| Colobops | |
|---|---|
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| A 3D reconstruction of the skull ofColobops, based on scan data obtained by Pritchardet al. (2018) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Lepidosauria |
| Order: | Rhynchocephalia |
| Suborder: | Sphenodontia |
| Genus: | †Colobops Pritchardet al.,2018 |
| Species: | †C. noviportensis |
| Binomial name | |
| †Colobops noviportensis Pritchardet al., 2018 | |
Colobops is agenus ofreptile from theLate Triassic ofConnecticut.[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] Thegeneric name,Colobops, is a combination ofκολοβός (kolobós), meaning shortened, andὤψ (ṓps), meaning face. This translation, "shortened face", refers to its short and triangular skull.Colobops is known from a single species,Colobops noviportensis. Thespecific name,noviportensis, is alatinization of New Haven, the name of both the geological setting of its discovery (the New Haven Arkose) as well asa nearby large city. The phylogenetic relations ofColobops are controversial. Its skull shares many features with those of the groupRhynchosauria, herbivorousarchosauromorphs distantly related to crocodilians and dinosaurs. However, many of these features also resemble the skulls of the groupRhynchocephalia, an ancient order of reptiles including the moderntuatara,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. AsColobops is only known from a skull, it is not certain which one of these groups it belonged to. Pritchardet al. (2018) interpreted it as a basal rhynchosaur,[1] while Scheyeret al. (2020) reinterpreted it as a rhynchocephalian.[2]
Theholotype skull ofColobops,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 ofMiddletown andMeriden.[3] This locale is part of theNew Haven Arkose, a subdivision of theNewark 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 beenUranium-Lead dated to the midNorian 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 byG.R. Case in 1982. A formal study of the specimen byHans-Dieter Sues andDonald 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 alacrimal bone, and noted that it originally possessed supposed fang-likepremaxillary teeth at the tip of the snout which were accidentally destroyed during preparation. These features led Sues & Baird to assign the skull toSphenodontia, a group containing most rhynchocephalians.[3]
The specimen finally received a formal name in early 2018, when a group led byAdam Pritchard provided new preparation and discussion of the skull, as well as giving it the nameColobops noviportensis. This study also included CT-scans of the specimen, proportional and numerical analyses of the enlarged temporal region, and aphylogenetic analysis in order to determine its relations. The mostparsimonious 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 byTorsten Scheyeret 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]
The snout ofColobops 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, thenasal bones (on the upper side of the snout) droop down to internally brace themaxillae (bones of the side of the snout). This feature is also known in rhynchosaurs and rhynchocephalians. The maxillae are also protected by the largeprefrontals (bones in front of the eyes), similar to the condition inturtles. The prefrontals are also contacted by the widepalatine bones of the roof of the mouth, similar tolepidosaurs (squamates and rhynchocephalians), as well as turtles. All of these features exist to strengthen the front part of the skull, which explains how theyconvergently evolved in multiple different types of reptiles.[1]
Colobops also possesses largeorbits (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 thefrontals (bones of the skull roof between the eyes) are separated from the orbit, a feature which is known to a lesser degree inSphenodon andClevosaurus, but not rhynchosaurs.[2] Another diagnostic feature ofColobops is the fact that the skull roof possesses a very large, diamond-shaped gap between its bones, referred to as afontanelle. 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 moderniguanians retain their fontanelles in adulthood, and it is conceivable thatColobops 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 inColobops, similar to derived rhynchosaurs.[1] However, later analyses argued that this apparent expansion was a misinterpretation due to thesquamosal being displaced and thepostorbital 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 twoparietal bones, has a thinsagittal crest running down its midline. This crest would have attached to powerful muscles for closing the jaw, such as them. adductor mandibulae profundus and them. 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 thebraincase 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 fullyossified thin and tall plate-like bone known as aparasphenoid rostrum, which extends forward along the midline of the rear part of the roof of the mouth. The epipterygoids (column-like bones between thepterygoids and braincase) are large and tall, and would have been the lower attachment point for them. pseudotemporalis superficialis. The only preserved portion of the mandible (lower jaw) was a large and pointedcoronoid process near the rear part of the skull. It would have been the lower attachment point for them. adductor mandibulae profundus.[1]
