Ornithomimosauria, Barsbold
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publication ID |
https://doi.org/ 10.5252/geodiversitas2022v44a25 |
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publication LSID |
urn:lsid:zoobank.org:pub:EA12DCB7-A5BE-4763-B805-25087EBD726D |
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DOI |
https://doi.org/10.5281/zenodo.6928928 |
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persistent identifier |
https://treatment.plazi.org/id/03C887B9-FF93-FFC3-76E4-A10DFC21F93D |
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treatment provided by |
Felipe |
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scientific name |
Ornithomimosauria |
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Ornithomimosauria View in CoL indet.
DESCRIPTION
Ornithomimosaurs are by far the most commonly represented vertebrates in Angeac-Charente, with more than 3800 macroremains collected ( Figs 29-30 View FIG View FIG ), accounting for more than 50% of the identified vertebrate material ( Rozada et al. 2021). The minimum number of individuals (MNI) is approximately 70 based on the distal end of left tibiae. Ornithomimosaur remains are mainly concentrated in the CG1 and CG3 loci, in which they represent 70 % of all the ornithomimosaur remains identified. Such a concentration and high number of individuals are congruent with a mass mortality event of an ornithomimosaur herd ( Allain et al. 2011, 2014; Néraudeau et al. 2012; Rozada et al. 2021). However, no articulated skeletons have been observed due to the intense trampling (dinoturbation) affecting this area ( Rozada et al. 2021). The only articulated remains of ornithomimosaurs found so far come from the northwestern part of the quarry (CG9 plot) and they include the zeugopod and the autopod of the forelimb of a single individual, as well as the the zeugopod and autopod of the hindlimb of another single individual.
Except for the most fragile elements such as the maxillary and palate bones, which have probably suffered from trampling and have not yet been identified, the skeleton of the Angeac-Charente ornithomimosaur is virtually complete ( Fig. 31 View FIG ). A complete description of the entire skeleton of this new taxon is beyond the scope of this study. Nevertheless, it seems important to highlight here key anatomical features of the Angeac-Charente ornithomimosaur:first, because this clade was hitherto unknown in Europe at the beginning of the Cretaceous ( Allain et al. 2014); secondly because it may be the oldest known ornithomimosaur to date ( Choiniere et al. 2012; Cerroni et al. 2019); thirdly, because it shows very close anatomical similarities to Limusaurus , which is a Late Jurassic Chinese theropod that is not considered a member of the Ornithomimosauria, but a ceratosaurian ( Xu et al. 2009). These similarities include a very large external mandibular fenestra and short forelimbs with manual digit reduction (RA pers. obs.). Relationships between ceratosaurians and ornithomimosaurs have long been confusing (e.g. Marsh 1895; Janensch 1925; Galton 1982; Holtz 1994; Rauhut 2003). Some taxa,including Elaphrosaurus , Deltadromeus , Limusaurus , Nqwebasaurus and probably the Angeac-Charente taxon do not have a clearly established phylogenetic position, and their anatomy may also reflect unexpected and unrecognized relationships between ceratosaurians and ornithomimosaurs. Pending a comparative and detailed phylogenetic study, we provide herein some anatomical features that clearly indicate the ornithomimosaurian affinity of Angeac-Charente material.
Besides the features already mentioned byAllain et al. (2014), we mainly used the anatomical characters discussed in the recent reappraisal of the phylogenetic position of Afromimus byCerroni et al. (2019).The edentulous and downturned dentary ( Fig.29A View FIG ) is an ornithomimosaurian synapomorphy convergently acquired by numerous other coelurosaurian groups ( Zanno & Makovicky 2010). It is worth noting that outside coelurosaurs only Limusaurus displays a toothless skull and mandible in mature individuals ( Wang et al. 2017). The pedal unguals of the Angeac-Charente theropod have a weak longitudinal curvature and exhibit the reduction of the flexor tubercle to a ventral platform seen in ornithomimosaurs, but also in abelisauroids ( Fig. 29B, C View FIG ; Cerroni et al. 2019: fig. 7). Nevertheless, they are more reminiscent of ornithomimosaurs, being slender, and having a triangular cross-section and a single ventral groove ( Longrich 2008), whereas pedal unguals of Afromimus and Masiakasaurus are shorter and possess a dorsal vascular groove.
