually increases posteriorly (Fig. 1A3). The posterior margin presents a steeped slope, resembling the triradiate osteoderm of Dongyangopelta (Chen et al., 2013), as well as the scute MPCAPv-41 from Salitral Moreno described by Coria and Salgado (2001). The margins of the scute are expanded radially. Dorsally, the osteoderm surface is ornamented with furrows and foramina. The ventral surface is smooth and slightly concave (Fig. 1A2). The second type of osteoderm (Fig. 1B) could be identified as a caudal plate (primary caudal osteoderm according to Ford, 2000). The piece is conical and slightly transversely compressed (Fig. 1B1,2). It is asymmetrical with the apex displaced posteriorly (Fig. 1B2). Therefore the anterior edge rises more gradually to the apex than the posterior. The osteoderm is symmetrical in anterior view (Fig. 1B3). The posterior margin of this plate is smoothly rounded whereas the anterior border is 2
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Fig. 1. Osteoderm types of an indeterminate ankylosaur dinosaur (MACN-PV-RN-1149). A. Sacral osteoderm in dorsal (A1), ventral (A2), and lateral (A3) views. B. Caudal osteoderm in dorsal (B1), lateral (B2) and anterior (B3) views. Anterior is up in A1,2 and B1; anterior is to the left in A3, B2; dorsal is up in A3 and B2,3.
(such as Ankylosauria and Hadrosauridae) together with Gondwanan groups such as Titanosauria and Abelisauridae (also present in the Late Cretaceous of North America). This is consistent with the biogeographic hypothesis proposed by Bonaparte (1986) who hypothesized that these local fauna were the result of a biotic interchange between both Americas during the Campanian. Gasparini et al. (1996), proposed the alternative hypothesis suggesting that hadrosaurs and ankylosaurs may belong to an endemic fauna in South America. In this sense, ankylosaur remains were recovered from the Lower Cretaceous of Australia (e.g. Minmi, Kunbarrasaurus; Molnar, 1980; Molnar, 2001; Leahey et al., 2015) and footprints from the Jurassic of Brazil (Francischini et al., 2017). This implies that a land connection between the Americas is not
from the Upper Cretaceous of Antarctica, and Kunbarrasaurus ieversi and Minmi paravertebra from the Lower Cretaceous of Australia (Thompson et al., 2012; Arbour and Currie, 2015; Arbour et al., 2016; Brown et al., 2017). Thus, the phylogenetic position of the gondwanan ankylosaurs is unclear and its evolutionary story is far from being understood. Therefore, it is necessary to make an effort in searching for remains to increase the ankylosaur fossil record of southern hemisphere. Ankylosaurs are regarded as part of the faunal assemblages coming from the Allen Formation (Leanza et al., 2004), as well as similar contemporary fauna observed at La Colonia Formation (Gasparini et al., 2015). This assemblage has been named Allenian (Leanza et al., 2004), and includes a mixture of taxa typical from the Northern Hemisphere
Fig. 2. Osteoderm types of an indeterminate ankylosaur dinosaur (MACNPV-RN-1149). A. Curved thoracic osteoderm in anterior (A1), lateral (A2) and ventral (A3) views. B. Thoracic osteoderm in posterior (B1, showing a vertical groove, g) and lateral (B2) views. C. Thoracic osteoderm in dorsal view. Anterior is up in A3, C; anterior is to the left in A2, B2; dorsal is up in A1,2 and B1,2.
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mandatory to explain the presence of ankylosaurs in the Upper Cretaceous of South America and Antarctica.
