New publication ( New information on the dissorophid Conjunctio (Temnospondyli) based on a specimen from the Cutler Formation of Colorado, U.S.A.; Gee et al., 2021, JVP)

Title: New information on the dissorophid Conjunctio (Temnospondyli) based on a specimen from the Cutler Formation of Colorado, U.S.A.
​Authors: B.M. Gee, D. S Berman, A. C. Henrici, J.D. Pardo, A. K. Huttenlocker
Journal: ​Journal of Vertebrate Paleontology
DOI: ​10.1080/02724634.2020.1877152

General summary: Much of the early evolution of the armoured dissorophoids, the dissorophids, remains poorly known. Many taxa are what we call 'wildcards,' which means that they shift around a lot in phylogenetic analyses between different studies and can sometimes have a destabilizing effect on the entire tree. One of these longtime wildcards is Conjunctio, a taxon first reported from New Mexico, that has a mixture of primitive and derived features, something that often leads to 'wildcard' status. Like many other dissorophids, Conjunctio is essentially known from one very spatially restricted region (part of New Mexico in this case). Is this a real phenomenon (endemism) that perhaps explains why there are so many different dissorophid taxa, or is it a bias in the fossil record? Our study reports Conjunctio from Colorado for the first time, a region that is very light on temnospondyl fossils in general. While spatially separated, the fossil occurs in the same general stratigraphic horizon as the previously known specimens (the Cutler Formation).  It suggests that some of these less sampled Four Corners states might have been home to widely dispersed dissorophids (see also: Platyhystrix).

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The Red River

The Red River is a mostly east-west oriented, eastward-flowing indirect tributary of the Mississippi River, stretching from the Texas Panhandle all the way to Louisiana (over 1,350 miles). It forms the border between Texas and Oklahoma (hence where the "Red River Rivalry" in collegiate football between University of Texas and the University of Oklahoma derives from), two of the most historically productive states for early Permian tetrapod fossils (especially the famed Texas redbeds). Much of our record of North American temnospondyls from this time comes from these two states, with more minor contributions from New Mexico, Utah, and Ohio. Dissorophids, the armoured dissorophoids, are among the best known taxa from these redbed deposits, and in many instances, the entire record of a given species is restricted to one or two states. Whether this is an artifact of the fossil record or an accurate representation of a truly restricted geographic range is often unclear; many taxa are in fact known from only one specimen or one site (the two species of Cacops below are examples; figures from Gee & Reisz, 2018 and Anderson et al., 2020). 

Land of Enchantment

Conjunctio multidens (reconstruction from Schoch & Sues, 2013) is one such dissorophid, known only from two sites (but the same county) in New Mexico. Although New Mexico and Texas are adjacent states, their faunal assemblages were somewhat different, a result of regional geography no longer present that would have created some degree of spatial segregation. Other examples can be found in the trematopids Anconastes and Ecolsonia, only found in New Mexico, or the dissorophid Broiliellus reiszi, which differs starkly from other members of the genus that are known from Texas. Because the Texas redbeds have been so extensively sampled, with at least a dozen other dissorophids known, it can be reasoned that indeed Conjunctio also did not occur in Texas. But does that mean that it was only found in New Mexico? Not necessarily. The rest of the Four Corners region and the midcontinent is less well-explored, probably because the early Permian deposits that yield tetrapod fossils are less extensive. One of the well-known productive localities in the Four Corners region is in the Placerville area in Colorado, where the Cutler Formation (also found in New Mexico) is several hundred meters thick. First reported by Lewis & Vaughn (1965), the assemblage preserves one of the few sails of the dissorophid Platyhystrix (below on right), the holotype of Diadectes sanmiguelensis, a diadectomorph stem amniote (below on left), and a number of other tetrapods also known from other redbeds deposits. Synapsid fans may also know this as the type locality for the sphenacodontid Cutleria.

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Odd man out

Core members

Dissorophids have historically been divided into three or four main groups, which is reflected in the taxonomy of Schoch & Milner (2014) and shown in the figure from Schoch (2012) on the left. Platyhystricines are identified by the hyperelongation of the spine and a single osteoderm closely adhered to the spine (or maybe no osteoderm, just an ornamented spine), as in Platyhystrix (but also Astreptorhachis). Aspidosaurines are also largely defined by the postcranial skeleton due to a paucity of cranial material; their osteoderms are distinctly shingle-shaped and attach to a spine that expands at the top to meet the underside of the osteoderm. The two derived groups, dissorophines and eucacopines are both represented by abundant cranial material and have markedly different skull morphologies that set them apart. One of the main challenges has been resolving the position of various dissorophids that don't clearly fit into these categories, which is predictable because evolution doesn't just hop been discrete bins in the blink of an eye. 

Conjunctio multidens, the only species of Conjunctio, has long been one of these taxa that doesn't clearly fit into the existing dissorophid categories (see figures of C. multidens  from Schoch & Sues, 2013, on the right). It has a weird mixture of plesiomorphies (e.g., high marginal tooth count, single series of narrow osteoderms) and apomorphies (e.g., rectangular postorbital, shortened posterior skull table) that defy clear placement within any of the established clades. The intuitive proposal is that it represents one of the earliest diverging members of one of these clades (a sort of 'transitional' form), combining primitive and derived features in a unique way. Animals like this tend to be troublesome in phylogenetic analyses.

