| Terminocavus | |
|---|---|
 | |
| Holotype parietals from the front and back | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ceratopsia |
| Family: | †Ceratopsidae |
| Subfamily: | †Chasmosaurinae |
| Genus: | †Terminocavus Fowler & Freedman Fowler,2020 |
| Type species | |
| †Terminocavus sealeyi Fowler & Freedman Fowler, 2020 | |
Terminocavus is agenus ofceratopsiddinosaur from the lateCretaceousPeriod of what is nowNorth America. The genus contains a single species, thetype speciesTerminocavus sealeyi, known from a parietal and some other associated fragments. Theholotype specimen was discovered in theKirtland Formation ofNew Mexico in 1997, and was later described and named in a 2020 study. It was similar in anatomy toPentaceratops andAnchiceratops, which it was closely related to, but had a distinctive heart-shaped upper frill with very narrow notch. It has been hypothesized to form an anagenetic series with several other chasmosaur species.
The holotype specimenNMMNH P-27468, collected in 1997, consists of a parietal (or fused paired parietals), other skull fragments, a partial sacrum, and vertebral fragments. It was discovered in grey siltstone deposits from theCampanian Hunter Wash Member of theKirtland Formation of theSan Juan Basin in New Mexico. It is the only diagnostic chasmosaurine specimen discovered from the middle or upper part of the Hunter Wash Member. The age of the specimen is undetermined; its frill texture indicates it is a young subadult, but its large size and epiparietal fusion would indicate it represents an adult.[1]
A 2005 abstract referred the specimen to the genusPentaceratops, though noted as aberrant for the genus.[1] Joshua Fry questioned the referral in a 2015 masters thesis, with a phylogenetic analysis failing to group it alongside otherPentaceratops specimens.[2] It was informally named as a distinct genus in 2016.[3] Later, in 2020, it was formally named and described by Denver W. Fowler and Elizabeth A. Freedman Fowler. The nameTerminocavus means "coming to the end of the cavity", referring to the parietal embayment being nearly closed off before being lost completely in more derived taxa. The specific name,sealeyi, refers to the discoverer of the holotype specimen, Paul Sealey. Naming the specimen as a new species ofPentaceratops was decided against to prevent the genus becoming paraphyletic.[1]
The nearly complete, but highly distorted skull PMU 24923 may belong to the species as well. Also hailing from the Kirtland Formation,Charles H. Sternberg discovered it in 1921. It was later named as the new species "Pentaceratops fenestratus" in 1930, but by future authors have considered its distinctiveness to be a result of pathology. Though sometimes considered a synonym ofPentaceratops sternbergi, Fowler and Freedman Fowler considered it more likely to belong to eitherNavajoceratops orTerminocavus due to its deep, narrow median embayment and broad parietal bar. The specimen is too distorted to allow confident referral to either, however.[1]
Known from limited material,Terminocavus is distinguished from close relatives such asPentaceratops andAnchiceratops by the anatomy of its parietal (the upper portion of its frill), which forms a heart shape. The prominent median embayment (a large notch in the middle of the top of the frill) of earlier relatives is heavily reduced, being very narrow as opposed to wide and U-shaped.Terminocavus' parietal bars (the top edges of the frill) are thin and extremely broad compared to earlier relatives; they are more plate-shaped than bar-shaped. Its median bar (the middle strut) has also expanded, bearing more pronounced flanges than its ancestorNavajoceratops. The parietal fenestrae (the holes in the frill) have a more rounded shape than the ancestral angular state, and are smaller due to the expanded parietal and median bars. Overall, the anatomy is intermediate between that of more primitive genera likePentaceratops and that of more derived ones likeAnchiceratops andtriceratopsins.[1]
Like most other chasmosaurs, its paired fused parietals combined bear six epiparietals (small horns along the parietal), symmetrically arranged with three on each side. The first pair, small and triangular, project from the top edge of the median embayment, and in life would have touched each other. The second pair are a larger set of triangles, whereas the third epiparietals have a rounded, "D" shape; both project upwards, angled in line with the rest of the parietal. The preserved right squamosal (bone which forms the right side of the frill) itself is long, indicating adultTerminocavus had a very large frill similar to that of its relatives. A singular, fused episquamosal (small horns along the squamosal) is also known from the holotype; it is rugose and indistinct from that of other ceratopsids. The left epijugal horn is known as well, fused to the jugal and quadratojugal bones; it robust and large, but unlike that ofPentaceratops is not especially long.[1]
Terminocavus known remains are slightly smaller than those ofUtahceratops andPentaceratops, indicating it was an animal of roughly similar adult size.[1]
Terminocavus was a member of the ceratopsid subfamilyChasmosaurinae. Fowler and Freedman Folwler (2020) divide chasmosaurs into two lineages; a "Chasmosaurus-line" leading toKosmoceratops and a "Pentaceratops-line" leading to more derived taxa.Terminocavus belongs to the latter group. Phylogenetic analysis found it to be relatively derived, more basal thanAnchiceratops but more derived thanNavajocertops. The tree, however, is unstable; removing some taxa from the analysis caused much of thePentaceratops-line to collapse into an unresolved polytomy. Additionally, it was noted the coding of thePentaceratops andChasmosaurus data may requires revision, as it likely contains specimens of more than one species; it was noted this could be impacting the results negatively. Several taxa had been named too recently to be included in the study describingTerminocavus, and their inclusion in an analysis could also shift its position. One analysis from Fowler and Freedman Fowler (2020) is reproduced below:[1]
It was proposed thatTerminocavus is a part of a longanagenetic lineage of chasmosaurs. The genera (in sequence)Utahceratops,Pentaceratops,Navajoceratops,Terminocavus, andAnchiceratops would, under this model, represent a single population of organisms changing in form over time, rather than as a diverse assemblage of close relatives. This is evidenced by their parietal morphology, which show consistent trends of change. These are: the gradual closure of the initially large medial embayment, flattening and expansion of the parietal bar, reduction of size of the parietal fenestrae, the change of its shape from (sub)angular to round, and development of flanges on the medial bar. Geometric morphometric analysis supported the sequence, finding them to plot in the expected order. The phylogenetic analysis (see above), however, complicated matters, with the generaCoahuilaceratops andBravoceratops plotting within the supposed anagenetic lineage. It was noted, however, that these are fragmentary and undiagnostic respectively, and so they may not be of significance.[1]
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