New publication: A new varanopid synapsid from the early Permian of Oklahoma and the evolutionary stasis in this clade (Maho, Gee, & Reisz, Royal Society Open Science)

Title: A new varanopid synapsid from the early Permian of Oklahoma and the evolutionary stasis in this clade
​Authors: S. Maho, B.M. Gee, R.R. Reisz
Journal: Royal Society Open Science vol. 6, article #191297 
DOI to paper: 10.1098/rsos.191297

Holotype skull of Mesenosaurus efremovi sp. nov. in right lateral profile (scale bar = 2 cm; figure from Maho et al., 2019).

General summary: My first paper on an unequivocal crown amniote! I've gone to the dark side... But fear not, the temnospondyls will be back in short order and in force over the next few months! I tagged onto this project that Sigi Maho, our current M.Sc. student, former undergrad, and future med student (sorry if you were looking for a potential Ph.D. student) completed for her undergraduate thesis this past academic year (2018-2019). We described a new species of varanopid synapsid from Richards Spur that's been lying on the back-burner for some time now. For the uninitiated, varanopids are one family of 'pelycosaurs,' a grade of early synapsids that span the late Carboniferous to the middle Permian, and synapsids are the group that gives rise to mammals like us. Don't think that pelycosaurs looked like us though - they include the charismatic, sail-backed Dimetrodon (and the slightly less charismatic but similarly sail-backed Edaphosaurus), the herbivorous caseids (which have stupidly sized head-to-body proportions), and the small, carnivorous eothyridids. You would probably be more inclined to closely associated varanopids with reptiles if you saw one running around today (which is an ongoing debate but not the point of this paper).

Reconstruction of the middle Permian Russian varanopid, Mesenosaurus romeri in dorsal (A) and left lateral (B) views (figure from Reisz & Berman, 2001).

Today’s inktober sketch is from a Varanopid, a group of synapsids that may be actually reptiles! https://t.co/ZvjmtWNnZA

— Gabriel N. U. (@SerpenIllus) October 9, 2018

Previous reports of synapsids from Richards Spur are very fragmentary (see below) - restricted to isolated postcranial elements or tooth-bearing fragments, which really lags behind the record of almost every other tetrapod group that we have at the locality (e.g., temnospondyls, captorhinid eureptiles, parareptiles). In contrast, the new varanopid is represented by three complete skulls with associated jaws. Now that I think about it, that's better than 7/8 temnospondyls that we have at Richards Spur... Anyway, in the typical paleontological fashion, we made detailed anatomical comparisons to previously described varanopids and ran a phylogenetic analysis, which recovered our varanopid as the closest relative of a varanopid called Mesenosaurus romeri from Russia (see above). In fact, there are only four minor differences that separate these two species, so we placed the Richards Spur taxon in Mesenosaurus​ as a new species, Mesenosaurus efremovi (named for the Russian paleontologist Ivan Efremov who named M. romeri in 1938). The kicker is that M. romeri is from the middle Permian - Richards Spur is early Permian, so a rough estimate puts them at more than 15 million years and who knows how many thousands of kilometers apart! Varanopids are unusual among pelycosaurs in persisting to the end of the middle Permian - most other groups are replaced by the more derived therapsids.  This longevity, combined with the lack of substantial changes (stasis) to the anatomy of this genus, is quite remarkable in the fossil record - it isn't found among temnospondyls until the Triassic, for example (see: Gerrothorax). We propose that the evident persistence of Mesenosaurus reflects a conserved ecological niche that varanopids were really good at (being an appreciably small tetrapod that ate other tetrapods) - they basically did the same thing for dozens of millions of years and didn't run into substantial competition for that role for some time.

Above are two examples of the previously known varanopid material from Richards Spur (from Maddin et al., 2006 on the left and from Evans et al., 2009 on the right). As you can see, it's not very much. Also note that I am evidently the latest in a lineage of distinctly non-synapsid workers to publish on a Richards Spur synapsid; Hillary Maddin works on temnospondyls and modern amphibians, and David Evans works on dinosaurs. 

How do you determine if something is a new genus or species, if either? 
There are no formal rules for how much difference you need to identify in order to name a new genus or species, which naturally invites a lot of controversy because some people (like me) are more conservative and tend to attribute minor differences to intraspecific variation, whereas other people will accept almost a single difference as sufficient grounds to name a new taxon. This is obviously one of the major challenges of working with the fossil record because our concepts of taxonomic differentiation are far more difficult to test. Here, we identified a few differences, so we felt comfortable saying that the Richards Spur taxon was a new species, but the animal fits all of the previously diagnostic features listed for the genus, so we placed it within Mesenosaurus​.

Holotype of Mesenosaurus efremovi in left lateral profile (scale bar = 2 cm; figure from Maho et al., 2019)

Referred specimen of Mesenosaurus efremovi (scale bar = 2 cm; figure from Maho et al., 2019).

