| Eoabelisaurus | |
|---|---|
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| Reconstructed skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Abelisauroidea |
| Genus: | †Eoabelisaurus Pol & Rauhut,2012 |
| Type species | |
| †Eoabelisaurus mefi Pol & Rauhut, 2012 | |
Eoabelisaurus (/ˈioʊəˌbɛlɪˈsɔːrəs/) is agenus ofabelisauroidtheropoddinosaur from theLower JurassicCañadón Asfalto Formation of theCañadón Asfalto Basin inArgentina,South America. The generic name combines a Greek ἠώς, (eos), "dawn", with the nameAbelisaurus, in reference to the fact it represents an early relative of the latter. Only one species is currently recognized,E. mefi; thespecific name honours the MEF, theMuseo Paleontológico "Egidio Feruglio", where discoverer Diego Pol is active.[2] It is characterized by reduced forelimb proportions that show primitive characteristics of theAbelisauridae family.
In 2009, Argentinianpaleontologist Diego Pol discovered the skeleton of a theropod near the village ofCerro Cóndor inChubut Province. The remains were found in the Jugo Loco locality that is placed in a series of fine beds of mudstone, marlstone, and limestone in theCañadón Asfalto Formation. In 2012, based on these remains, thetype speciesEoabelisaurus mefi was named and described by Pol and his German colleague Oliver Rauhut.[2]
Before discovery, the oldest known abelisaurids were represented only by fragmentary remains from the late Early Cretaceous of South America and Africa and older records of abelisauroids in general were questionable. With the discovery ofEoabelisaurus, the temporal range of the clade was extended for more than 40 million years into theToarcian. The existence of a derived ceratosaur at that time indicated a rapid diversification of ceratosaurs during that time period, as all the major ceratosaurian lineages (ceratosaurids, noasaurids, and abelisaurids) had already been established.[2]
It was abipedalcarnivore that was estimated to have reached 6–6.5 metres (19.7–21.3 ft) in length,[2] although a comprehensive analysis of abelisaur size conducted in 2016 yielded a size estimate of 5.8 metres (19 ft).[3] It is estimated to have weighed up to 730–800 kilograms (1,610–1,760 lb).[4][5] Theholotype specimen,MPEF PV 3990, was uncovered in a layer of theCañadón Asfalto Formation, a lacustrine deposit dating originally from theAalenian-Bajocian, roughly 170 million years old, yet a recent advanced zircon datation constrained its age to the Middle-Late Toarcian (179-178 million years).[6] It consists of a nearly complete skeleton with skull, of a subadult or adult individual.[2]
The posterior half of the skeleton was found in articulation and the anterior dorsal and cervical vertebrae and forelimbs were found partially disarticulated prior to burial. The skull was discovered slightly separated from the vertebral column. The skull and anterior presacrals were also exposed at the time of discovery and had been partially been destroyed by erosion. From the material known of the snout, only a small fragment of the right maxilla has been recovered and shows that the interdental plates are fused, but not striated. The known posterior part of the skull is high, with an oval orbit and an enlarged infratemporal fenestra, similar to otherceratosaurs. The skull roof is not notably thickened and no cranial ornamentation is present.[2]
Both humeri from the known material are poorly preserved, but show primitive characters. The articular head is slightly rounded, but not a globular shape that is commonly seen innoasaurids andabelisaurids. The radius and ulna are short and the ulna has a large olecranon process. The manus is very foreshortened, retains four digits, and has short metacarpals. Metacarpal I is about half the length of metacarpal II and considerably more slender. The arm also bears a stout phalanx that is slightly longer than the metacarpal. The non-terminal manual phalanges are about as long as wide and lack any constriction between the articular ends, and manual unguals are reduced. It is these reduced limb proportions that demonstrateEoabelisaurus was indeed a primitive abelisaurid.[2]
The exact number of vertebrae is unknown due to several gaps in the holotype's spine, but its cervical vertebrae are short and have two pneumatic foramina on either side of the centra. The length of vertebral centra remains constant over the preserved portion of the tail, but middle and posterior caudals are considerably lower.[2]
Eoabelisaurus was assigned to the basalmost position inAbelisauridae by its describers. It would, then, be the oldest abelisaurid species known by forty (40) million years. The describers indicated that in thecladistic analysis a difference of only a single trait would have resulted in a position lower in the evolutionary tree, basal in theAbelisauroidea. The followingcladogram follows their analysis.[2]
In 2018, Rafael Delcourt placedEoabelisaurus at the basic position withinCeratosauridae, noting that the topology of this genus is still problematic.[7] The following cladogram demonstrates the relationships ofEoabelisaurus according to latter publication. However, such classification is not well-supported.
| Ceratosauroidea |
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Subsequent analyses placedEoabelisaurus as a basal abelisaurid,[8][9] a sister taxon of the Abelisauroidea,[10] or an abelisauroid outside the abelisaurid family.[11]
Before the discovery ofEoabelisaurus, abelisaurid anatomy was only known from a handful ofLate Cretaceous taxa that were aberrant in their morphology, such as their unusual skull structure and reduction of their forelimbs.Eoabelisaurus shows what was previously an unknown stage in the evolution of abelisaurids, having only some of the cranial modifications and a unique combination of features in its forelimbs. The manus ofEoabelisaurus have a derived morphology, with short and robust metacarpals, non-terminal phalanges, and reduced manual unguals. The humerus is unreduced, and the ulna and radius are shortened but do not differ from more basal ceratosaurs. Therefore, the modification of the forelimbs in the evolution of abelisaurids seems to have occurred in a modular fashion, starting with the distal elements such as the fingers, and only later affecting the more proximal elements. It is with this evidence thatEoabelisaurus confirms a recently proposed evolutionary scenario of abelisaurid limbs based on the incomplete evidence of Cretaceous abelisauroids.[2]
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