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Acknowledgements We want to thank Sebastián Apesteguía, Pablo Gallina, Leonardo J. “Harry” Pazo, Fernando Garberoglio, Stella M. Álvarez, Martín D. Ezcurra, Federico L. Agnolín, Agustín Martinelli, James I. Kirkland, Ralph E. Molnar, Attila Ösi, Michael E. Burns, Victoria Arbour, Tracy Ford and Penélope Cruzado-Caballero for their extremely valuable assistance in this project, either through observations, advice or support. We also thank the Marcos Becerra and an anonymous reviewer for its valuable recommendations to make a better presentation of this research. References Arbour, V.M., Burns, M.E., Currie, P.J., 2011. A review of pelvic shield morphology in ankylosaurs (dinosauria: ornithischia). J. Paleontol. 85 (2), 298–302. Arbour, V.M., Currie, P.J., 2013. The taxonomic identity of a nearly complete ankylosaurid dinosaur skeleton from the Gobi Desert of Mongolia. Cretac. Res. 46, 24–30. Arbour, V.M., Lech-Hernes, N.L., Guldberg, T.E., Hurum, J.H., Currie, P.J., 2013. 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New ankylosaur (Thyreophora, ornithischia) remains from the Upper Cretaceous of Patagonia Alejandro Murraya, Facundo Riguettia,b, Sebastián Rozadillac,b,
T
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Fundación Azara, Hidalgo 775, 7th floor, 1405, Buenos Aires, Argentina Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina c Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ankylosauria Ornithischia Upper cretaceous Gondwana Patagonia
Fossil evidences of the presence of ankylosaurian dinosaurs in Gondwana are scarce but consistent, being found in Antarctica, Oceania and South America. In spite that there are no nominated species in South America, the ankylosaur fossil record has increased in the last years. Indeterminate nodosaurid specimens, some isolated osteoderms and many trackways are known from the Upper Cretaceous of South America. The aim of the present contribution is to report new ankylosaurian remains from the Allen Formation (Campanian-Maastrichtian) at the Salitral Moreno locality, Northern Patagonia, Argentina. These osteoderms are small and conical, and includes thoracic, sacral and caudal scutes. The thoracic and sacral pieces are similar to those belonging to nodosaurids. The caudal osteoderm is a new element for the record of South American ankylosaurs. It resembles the caudal plates of Kunbarrasaurus and some ankylosaurids. The scutes show a mixture of characters so it is not possible to assign these pieces to a nodosaurid-like or ankylosaurid ankylosaur. These elements are consistent with the previously known ankylosaur fossil record of the Upper Cretaceous of Argentina, being a new sample of the diversity of the latest Cretaceous from South America.
1. Introduction The fossil record of dinosaurs from the Late Cretaceous of South America is characterized by a high diversity and abundance of Theropoda and Sauropoda. In Argentina, many species identified as members of these clades have been described, however, ornithischian remains are scarce when compared with saurischian diversity (Coria, 2016). In the South American Senonian (Campanian-Maastrichtian), the ornithischians are mainly represented by hadrosaurids, counting with an abundant record of specimens, nested in a few species of Hadrosauridae (Brett-Surman, 1979; Bonaparte et al., 1984; Juárez Valieri et al., 2010; Coria et al., 2012; Cruzado-Caballero and Powell, 2017). Nonetheless an unidentified species of Ankylosauria have been described by Salgado and Coria (1996) being the first undoubted record for this clade in South America. The ankylosaur remains have been collected in the Salitral Moreno locality, at Río Negro Province, from beds corresponding to the Maastrichtian Allen Formation. A diverse Latest Cretaceous tetrapod assemblage occurs in this formation (Leanza et al., 2004), that shows
some faunistic similarities with the Northern Hemisphere; having coetaneous hadrosaurian and ankylosaurian remains. The ornithischians from Allen Formation are represented by the hadrosaurs Willinakaqe salitralensis (Juárez Valieri et al., 2010; Cruzado-Caballero and Coria, 2016), Lapampasaurus cholinoi (González Riga and Casadío, 2000; Coria et al., 2012), Bonapartesaurus rionegrensis (Cruzado-Caballero and Powell, 2017), and ankylosaurian materials referred to Nodosauridae indet. (Coria and Salgado, 2001). The Allen Formation has also yielded remains of several taxa assigned to Titanosauria (Salgado and Coria, 1993; Salgado and Azpilicueta, 2000; Martinelli and Forasiepi, 2004; Garcia and Salgado, 2003). Bonaparte (1986) suggested a faunistic interchange with North America during the Late Cretaceous, in which hadrosaurs and ankylosaurs immigrated to South America, whilst the South American titanosaurs emigrated to North America. Therefore, the Allen Formation reflects a complex community, consisting of a mixed association of local and immigrant taxa. Since the description of ankylosaur remains by Salgado and Coria (1996), the knowledge of gondwanan ankylosaurs has been gradually increasing with the description of Antarctopelta oliveroi (Salgado and
⁎ Corresponding author. Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina. E-mail addresses: [email protected] (A. Murray), [email protected] (F. Riguetti), [email protected] (S. Rozadilla).