Terminology: While 'transitional form' or 'transitional fossil' is a commonly used term that even the general public knows (e.g., Archaeopteryx), scientists are moving away from it because it's misleading, suggesting that one animal directly turns into another (one of the most common misconceptions about evolution). In fact, this very rarely occurs (a process called anagenesis), and it does not capture major transitions like the theropod-bird transition or the fish-tetrapod transition. Instead, continual splitting of new species that go extinct is what gradually leads to a shift in a group's anatomy. A given species' suite of features may be transitional insofar as it captures part of this shift, but the species or individual itself is not.

Agree to disagree

Comparison of different phylogenetic results with Conjunctio. Boxes refer to dissorophid subfamilies: green = cacopines, blue = dissorophines.

As one might expect from a taxon with a weird mixture of features, there is no agreement on the position of Conjunctio in phylogenetic analyses of dissorophids, as summarized on the right. Historically, the phylogeny did not support that they were conspecific (expected recovery as exclusive sister taxa), as they were frequently scored separately due to uncertainty over Carroll's referral (the referred specimen was called the "Rio Arriba Taxon"). It doesn't really help that the holotype (C-D) and the much smaller referred specimen (A-B) differ from each other in cranial proportions, somewhat questioning whether they are actually the same taxon (since Bob Carroll created Conjunctio in 1964). You don't have to be a paleontologist to look at the two specimens above and kinda wonder whether they are actually the same (or whether the bottom one can be said to be much of anything really). Sometimes both specimens were recovered as eucacopines (Holmes et al., 2013), sometimes only one was recovered as a eucacopine (Schoch, 2012), sometimes Eucacopinae didn't exist (Maddin et al., 2013), sometimes the composite was recovered as a eucacopine (Schoch & Sues, 2013), and sometimes the composite was recovered as a dissorophine (Liu, 2018). So basically every possible result short of it being recovered as a non-dissorophid!

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The Centennial State

The new specimen that we report here was collected from what's known as the Placerville locality by my coauthors, Adam Huttenlocker and Jason Pardo, a few years back. It is not nearly as complete as the other two specimens, but it is well-preserved, enough to show some distinctive features not found in most or all of the other dissorophids. For example, the two bones called the postorbital (right behind the eye) and the supratemporal (a little farther back) usually touch, but here they do not (this is found in two of the species of Cacops as well). The jaw articulation (marked by the quadrate) also sits in front of the level of the back of the skull (marked by the postparietal); this is something found only in Dissorophus and Scapanops. So there is actually quite a lot of information in this little skull despite it being at best 25% complete!

Our phylogenetic analysis, which combines two previous matrices (it was originally intended for a larger sample), finally recovers not just the original two specimens, but also this third one as a proper clade (i.e. what you expect if they really do all belong to the same species)! The statistical support is not very good though, and it's quite possible that a future study could find a different result. Dissorophid phylogenetics remains very much in flux outside of just Conjunctio  (something that I'm working on right now).

COVID addendum: Lest someone think that all of us are superhumans who pumped out this paper in the pandemic, it was mostly completed before the pandemic really set in (we submitted the first version in May 2020). Jason and I in particular as current and recently graduated PhD students have really slowed down in putting new work into the pipeline during the pandemic, which I think is important to note since a lot of people are really struggling and should not be concerned with their perceived relative productivity!

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Refs

• ​​Anderson, J.S., Scott, D. and Reisz, R.R., 2020. The anatomy of the dermatocranium and mandible of Cacops aspidephorus Williston, 1910 (Temnospondyli: Dissorophidae), from the Lower Permian of Texas. Journal of Vertebrate Paleontology, 40(2), p.e1776720.
  
• Carroll, R.L., 1964. Early evolution of the dissorophid amphibians. Bulletin of the Museum of Comparative Zoology, 131, p. 163-250.
• Dilkes, D.W., 2020. Revision of the Early Permian Dissorophid ‘Dissorophus’ angustus (Temnospondyli: Dissorophoidea). Journal of Vertebrate Paleontology, 40(4), p.e1801704.
• Gee, B.M. and Reisz, R.R., 2018. Cranial and postcranial anatomy of Cacops morrisi, a eucacopine dissorophid from the early Permian of Oklahoma. Journal of Vertebrate Paleontology, 38(2), p.e1433186.
• Holmes, R., Berman, D.S. and Anderson, J.S., 2013. A new dissorophid (Temnospondyli, Dissorophoidea) from the Early Permian of New Mexico (United States). Comptes Rendus Palevol, 12(7-8), pp.419-435.
• Lewis, G.E., Baird, D. and Vaughn, P.P. 1965. Early Permian vertebrates from the Cutler Formation of the Placerville Area, Colorado. Geological Survey Professional Paper 503-C.
• Liu, J., 2018. Osteology of the large dissorophid temnospondyl Anakamacops petrolicus from the Guadalupian Dashankou Fauna of China. Journal of Vertebrate Paleontology, 38(5), p.e1513407.
• Maddin, H.C., Fröbisch, N.B., Evans, D.C. and Milner, A.R., 2013. Reappraisal of the Early Permian amphibamid Tersomius texensis and some referred material. Comptes Rendus Palevol, 12(7-8), pp.447-461.
• Schoch, R.R., 2012. Character distribution and phylogeny of the dissorophid temnospondyls. Fossil Record, 15(2), pp.121-137.
• Schoch, R.R. and Sues, H.D., 2013. A new dissorophid temnospondyl from the Lower Permian of north-central Texas. Comptes Rendus Palevol, 12(7-8), pp.437-445.
• Schoch, R.R. and Milner, A.R. 2014. Handbook of Paleoherpetology: Temnospondyli I (Part 3A2). München: Verlag Dr. Friedrich Pfeil.