# Trihecaton

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***Trihecaton*** is an extinct [genus](/source/Genus) of [microsaur](/source/Microsaur) from the Late [Pennsylvanian](/source/Pennsylvanian_(geology)) of [Colorado](/source/Colorado). Known from a single species, ***Trihecaton howardinus***, this genus is distinctive compared to other microsaurs due to possessing a number of [plesiomorphic](/source/Plesiomorphy_and_symplesiomorphy) ("primitive") features relative to the rest of the group. These include large intercentra (wedge-like components of the [vertebrae](/source/Vertebra)), folded [enamel](/source/Tooth_enamel), and a large [coronoid process](/source/Coronoid_process_of_the_mandible) of the jaw. Its classification is controversial due to combining a long body with strong limbs, features which typically are not present at the same time in other microsaurs. Due to its distinctiveness, *Trihecaton* has been given its own [monospecific](/source/Monotypic_taxon) [family](/source/Family_(taxonomy)), **Trihecatontidae**.[1][2]

## Discovery

*Trihecaton* is known from well-preserved fossils discovered in [Fremont County](/source/Fremont_County,_Colorado), [Colorado](/source/Colorado) by a [UCLA](/source/University_of_California,_Los_Angeles) field expedition in 1970. These fossils were found at a quarry near the town of [Howard](/source/Howard,_Colorado). The quarry preserved sediments from the [Sangre de Cristo Formation](/source/Sangre_de_Cristo_Formation), a geological formation dated to the Late [Carboniferous](/source/Carboniferous) period, specifically the Missourian subsection near the end of the [Pennsylvanian subperiod](/source/Pennsylvanian_(geology)). *Trihecaton* fossils are known from a narrow band of shale in the quarry, a layer known as "Interval 300". The [generic name](/source/Genus) *Trihecaton* references Interval 300 while the [specific name](/source/Specific_name_(zoology)) *T. howardinus* is named after the nearby town.[1]

The [holotype](/source/Holotype) fossil, originally called UCLA VP 1743, is a well-preserved skeleton lacking most of the skull and large portions of the hip, hindlimbs, and tail. However, a second fossil, UCLA VP 1744, incorporates a string of tail vertebrae and was found right next to the holotype, indicating that it was probably from the same individual. These fossils were described in 1972 as part of one of [Peter Paul Vaughn](/source/Peter_Paul_Vaughn)'s reviews of new Sangre de Cristo fauna. Vaughn also named a new family, Trihecatontidae, specifically for the new genus.[1] In 1987, Vaughn's collections were moved to the [Carnegie Museum of Natural History](/source/Carnegie_Museum_of_Natural_History),[3] with specimens UCLA VP 1743 and 1744 now known as CM 47681 and 47682.[4]

## Description

Most of the skull bones were not preserved, with the exception of both lower jaws as well as a toothed maxilla bone. All of the teeth are slender and conical, and their internal structure has shallowly folded [enamel](/source/Tooth_enamel) unlike almost all other microsaurs. The teeth are largest right behind the tip of the snout and diminish in size towards the back of the mouth. The complete left mandible was 2.6 centimeters (1.0 inches) in length, indicating that the skull was similar in size. The mandible would have preserved approximately nineteen teeth, further back it rises up into a pronounced [coronoid process](/source/Coronoid_process_of_the_mandible).[1]

The body was fairly long, with about 36 vertebrae in the presacral vertebral column (i.e. the portion between the head and the hip). The holotype's presacral vertebral column was 16 cm (6.3 inches) long. The presacral vertebrae have plate-like neural spines on top which alternate in appearance. Some neural spines are low and ridge-like while others are taller towards the rear of their respective vertebrae. This switch between "short" and "tall"-type neural spines seems to alternate through most (but not all) of the backbone. The "tall"-type neural spines towards the rear of the body split longitudinally, while those towards the front stay intact. The [atlas vertebra](/source/Atlas_(anatomy)) was similar to that of other microsaurs, with a central spine-like knob (known as an odontoid process), a pair of adjacent wing-like facets for the braincase, and a deep pit for the [notochord](/source/Notochord) visible from behind. The rest of the vertebrae are gastrocentrous, meaning that they have large main portions known as pleurocentra, as well as somewhat smaller crescent-shaped bones known as intercentra, which wedge between the pleurocentra throughout the body. Most microsaurs have diminished or absent intercentra, but *Trihecaton* has somewhat large ones, albeit not as large as the pleurocentra. Ribs, when preserved, contact both the intercentra and pleurocentra. The ribs are tapered towards the rear of the body and have expanded tips towards the front, although the first few ribs were missing. The referred tail vertebrae are simple, with short neural spines and [haemal arches](/source/Haemal_arch) fused to the intercentra.[1]

The [interclavicle](/source/Interclavicle) (the middle element of the [shoulder girdle](/source/Shoulder_girdle)) was broad and T-shaped, with a remarkably short rear prong. The [humerus](/source/Humerus) (upper arm bone) was robust and twisted, with a distinct [entepicondylar foramen](/source/Entepicondylar_foramen). Other preserved bones of the shoulder and arm were similar to those of the large microsaur *[Pantylus](/source/Pantylus)*. The hip was not well preserved, but the [femur](/source/Femur) (thigh bone) was present, with an S-shaped shaft constricted in the middle. The skeletal remains as a whole were covered with thin, oblong scales.[1]