https://doi.org/10.1016/j.jsames.2019.102320 Received 4 June 2019; Received in revised form 15 August 2019; Accepted 15 August 2019 Available online 19 August 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.
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sharp from the apex to the base. The base is deeply concave and their edges are not complete. Dorsally its surface is ornamented by grooves and foramina. The remaining osteoderms are putatively considered as thoracic scutes. In dorsal view the elements are oval or oblong (Fig. 2C). The keel is high, posterodorsally oriented and laterally displaced. One piece is conical (Fig. 2A), resembling a spine but without exceeding in height the scute diameter (the term spine sensu Blows, 2001 represents an osteoderm higher than the longest diameter of the base). This “spine” presents a gradual dorsoventral curvature (Fig. 2A1). The anterior edge of the “spine” is acute from the apex to the base in anterior view, although the posterior edge is rounded. In lateral view the posterior margin is steeper, almost vertical (Fig. 2A2). The base is slightly concave (Fig. 2A3). The ventral surface and edges of this scute are rugose, whereas its dorsal surface is ornamented by grooves and foramina. It is similar in aspect to the spines of MPC 100/1305 from the Upper Cretaceous of North America (Carpenter et al., 2011; Arbour and Currie, 2013) and the problematic spike of Struthiosaurus austriacus (PIUW 2349/15) (Pereda-Suberbiola and Galton, 2001; Ösi, 2015; Ösi and Pereda-Suberbiola, 2017). Other two scutes (Fig. 2B and C) possess a straight and pointed dorsal keel, which in lateral view shows its anterior edge that rises gradually above the base, whereas the posterior edge descends almost vertically. One of these scutes (Fig. 2C) lacks its apex and the posterior border of its base. The anterior border of this piece is sharp in anterior view. The more complete scute displays a vertical groove on its posterior edge (Fig. 2B1). The base of this scute displays a sub-pentagonal outline in ventral view. The edges of its base are slightly expanded and ventrally smooth, with few small foramina. The base of this scute is deeply concave. Dorsally, both scutes are ornamented with foramina interconnected by small shallow radial grooves.
Gasparini, 2006) from the Campanian of Antarctica, and Kunbarrasaurus ieversi (Leahey et al., 2015) from the lower Cretaceous of Australia. These taxa, together with the previously described Minmi paravertebra (Molnar, 1980) from the Lower Cretaceous of Australia, exhibit a mix of features that make them focus of hard phylogenetic debate that ends in an unstable phylogenetic position for these dinosaurs (Carpenter, 2001; Salgado and Coria, 1996; Arbour and Currie, 2015; Arbour et al., 2016). Nevertheless, since the work of Salgado and Coria (1996) the osteological record of South American ankylosaurs has barely increased, only summing up a couple of isolated osteoderms, vertebrae and a tooth (Coria and Salgado, 2001; De Valais et al., 2003; Gasparini et al., 2015). We here describe new ankylosaur materials from the Allen Formation (Campanian-Maastrichtian) of the Rio Negro province, Argentina, and discuss both its phylogenetic affinities and palaeobiogeographic implications. Institutional abbreviations: MACN Pv, Colección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina. 2. Systematic palaeontology DINOSAURIA Owen (1842). ORNITHISCHIA Seeley (1887). THYREOPHORA Nopcsa (1915). ANKYLOSAURIA Osborn (1923). gen. et sp. indet Referred material: MACN Pv RN1149, five isolated osteoderms. 3. Locality and horizon
5. Discussion and conclusions
These materials come from strata belonging to the Allen Formation (late Campanian – early Maastrichtian; Hugo y Leanza, 2001) which are exposed in the Eastern region of the Salitral Moreno, about 50 km South of the city of Gral. Roca, Río Negro Province, Argentina. These materials were collected from the Lower Member of the Allen Formation, conformed by light grey to yellowish mid to coarse-grained sandstones, well to moderately selected, interpreted as a fluvial palaeoenvironment with moderate energy (Hugo and Leanza, 2001; Simon, 2006). The ankylosaur bone plates were recovered together with hadrosaur and titanosaur materials, and these lack a clear association each other. In this regard, is hard to tell if the ankylosaur materials belong to a unique or different individuals.
