Bolosaurid thecodont tooth implantation

Snyder et al. 2020 report,
“Analysis of the specialized dentition of the bolosaurid parareptiles Bolosaurus from North America and Belebey from Russia, utilizing a combination of histological and tomographic data, reveals unusual patterns of tooth development and replacement. The data confirm that bolosaurid teeth have thecodont implantation with deep roots, the oldest known such example among amniotes,”

Bolosaurus is known from the Early Permian. The oldest amniote, Silvanerpeton, from the Viséan (Early Carboniferous) needs to be examined for tooth implantation before making this statement.

“and independently evolved among much younger archosauromorphs (including dinosaurs and crocodilians) and among synapsids (including mammals).”

In the large reptile tree (LRT, 1690+ taxa) synapsids and dinosaurs are both archosauromorphs. Given that the last common ancestor of all amniotes may also have had thecodont tooth implantation, this is not confirmed as ‘independently evolved.’ 

Co-author Reisz still believes in the invalid clade Parareptilia and is not aware of the basal dichotomy splitting Reptilia (Amniota is a junior synonym) between Archosauromorpha and Lepidosauromorpha.

Figure 2. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Figure 1. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Snyder et al. report,
“Finally, the nearly complete Eudibamus cursoris (Berman, 2000) of Germany represents the only other known taxon within the group.”

In the LRT Eudibamus is an unrelated diapsid (Fig. 1). Adding taxa reveals this interrelationships. In the LRT bolosaurids are sisters to diadectids, all derived from Milleretta and more ancestral taxa. None of these are bipedal with a parasagittal gait.

Snyder et al. report,
“Uninterrupted marginal tooth rows in the fossil record of Paleozoic tetrapods are rare, usually associated with unusual attachment and replacement cycles, such as in captorhinids.”

In the LRT bolosaurids and Milleretta are derived from captorhinids. I can’t find any examples in the LRT of interrupted marginal tooth rows in taxa surrounding bolosaurids.

Snyder et al. report,
“The level of heterodonty in bolosaurids is also remarkable, with mesial incisiform teeth transitioning to large, transversely bulbous cheek teeth distally along the tooth row. Tooth cusps slope posteriorly and have a conical apex that is more obvious in the larger, posterior teeth.”

Perhaps not so remarkable. Sister taxa, like Diadectes, have long, flat-tipped ‘incisors’ and short conical ‘molars’. Out-group Limnoscelis likewise has long anterior teeth and short posterior teeth. Other outgroups (captorhinids, Colobomycter, Tetraceratops, etc. are likewise heterodont.

In the discussion section, Snyder et al. report, 
“One comparable taxon are the diadectids. Dentitions in both diadectids and bolosaurids show heterodonty, thecodont implantation, as well as evidence of occlusal wear. The molariform cheek teeth of Diadectes are mediolaterally expanded compared to the more teardrop-shaped of bolosaurids, but like bolosaurids, they also frequently possess numerous replacement pits along the length of their jaws, suggesting a similar origin of tooth buds in both taxa.”

An inadequate taxon list prevented Snyder et al. from realizing the close relationship of diadectids and bolosaurids, as recovered several years ago in the LRT. Phonodus (Early Triassic) is also a bolosaurid in the LRT, unacknowledged by Snyder et al.


References
Snyder AJ, LeBlanc ARH, Jun C, Bevitt JJ and Reisz RR 2020. Thecodont tooth attachment and replacement in bolosaurid parareptiles. PeerJ 8:e9168
doi: Âhttps://doi.org/10.7717/peerj.9168 – https://peerj.com/articles/9168/

2 thoughts on “Bolosaurid thecodont tooth implantation

  1. Hey David Peters, a have a quastion to you.

    It’s not about the topic above that you talk about, but it’s someting else.

    I wonder namely if you have good pictures of the head from the Longisquama insignis holotype (not from some cast of the holotype, please).

    I don’t know if you have taken pictures of the Longisquama insignis holotype yourself, but if so, it would be very good.

    I work on a reconstruction of Longisquama insignis namely, and it would be very good to have refurences alongside. The thing I want to be best is the head.

    – Thanks!

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