## Classification

In various aspects of the general body shape, vertebral construction, and limbs, *Trihecaton* is clearly a member of a group of lizard-like Paleozoic amphibians called microsaurs. Although it is far from the oldest member of the group, *Trihecaton* possesses several [plesiomorphic](/source/Plesiomorphy_and_symplesiomorphy) ("primitive") features which indicate that it had a very [basal](/source/Basal_(phylogenetics)) position compared to other microsaurs. For example, no other microsaurs possessed large intercentra with rib facets (in the body) or haemal spines (in the tail), though *[Microbrachis](/source/Microbrachis)* did have small intercentra and *Pantylus* did have small haemal spines. In addition, *Trihecaton*'s folded enamel is more consistent with larger "[labyrinthodonts](/source/Labyrinthodontia)", a [paraphyletic](/source/Paraphyly) [grade](/source/Evolutionary_grade) of crocodile-like amphibians which the smaller and more specialized microsaurs are probably descended from. The jaw generally resembles that of the eel-like [adelogyrinids](/source/Adelospondyli) considering its large coronoid process, which is small in most microsaurs.[1]

*Trihecaton*'s relation to specific microsaur subgroups is uncertain. The elongated body and retention of intercentra is akin to the feeble-limbed [microbrachids](/source/Microbrachis), but the robust limbs of *Trihecaton* resemble those of short-bodied microsaurs like pantylids and tuditanids.[1] Carroll & Gaskill (1978) noted that the proportions and intercentra of *Trihecaton* were also shared with goniorhynchids. However, there is not enough shared material to provide specific comparisons, and in some aspects, such as the construction of the shoulder girdle, *Trihecaton* clearly differed from goniorhynchids. Carroll & Gaskill preferred not to consider *Trihecaton* close to any other family of microsaurs, instead considering it an independent relic of the origin of microsaurs.[2]

A series of [phylogenetic analyses](/source/Phylogenetics) by Marjanovic & [Laurin](/source/Michel_Laurin) (2019) included *Trihecaton*, though with inconclusive results. Like many other studies, they concluded that microsaurs were a paraphyletic grade of amphibians, with most forming a group with a single ancestor, yet a few primitive members (i.e. *Microbrachis* and *Hyloplesion*) formed a branch with non-microsaurian [holospondyls](/source/Holospondyli) like [diplocaulids](/source/Diplocaulidae) and [aistopods](/source/Aistopoda). *Trihecaton* jumps between these two branches based on different hypotheses for the position of [lissamphibians](/source/Lissamphibia) (modern amphibians like [frogs](/source/Frog) and [salamanders](/source/Salamander)). When all lissamphibians are considered to be descended from microsaurs, *Trihecaton* is equally likely to be close to the *Microbrachis* + *Hyloplesion* + Holospondyli branch, or alternatively in the main microsaur group intermediate between [ostodolepidids](/source/Ostodolepididae), [gymnarthrids](/source/Gymnarthridae), and *[Saxonerpeton](/source/Saxonerpeton)*. When lissamphibians are all considered to be [temnospondyls](/source/Temnospondyli) unrelated to microsaurs, the structure of Microsauria alters to cement *Trihecaton* as closer to the *Microbrachis* branch. Oddly enough, re-adding [caecilians](/source/Caecilian) to Microsauria moves *Trihecaton* much closer to ostodolepidids and gymnarthrids. This is also the position found by a [bootstrap](/source/Bootstrapping_(statistics)) and [bayesian](/source/Bayesian_inference) analyses of Marjanovic & Laurin's data.[4] Evidently there is still a lack of resolution for *Trihecaton*'s position, especially considering how interpretations of lissamphibian origins differ wildly between different amphibian-oriented paleontologists.

## References

1. Vaughn, Peter Paul (23 February 1972). ["More vertebrates, including a new microsaur, from the upper Pennsylvanian of Central Colorado"](https://www.biodiversitylibrary.org/page/52087854#page/137/mode/1up). *Contributions in Science*. **223**: 1–19. [doi:10.5962/p.241208](https://doi.org/10.5962/p.241208). [S2CID 134446055](https://api.semanticscholar.org/CorpusID:134446055)

1. Carroll, Robert L. & Gaskill, Pamela (1978). [*The Order Microsauria*](https://books.google.com/books?id=mjcyaQw78X4C&pg=PA99). Philadelphia: The American Philosophical Society. ISBN 978-0-87169-126-2.

1. Berman, David S. & Sumida, Stuart S. (15 November 1990). ["A new species of *Limnoscelis* (Amphibia, Diadectomorpha) from the late Pennsylvanian Sangre de Cristo Formation of Colorado"](https://archive.org/details/annalsofcarnegie5919carn/page/304). *Annals of the Carnegie Museum*. **59** (4): 303–341. [doi:10.5962/p.240774](https://doi.org/10.5962/p.240774). [S2CID 92022042](https://api.semanticscholar.org/CorpusID:92022042)

1. Marjanović, David & Laurin, Michel (2019-01-04). "Phylogeny of Paleozoic limbed vertebrates reassessed through revision and expansion of the largest published relevant data matrix". *PeerJ*. **6**. [doi:10.7717/peerj.5565](https://doi.org/10.7717/peerj.5565). [ISSN 2167-8359](https://www.worldcat.org/issn/2167-8359). [PMC 6322490](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6322490). [PMID 30631641](https://pubmed.ncbi.nlm.nih.gov/30631641)

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Adapted from the Wikipedia article [Trihecaton](https://en.wikipedia.org/wiki/Trihecaton) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Trihecaton?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.