The isolated and fragmentary nature of the specimens here described precludes an assignation to the genus level. The material is confidently identified as an ankylosaur. Conical scutes of several heights with a concave base of different depths and extended basal borders are characteristic of ankylosaurian osteoderms (see Ford, 2000). Although there have been many studies that tried to develop a comprehensive ankylosaur osteoderm systematics (Ford, 2000; Scheyer and Sander, 2004; Burns, 2008; Hayashi et al., 2010; Arbour et al., 2011; Burns and Currie, 2014; Ösi and Pereda-Suberbiola, 2017), we are cautious to assign these elements to some of the major ankylosaur subclades (e.g. Nodosauridae or Ankylosauridae). Excepting the caudal osteoderm, the other pieces are similar to the scutes described as “first type” by Coria and Salgado (2001) from Salitral Moreno. The depressed scute with projected borders is similar to those from the sacral region of the Nodosauridae Stegopelta (Moodie, 1910: pl. LV), Gastonia (Kirkland, 1998; Kinneer et al., 2016) and Edmontonia (Gilmore, 1930), and to the dorsosacral scutes of Sauropelta (Ostrom, 1970) and Borealopelta (Brown et al., 2017). On the other hand, by comparison with the above mentioned taxa and Asian Ankylosauridae such as Dyoplosaurus (Maryanska, 1970) or Saichania (Maryanska, 1977), the conical scutes could pertain to in the thoracic region, and the curved spine in the cervico-pectoral region (following the regionalization proposed by Ford, 2000). Finally, the caudal osteoderm is similar to the lateral caudal plates of Ankylosaurus (Carpenter, 2004), Kunbarrasaurus (Molnar, 2001), Saichania (Carpenter et al., 2011) and Tarchia (Arbour et al., 2013). Up to date, caudal armour was still unknown in South American ankylosaurs. The referral of available material to a subclade within Ankylosauria is complex, since present osteoderms display characteristics shared with both nodosaurids and ankylosaurids. A similar combination of nodosaurid and ankylosaurid characters is shared with southern ankylosaurs, including Antarctopelta oliveroi (Salgado and Gasparini, 2006)
4. Description The recovered osteoderms can be grouped in three different morphotypes. The first morphotype (Fig. 1A0) resembles a ridged scute (see Ford, 2000: Fig. 2B, and Scheyer and Sander, 2004: Fig. 2C). It presents an elliptical contour in dorsal view (Fig. 1A1). There is a domed, low central keel, which runs along the major axis of the scute. The height of the central keel gradually increases posteriorly (Fig. 1A3). The posterior margin presents a steeped slope, resembling the triradiate osteoderm of Dongyangopelta (Chen et al., 2013), as well as the scute MPCAPv-41 from Salitral Moreno described by Coria and Salgado (2001). The margins of the scute are expanded radially. Dorsally, the osteoderm surface is ornamented with furrows and foramina. The ventral surface is smooth and slightly concave (Fig. 1A2). The second type of osteoderm (Fig. 1B) could be identified as a caudal plate (primary caudal osteoderm according to Ford, 2000). The piece is conical and slightly transversely compressed (Fig. 1B1,2). It is asymmetrical with the apex displaced posteriorly (Fig. 1B2). Therefore the anterior edge rises more gradually to the apex than the posterior. The osteoderm is symmetrical in anterior view (Fig. 1B3). The posterior margin of this plate is smoothly rounded whereas the anterior border is 2
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Fig. 1. Osteoderm types of an indeterminate ankylosaur dinosaur (MACN-PV-RN-1149). A. Sacral osteoderm in dorsal (A1), ventral (A2), and lateral (A3) views. B. Caudal osteoderm in dorsal (B1), lateral (B2) and anterior (B3) views. Anterior is up in A1,2 and B1; anterior is to the left in A3, B2; dorsal is up in A3 and B2,3.