In order to determine which reptile groupColobops truly belonged to, its describers (Pritchardet al.) included it within a phylogenetic analysis. Their analysis was a modified version of one originally designed by Pritchard & Nesbitt (2017) to test the affinities of the beakeddrepanosaurAvicranium. WithColobops incorporated into the analysis and several character scores updated, themost parsimonious tree found that Colobops was an archosauromorph as the earliest diverging member of Rhynchosauria. This position was supported by three features of the snout and one feature of the supratemporal area. Like rhynchosaurs,Colobops had a shortened snout with a maxilla that overlaps the nasal. In addition, the supratemporal fenestrae of Colobops and rhynchosaurs are positioned high on the skull, about the same level as the upper edge of the orbit. Other archosauromorphs, such asProlacerta, had supratemporal fenestrae in a slightly lower position, with the bones forming the outer edge of the holes being positioned about midway up the orbit.[1]
However, this classification was only slightly better supported than certain alternative interpretations. A phylogenetic analysis constructs thousands of family trees, each of which include hundreds of "steps" in evolution where analyzed traits are evolved, lost, or reacquired. The family tree with the fewest "steps", known as the most parsimonious tree (MPT), is generally considered to be the most accurate under the principle ofOccam's razor. In the case of this analysis, the MPT consideredColobops to be a basal rhynchosaur. However, some family trees look completely different from the MPT despite only a being few evolutionary steps more complex. If new data is incorporated into the analysis, one of these alternative trees may become a new MPT, rewriting our knowledge of reptile classification in the process. The MPT given by Pritchardet al. (2018) is given below:[1]
Prior to receiving a formal name and description, the holotype ofColobops noviportensis was actually believed to be a rhynchocephalian upon its discovery and preliminary description by Sues & Baird (1993).[3] This alternative position forColobops was tested by Pritchardet al. in their phylogenetic analysis. The analysis found that the simplest family trees includingColobops within Rhynchocephalia were only 2 steps more complex than the MPT of the analysis, which considered it a rhynchosaur. In these trees, the closest relatives ofColobops wereclevosaurs such asClevosaurus. Two of the features which supported the assignment ofColobops as a basal rhynchosaur also happen to support its assignment as a rhynchocephalian, an example of convergent evolution between the two groups. These features include a maxilla which overlaps the nasal, and supratemporal fenestrae positioned high on the skull. In addition, the unfused frontal bones ofColobops also support a place among rhynchocephalians. The strict consensus (average result) of the simplest family trees which includeColobops within Rhynchocephalia is given below. In this strict consensus tree, the structure of Archosauromorpha is reduced to apolytomy, depicting a compromise between many family trees with competing structures but equal complexity.[1]
Many aspects of the anatomy ofColobops makes it difficult to evaluate its classification. One possibility is that the specimen is an infant, as supported by its large eyes, small size, and massive fontanelle in the skull roof. Juvenile specimens are notorious for jeopardizing the results of phylogenetic analyses, as diagnostic traits within adult species would not have developed yet.[1] However, the supposed massive jaw musculature ofColobops would be highly unusual for a young reptile, even compared to other rhynchosaurs (which are known to develop diagnostic traits at a young age).[5]
The redescription by Scheyeret al. (2020) expanded the data matrix with additional lepidosauromorph characteristics and taxa. In this expansion,Colobops is positioned as a rhynchocephalian next toSphenodon (the tuatara), with a minimum of 17 steps required to place it back as a basal rhynchosaur.[2]
The interpretation of Pritchardet al. (2018) supports the idea thatColobops possessed large jaw muscles. Most modern reptiles enlarge their jaw musculature by two methods, either developing large muscle receptor areas on the parietals bones in the middle of the skull, or by the supratemporal fenestrae being widened.Colobops, however, may have developed both of these methods at the same time, giving it a bite force unprecedented for its body size. This would have been further assisted by the tall coronoid process of the lower jaw. The heavily reinforced snout likely evolved in conjunction with the development of strong jaw muscles. Based on comparisons with both rhynchosaurs and rhynchocephalians,Colobops can safely be presumed to have fed using precise and strong bites, although it cannot be determined whether this was forcarnivory (as in the tuatara) orherbivory (as in rhynchosaurs), as no teeth have been preserved. The bones at the edge of the jaws were broad, a condition which is shared by livinglizards such asChamaeleolis chamaeleonides (the Cuban false chameleon) andDracaena guianensis (the Northern caiman lizard). These lizards specialize in hard-shelled prey such as crustaceans and snails.[1]
However, Scheyer et al. (2020) reinterpreted the supratemporal fenestrae as much narrower in life, with crushing and bone displacement artificially expanding the fossil. PlottingColobops with other reptiles according to these new proportional estimates shows thatColobops did not have unusually large jaw muscles for its size.[2]
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