Is Palacrodon a rhynchocephalian? – SVP abstract 2016

Originally (Broom 1906)
considered what little is known of Palacrodon browni (= Fremouwsaurus geludens; Early Triassic; Fig. 1) a member of the Rhynchocephalia. This year, Jenkins and Lewis 2016 tested Palacrodon against rhynchcephalians and procolophonids and found it nested with the former. This genus is so obscure that Wikipedia ignores it. The few specimens are poorly known, only a few fragments of skull + teeth from South Africa and Antarctica.

Here in a large gamut analysis Palacrodon nests
in the large reptile tree (LRT) at the base of the Placodontia (Fig. 1) between sharp-toothed and big-eyed Palatodonta + Pappochelys and the much larger, pavement-toothed, smaller-eyed Paraplacodus.

Figure 1. A comparison of basal placodonts to scale (and Paraplacodus reduced to one-third shows how Fremouwsaurus (Palacrodon) is transitional between the small spike-tooth ancestors like Palatodonta and Pappochelys and the pavement toothed Paraplacodus.

Figure 1. A comparison of basal placodonts to scale (and Paraplacodus reduced to one-third shows how Fremouwsaurus (Palacrodon) is transitional between the small spike-tooth ancestors like Palatodonta and Pappochelys and the pavement toothed Paraplacodus.

Unfortunately recent work by Jenkins and Lewis 2016
did not include basal placodonts in their limited taxon analysis. The anterior maxillary teeth are still needle-like as in ancestral taxa. One can readily wonder if this is how the transition from one tooth type to the other occurred. Note the anterior maxillary teeth of Paraplacodus are still a bit sharp. I flipped the drawing of the quadrate from its original concave posterior. We have no palatal material for Palacrodon, but ancestral taxa display short robust teeth.

From the Jenkins and Lewis 2016 abstract
“Palacrodon browni is an Early Triassic reptile found on both the South African and Antarctic continents. The taxon has been classified as a diapsid, rhynchocephalian, and procolophonid in descriptions dating from 1906 to 1999, and consensus has not been reached regarding its phylogentic relationship within Lepidosauria. A refined phylogenetic placement of this reptile would push back stem dates of Lepidosauria from the Middle to the Early Triassic. It is possible Palacrodon is part of the faunal assemblage that experienced a decrease in body size as a result of the Lilliput effect noted in several Early Triassic lineages. There is also a noted range shift which occurred within the first 20 million years of the Triassic. The change in size and range suggest Palacrodon was strongly affected by the Permian mass extinction. Using high-resolution computed tomography, two dentaries were scanned and digitally segmented using AMIRA 6.2 to examine tooth implantation type (i.e., acrodont or thecodont) and reveal characters for better resolving the phylogenetic position of Palacrodon. Thirteen additional tooth-bearing elements, made available by the Evolutionary Institute at the University of Witwatersrand in Johannesburg, were also assessed for externally visible characters. Characters were scored against known Rhynchocephalia and procolophonid specimens using MacClade 4.08 and using an apomorphy-based approach specific to characters relating to dentition and tooth-bearing bones. Preliminary data suggest rhynchocephalian association due to acrodont dentition implantation in combination with possible protothecodont dentition in posterior teeth, and additional posterior dentition typical of sphenodontians. Initial survey also exhibits extreme wear on the occlusal surface of the teeth, a pattern typical of acrodont vertebrates and certainly rhynchocephalians. Phylogenetic analysis reveals Palacrodon’s familial association to be within Lepidosauria and its close relationship to crown Rhynchocephalia. A better understanding of the taxa that survived the Permian extinction may be beneficial to understanding and predicting the survival patterns of the current extinction, which shares any similarities to the Permian event. Change in body size and range behavior may be examples of these patterns which can be assessed in Palacrodon.”

Neenan et al. 2014
looked at tooth replacement in placodonts and found, “The plesiomorphic Placodus species show many replacement teeth at various stages of growth, with little or no discernible pattern.  Importantly, all specimens show at least one replacement tooth growing at the most posterior palatine tooth plates, indicating increased wear at this point and thus the most efficient functional crushing area.”

When head-less taxa meet head-only taxa.
The nesting of head-only Palacrodon with head-less Majaiashanosaurus immediately leads to rampant speculation worthy of Dr. Frankenstein. So… what if we put the enlarged head of the former on the body of the latter. Well, it might work (Fig. 2).

Figure 2. The head of Palacrodon and the headless body of the Majiashanosaurus compared.

Figure 2. The head of Palacrodon and the headless body of the Majiashanosaurus compared at the same scale (left) and enlarged (at right). 

References
Broom R 1906. On a new South African Triassic rhynchocephalian. Transactions of the Philosophical Society of South Africa 16:379-380.
Gow CE 1992. An enigmatic new reptile from the Lower Triassic Fremouw Formation of Antarctica. Palaeontologia Africana 29:21-23.
Gow CE 1999. The Triassic reptile Palacrodon brown Broom, synonymy and a new specimen.
Jenkins KM and Lewis PJ. 2016. Triassic lepidosaur from southern Gondwana. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Neenan JM, Li C, Rieppel O, Bernardini F, Tuniz C, Muscio G and Scheyer TG 2014. Unique method of tooth replacement in durophagous placodont marine reptiles, with new data on the dentition of Chinese taxa. Journal of Anatomy 224(5):603-613.

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