(such as Ankylosauria and Hadrosauridae) together with Gondwanan groups such as Titanosauria and Abelisauridae (also present in the Late Cretaceous of North America). This is consistent with the biogeographic hypothesis proposed by Bonaparte (1986) who hypothesized that these local fauna were the result of a biotic interchange between both Americas during the Campanian. Gasparini et al. (1996), proposed the alternative hypothesis suggesting that hadrosaurs and ankylosaurs may belong to an endemic fauna in South America. In this sense, ankylosaur remains were recovered from the Lower Cretaceous of Australia (e.g. Minmi, Kunbarrasaurus; Molnar, 1980; Molnar, 2001; Leahey et al., 2015) and footprints from the Jurassic of Brazil (Francischini et al., 2017). This implies that a land connection between the Americas is not
from the Upper Cretaceous of Antarctica, and Kunbarrasaurus ieversi and Minmi paravertebra from the Lower Cretaceous of Australia (Thompson et al., 2012; Arbour and Currie, 2015; Arbour et al., 2016; Brown et al., 2017). Thus, the phylogenetic position of the gondwanan ankylosaurs is unclear and its evolutionary story is far from being understood. Therefore, it is necessary to make an effort in searching for remains to increase the ankylosaur fossil record of southern hemisphere. Ankylosaurs are regarded as part of the faunal assemblages coming from the Allen Formation (Leanza et al., 2004), as well as similar contemporary fauna observed at La Colonia Formation (Gasparini et al., 2015). This assemblage has been named Allenian (Leanza et al., 2004), and includes a mixture of taxa typical from the Northern Hemisphere
Fig. 2. Osteoderm types of an indeterminate ankylosaur dinosaur (MACNPV-RN-1149). A. Curved thoracic osteoderm in anterior (A1), lateral (A2) and ventral (A3) views. B. Thoracic osteoderm in posterior (B1, showing a vertical groove, g) and lateral (B2) views. C. Thoracic osteoderm in dorsal view. Anterior is up in A3, C; anterior is to the left in A2, B2; dorsal is up in A1,2 and B1,2.
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mandatory to explain the presence of ankylosaurs in the Upper Cretaceous of South America and Antarctica.
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Acknowledgements We want to thank Sebastián Apesteguía, Pablo Gallina, Leonardo J. “Harry” Pazo, Fernando Garberoglio, Stella M. Álvarez, Martín D. Ezcurra, Federico L. Agnolín, Agustín Martinelli, James I. Kirkland, Ralph E. Molnar, Attila Ösi, Michael E. Burns, Victoria Arbour, Tracy Ford and Penélope Cruzado-Caballero for their extremely valuable assistance in this project, either through observations, advice or support. We also thank the Marcos Becerra and an anonymous reviewer for its valuable recommendations to make a better presentation of this research. References Arbour, V.M., Burns, M.E., Currie, P.J., 2011. A review of pelvic shield morphology in ankylosaurs (dinosauria: ornithischia). J. Paleontol. 85 (2), 298–302. Arbour, V.M., Currie, P.J., 2013. The taxonomic identity of a nearly complete ankylosaurid dinosaur skeleton from the Gobi Desert of Mongolia. Cretac. Res. 46, 24–30. Arbour, V.M., Lech-Hernes, N.L., Guldberg, T.E., Hurum, J.H., Currie, P.J., 2013. 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