Colobops
From Wikipedia, the free encyclopedia
| Colobops Temporal range: Middle Norian | |
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| A 3D reconstruction of the skull of Colobops, based on scan data obtained by Pritchard et al. (2018) | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Lepidosauria |
| Order: | Rhynchocephalia |
| Suborder: | Sphenodontia |
| Genus: | †Colobops Pritchard et al., 2018 |
| Species: | †C. noviportensis |
| Binomial name | |
| †Colobops noviportensis Pritchard et al., 2018 | |
Colobops is a genus of reptile from the Late Triassic of Connecticut.[1][2] Only known from a tiny skull (estimated total length of 2.8 centimeters or 1.1 inches long),[3] this reptile has been interpreted to possess skull attachments for very strong jaw muscles. This may have given it a very strong bite, despite its small size.[1] However, under some interpretations of the CT scan data, Colobops's bite force may not have been unusual compared to other reptiles.[2] The generic name, Colobops, is a combination of κολοβός (kolobós), meaning shortened, and ὤψ (ṓps), meaning face. This translation, "shortened face", refers to its short and triangular skull. Colobops is known from a single species, Colobops noviportensis. The specific name, noviportensis, is a latinization of New Haven, the name of both the geological setting of its discovery (the New Haven Arkose) as well as a nearby large city. The phylogenetic relations of Colobops are controversial. Its skull shares many features with those of the group Rhynchosauria, herbivorous archosauromorphs distantly related to crocodilians and dinosaurs. However, many of these features also resemble the skulls of the group Rhynchocephalia, an ancient order of reptiles including the modern tuatara, Sphenodon.[3] Although rhynchosaurs and rhynchocephalians are not closely related and have many differences in the skeleton as a whole, their skulls are remarkably similar. As Colobops is only known from a skull, it is not certain which one of these groups it belonged to. Pritchard et al. (2018) interpreted it as a basal rhynchosaur,[1] while Scheyer et al. (2020) reinterpreted it as a rhynchocephalian.[2]
The holotype skull of Colobops, YPM VPPU 18835, is mostly complete, although flattened and missing tooth-bearing portions of the cranial bones. The specimen was discovered in 1965 during highway construction in central Connecticut between the towns of Middletown and Meriden.[3] This locale is part of the New Haven Arkose, a subdivision of the Newark supergroup. The Newark supergroup is a collection of Late Triassic formations along the eastern coast of North America, and the New Haven Arkose has specifically been Uranium-Lead dated to the mid Norian age, about 214.0 to 209.8 million years ago.[4]
The skull was not described in an academic context until 1993, although photographs of the specimen had been featured in "A pictorial guide to fossils", a natural history book published by G.R. Case in 1982. A formal study of the specimen by Hans-Dieter Sues and Donald Baird in 1993 offered a discussion of its classification, but did not provide a scientific name for the reptile in question. This study considered the skull to lack a lacrimal bone, and noted that it originally possessed supposed fang-like premaxillary teeth at the tip of the snout which were accidentally destroyed during preparation. These features led Sues & Baird to assign the skull to Sphenodontia, a group containing most rhynchocephalians.[3]
The specimen finally received a formal name in early 2018, when a group led by Adam Pritchard provided new preparation and discussion of the skull, as well as giving it the name Colobops noviportensis. This study also included CT-scans of the specimen, proportional and numerical analyses of the enlarged temporal region, and a phylogenetic analysis in order to determine its relations. The most parsimonious results of the phylogenetic analysis indicated that the reptile was a basal rhynchosaur, although the analysis also showed that a position within Rhynchocephalia was only slightly less likely to be true.[1] A 2020 reinterpretation by Torsten Scheyer et al. argued that the skull was crushed and several bones were displaced, and that it more closely resembled a rhynchocephalian once these issues were rectified.[2]
Description
The snout of Colobops is very short, with the portion of the skull in front of the eyes occupying only a quarter of the total length of the skull. This portion of the snout is also reinforced by overlapping bones. For example, the nasal bones (on the upper side of the snout) droop down to internally brace the maxillae (bones of the side of the snout). This feature is also known in rhynchosaurs and rhynchocephalians. The maxillae are also protected by the large prefrontals (bones in front of the eyes), similar to the condition in turtles. The prefrontals are also contacted by the wide palatine bones of the roof of the mouth, similar to lepidosaurs (squamates and rhynchocephalians), as well as turtles. All of these features exist to strengthen the front part of the skull, which explains how they convergently evolved in multiple different types of reptiles.[1]
Colobops also possesses large orbits (eye holes), although this may be a juvenile feature. The upper edge of each orbit is formed by the upper rear branch of a prefrontal and the upper forward branch of a postfrontal (bone behind the eye). This means that the frontals (bones of the skull roof between the eyes) are separated from the orbit, a feature which is known to a lesser degree in Sphenodon and Clevosaurus, but not rhynchosaurs.[2] Another diagnostic feature of Colobops is the fact that the skull roof possesses a very large, diamond-shaped gap between its bones, referred to as a fontanelle. Fontanelles typically can be used to characterize infant animals with skull roofs that are not completely fused. However, under the interpretation that the skull has overlapping bones and large sites for muscle attachment, the skull could be interpreted as belonging to a much older animal. A few species of modern iguanians retain their fontanelles in adulthood, and it is conceivable that Colobops was similar.[1] The presence of a fontanelle would be less unprecedented if the skull belonged to a juvenile.[2]
The rear part of the skull roof, formally known as the supratemporal area, has a pair of large holes known as supratemporal fenestrae. These holes were initially interpreted as quite broad in Colobops, similar to derived rhynchosaurs.[1] However, later analyses argued that this apparent expansion was a misinterpretation due to the squamosal being displaced and the postorbital being incomplete.[2] Only a small area of bone is present between the supratemporal fenestrae. This area of bone, formed by the fusion of the two parietal bones, has a thin sagittal crest running down its midline. This crest would have attached to powerful muscles for closing the jaw, such as the m. adductor mandibulae profundus and the m. pseudotemporalis superficialis. Colobops would have been the smallest known reptiles to possess such a powerful and expanded supratemporal area,[1] although uncertainty in the shape of the skull may oppose this interpretation.[2]
Although the braincase is only partially known, certain features can be recognized. The supraoccipital (upper part of the braincase) has small prongs which brace the parietals from behind. Unlike some lepidosaurs, Colobops possesses a fully ossified thin and tall plate-like bone known as a parasphenoid rostrum, which extends forward along the midline of the rear part of the roof of the mouth. The epipterygoids (column-like bones between the pterygoids and braincase) are large and tall, and would have been the lower attachment point for the m. pseudotemporalis superficialis. The only preserved portion of the mandible (lower jaw) was a large and pointed coronoid process near the rear part of the skull. It would have been the lower attachment point for the m. adductor mandibulae profundus.[1]