So if you name something new, how do you come up with the name?
You can't be overtly immature or obscene about it, but there aren't many guidelines on naming. You don't, for example, need to get someone's written permission to name something after them, although it is a common courtesy to at least tell them of your intent if they're still alive. My dream is to one day name something after Taylor Swift, but if we're being honest, I don't think there are any non-amniotes that really match her personality (something tells me she may not be the biggest temnospondyl fan either). Anyway, Ivan Efremov was active in the mid-20th century, so that wasn't a consideration for us. Efremov was one of the foremost Russian paleontologists and named the first species of Mesenosaurus (after the famous American paleontologist A.S. Romer), so that was how we arrived that the species name, efremovi, for the new species. We didn't name Mesenosaurus​, but the name for that one derives from the Mezen (sometimes 'Mesen') River complex in Russia where the genus was first discovered. It's more common to use the dramatic, awe/fear-inspiring name as the genus name and one honouring a person as the species name (not all species are named after people either).

This seems like a really long time for a genus to be around - is that possible?
Potentially 20 million years of separation is definitely a long time, and it's practically a truism that the observed time ranges of any given fossil organism are not the full ranges (but may closely approximate it), so Mesenosaurus could have been around for even longer. However, it's important to emphasize that there are plenty of examples of organisms that remain relatively unchanged - this is often where the "living fossil" term comes into play with coelacanths and lungfish, to name a few. It's also important to note, in line with the above sections discussing how we classify extinct organisms, that a lot of the taxonomy is arbitrary. Different people will have different opinions about whether X number of differences is enough for a new genus, a new species, or neither, and classically, finding any difference (even one that could just as easily be explained as an artifact of preservation), led to at least a new species if not a new genus. There can also be differences of opinion regarding the nature of differences - are differences that are clearly attributable to a meaningful biological difference (e.g., differences in tooth type) more important than those that don't clearly have any biological meaning? So it's possible that not everyone who reads the paper will agree that there are "only a few differences that don't merit erecting a new genus" (or conversely that there are "enough differences to merit erecting a new species"). The three reviewers agreed with us though, so that's a starter. But getting back to the point, longevity of taxa is more or less arbitrary. 

Referred specimen of Mesenosaurus efremovi (scale bar = 2 cm; figure from Maho et al., 2019).

Morphological stasis and varanopids
Among 'pelycosaurs,' varanopids are one of the longer-lived clades (Late Carboniferous to middle Permian) along with the herbivorous caseasaurs. They're also widely dispersed, from western North America to eastern Eurasia to South Africa. This suggests that whatever varanopids are doing, they did it pretty well, and this may be born out in the combination of their relatively conserved morphology and their taxonomic longevity. Although there's a fair bit of size variation among varanopids, the small ones in particular seem to do the best, or at least they're around the longest. The fact that our Mesenosaurus in the early Permian of North America is so similar to the Mesenosaurus in the middle Permian of Russia suggests that in spite of pretty drastic changes to environment and tetrapod community assemblage, varanopids found a way to keep thriving. In some modern environments, that can suggest a generalization (e.g., in dietary preference) that allows animals to make do in a wide range of conditions - think raccoons and coyotes, for example, which are some of the most successful urban wildlife. The inverse is that it can suggest a successful specialization in a particular niche that isn't challenged (at least not successfully) by other groups of animals, which is what we suggest here. Varanopids have not only sharp teeth, but sharp cutting edges that usually have serrations on them, similar to what we see in hypercarnivorous dinosaurs, so that suggests that they're probably not generalist feeders but instead very specialized for eating a fairly specific size range of other animals (probably other tetrapods), and this is our working hypothesis for how this genus seems to persist for so long.

A temnospondyl connection
This is a temnospondyl blog, so there has to be a temnospondyl collection! One of the newly reported specimens may look familiar to people who read my more typical flavour of papers. Below on the left is an annotated figure from my 2018 paper in Fossil Record on a large, headless dissorophid skeleton - in the red oval is the right lower jaw and maxilla of the largest of the new varanopid specimens, which is properly figured below on the right. Somewhat ironically, the varanopid has no associated postcrania, and the temnospondyl is missing its head but has a fantastic postcranial skeleton...

So does this mark a new chapter in the academic journey of Bryan? Are there future forays into amniotes?? Will I work on dinosaurs one day??? 
No, unfortunately yes, probably not.

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Refs

• Evans DC, Maddin HC, Reisz RR. 2009. A re‐evaluation of sphenacodontid synapsid material from the Lower Permian fissure fills near Richards Spur, Oklahoma. Palaeontology, 52(1): 219-227. doi: 10.1111/j.1475-4983.2008.00837.x
• Gee BM, Reisz RR. 2018. Postcrania of large dissorophid temnospondyls from Richards Spur, Oklahoma. Fossil Record, 21(1): 79-91. doi: 10.5194/fr-21-79-2018
  
• ​Maddin HC, Evans DC, Reisz RR. 2006. An Early Permian varanodontine varanopid (Synapsida: Eupelycosauria) from the Richards Spur locality, Oklahoma. Journal of Vertebrate Paleontology, 26(4): 957-966. doi: 10.1671/0272-4634(2006)26[957:AEPVVS]2.0.CO;2