The centrum of the middle and distal caudal vertebrae is long and low( Fig.29 View FIG D-L).The anterior and posterior articular surfaces are slightly wider than tall, with a reniform contour ( Fig. 29H, L View FIG ). A broad and shallow sulcus is present on the ventral surface, and it is laterally delimited by two prominent ridges ( Fig. 29E, J View FIG ). All these features are present in ornithomimosaurs ( Osmolska et al. 1972; Longrich 2008) but also in Elaphrosaurus ( Rauhut & Carrano 2016) . As in all ornithomimosaurs, the robust and tongue-shaped prezygapophyses of the Angeac-Charente taxon are elongated anteroposteriorly, up to three-quarters the length of the centrum. They are horizontally directed ( Fig. 29F, K View FIG ) and do not diverge laterally from the sagittal plane ( Fig. 29D, I View FIG ). Conversely, the zygapophyses of ceratosaurs are slender, shorter and directed anterodorsally ( Carrano et al. 2002; O’Connor 2007, Cerroni et al. 2019).
The tibia of the Angeac-Charente ornithomimosaur has already been described in detail ( Allain et al. 2014).Here, we figure new material to highlight the features that best differentiate it from a ceratosaur tibia ( Fig. 30 View FIG A-D). The proximal end of the tibia is markedly different from that of Ceratosaurus , Masiakasaurus , Carnotaurus , Majungasaurus , Afromimus and Elaphrosaurus having a fibular crest clearly separated from the proximal articular surface ( Fig. 30 View FIG A-C), as in tetanuran theropods and thus all the ornithomimosaurs. The elliptical scar present on the posterior surface of the proximal end of the tibia of some ceratosaurs is not visible in the Angeac-Charente taxon ( Cerroni et al. 2019). As in all ornithomimosaurs, the anterior surface of the distal end of the tibia of the Angeac-Charente taxon bears a tall and transversely expanded flat articular surface for the ascending process of the astragalus ( Fig. 30D View FIG ). There is no medial buttress to accommodate the ascending process as in many basal tetanurans and ceratosaurs, including Berberosaurus , Masiakasaurus , Majungasaurus and Ceratosaurus .
The medial face of the fibula bears a deep and proximodistally elongate elliptical fossa for the insertion of musculus popliteus. This fossa opens medially and is anteriorly and posteriorly bounded by sharp rims ( Fig. 30E View FIG ). Such a condition is only known in coelurosaurs and Elaphrosaurus , and markedly differs from the condition seen in coelophysoids and ceratosaurs, in which the fossa is covered anterodorsally by the tibial crest and thus opens posteriorly ( Rauhut 2003; Allain et al. 2007; Rauhut & Carrano 2016; Cerroni et al. 2019).
In common with the tibia and fibula, the astragalus has a morphology typical of the coelurosaurs and very different from that of the ceratosaurs ( Fig.30 View FIG F-H).In contrast to Ceratosaurus , Elaphrosaurus , Masiakasaurus , and abelisaurids, the astragalus is fused neither to the calcaneum nor the tibia or fibula ( Fig. 30H View FIG ). The height of the blade-like ascending process of the astragalus is more than twice the height of astragalar body and the process arises from the complete breadth of the astragalar body ( Fig. 30 View FIG F-G). In contrast, all ceratosaurs exhibit a low and narrow ascending process. In addition, the fibular facet on the astragalus is strongly reduced on the lateral side of the ascending process of the astragalus ( Fig. 30H View FIG ). In contrast, the distal end of the fibula of numerous abelisauroids including Berberosaurus , Masiakasaurus , Afromimus and Majungasaurus is transversely expanded and the flared distal end partially overlaps the ascending process of astragalus, the fibular facet of which is large. As previously stated ( Néraudeau et al. 2012, Allain et al. 2013, 2014), all surveyed anatomical features agree with assignment of the Angeac-Charente theropod to Ornithomimosauria.
Cerroni et al. (2019) have recently questioned the ornithomimosaurian phylogenetic affinities of the Early Cretaceous African Nqwebasaurus ( Choiniere et al. 2012) . If confirmed, it would imply that the Charentais taxon is the oldest known ornithomimosaur, based on the Berriasian age of the Lägerstatte of Angeac-Charente ( Benoit et al. 2017; Polette et al. 2018). Moreover, ornithomimosaurs would then have an exclusively Laurasian distribution. Nevertheless, based on first hand examination of fossil specimens by one of us (R.A.), the phylogenetic affinities of Limusaurus and Deltadromeus are far from certain. More detailed descriptions regarding their anatomy are required to draw conclusions regarding the origin and evolution of ornithomimosaurs.
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