Tetraceratops: Not a Basal Therapsid

Updated January 11, 2020
with better data from Spindler 2020.

Updated June 14, 2021
with a new tracing of Martensius, a new sister for Tetraceratops.

Tetraceratops (Matthew 1908) was a weird little bumpy-faced reptile with large fangs and a deeply curved jawline. Only the 9 cm skull of a single specimen is known (Figure 1). Laurin and Reisz (1996), Conrad and Sidor (2001) and Amson and Laurin (2011) all nested Tetraceratops within the Synapsida, between sphenacodontids and basal therapsids. They did not test other taxa.

Figure 4. Tetraceratops and LRT relatives including Saurorictus, Limnoscelis, Orobates and Milleretta.

Figure 1. Tetraceratops and LRT relatives including Saurorictus, Limnoscelis, Orobates and Milleretta.

Take Away the Assumptions and
Add More Taxa to See Where Tetraceratops Nests
Tetraceratops is “weird” because it does not closely resemble any other reptile tested by prior authors. The synapsids, Haptodus and Biarmosuchus (Figure 1), are not close matches.

Figure 5. Tetraceratops tracing using DGS and freehand illustration by Spindler 2020.

Figure 2. Tetraceratops tracing using DGS and freehand illustration by Spindler 2020.

However,
when Tetraceratops is tested against a larger inclusion set, it nests as a sister to Tseajaia (a limnoscelid and a basal reptile, not a synapsid). As a test, when you add Tseajaia to the Amson and Laurin (2011) data matrix, analysis recovers Tseajaia as a sister to Tetraceratops. Delete Tetraceratops and Tseajaia stays put between the pelycosaurs and therapsids. The character and taxon lists were too short to nest the diadectomorphs as outgroups.

Figure 3. Tetraceratops compared to several haptodine and basalmost therapsid taxa, the closest relatives according to Spindler 2020.

Figure 3. Tetraceratops compared to several haptodine and basalmost therapsid taxa, the closest relatives according to Spindler 2020.

The Temporal Region is Missing?
The entire back half of the skull of Tetraceratops appears to be missing. It is not missing. The posterior jugal process and the quadratojugal have rotated during taphonomy to the position formerly occupied by the postorbital and ascending process of the jugal. The squamosal is present in pieces, largely in the orbit.

Matthews (1908) imagined a small synapsid-like lateral temporal opening.  Laurin and Reisz (1996) and Amson and Laurin (2011) reported a larger lateral temporal fenestra, larger than the orbit based on their interpretation of a strip of bone they considered a squamosal. They curved it in their reconstruction.

New information on ‘pelycosaurian’ character variation and relationships indicates that Tetraceratops represents a haptodontine-grade or (less likely) sphenacodontid ‘pelycosaur’.

Figure 4. Tetraceratops fossil with tracing by Spindler and colors added using DGS methods.

Similar to Limnoscelis and Tseajaia
Tetraceratops was more similar to its limnoscelid sisters. Tseajaia shared a ventrally convex mandible, tusk/fangs in the same locations and the genesis of facial bumps. Limnoscelis shared a pinched rostrum, a robust premaxilla and dentary fangs.

The palates of Tetraceratops was more similar to those of Limnoscelis and Tseajaia. The purported reflected lamina of Tetraceratops represents a quadrate, not an articular.

The Pelycosaur/Therapsid Mistake
On a side note, Amson and Laurin (2011) nested therapsids with pelycosaurs, but they did not include Ophiacodon. The larger study nested Biarmosuchus with Ophiacodon, rather than Haptodus, Dimetrodon and Sphenacodon. We’ll look at that in a future blog.

References
Amson E and Laurin M 2011. On the affinities of Tetraceratops insignis, an Early Permian synapsid. Acta Palaeontologica Polonica 56(2):301-312. online pdf
Conrad J and Sidor CA 2001. Re−evaluation of Tetraceratops insignis (Synapsida: Sphenacodontia). Journal of Vertebrate Paleontology 21: 42A.
Matthew WD 1908. A four-horned pelycosaurian from the Permian of Texas.Bulletin of the American Museum of Natural History 24:183-185.
Laurin M and Reisz RR. 1996. The osteology and relationships of Tetraceratops insignis, the oldest known therapsid. Journal of Vertebrate Paleontology 16:95-102. doi:10.1080/02724634.1996.10011287.
Sidor CA and Hopson JA 1998. “Ghost lineages and “mammalness”: Assessing the temporal pattern of character acquisition in the Synapsida”. Paleobiology 24: 254–273.

wiki/Tetraceratops

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