Colobops and taxon exclusion issues

Too often workers fail to include the closest relatives of new specimens
in analysis and then report they have something new and different in the pantheon of tetrapods. Too often the analysis lacks the correct tree topology, also due to taxon exclusion.

The new genus, Colobops noviportensis
(Pritchard, Gauthier, Hanson, Bever and Bhullar 2018; Fig. 1) was described as a tiny (2.5 cm long skull) saurian reptile from the Triassic of Connecticut, USA. Taxonomically it suffers from taxon exclusion. It was nested by default because more closely related taxa were omitted from a previously published analysis (Pritchard and Nesbitt 2017; Fig. 2), which was an inadequate analysis to work from because it failed to show the basal dichotomy of the Reptilia (Lepidosauromorpha/Archosauromorpha; Fig. 3) revealed by increasing the number of taxa.

Figure 1. Colobops as originally presented and slightly restored.

Figure 1. Colobops as originally presented and slightly restored. Glad to see other workers are coloring bones or identification. These are from CT scans. The postorbital processes invading the supratemporal fenestrae are unique.

From the abstract
“The taxon possesses an exceptionally reinforced snout and strikingly expanded supratemporal fossae for adductor musculature relative to any known Mesozoic or Recent diapsid of similar size. Our phylogenetic analyses support C. noviportensis as an early diverging pan-archosaur. Colobops noviportensis reveals extraordinary disparity of the feeding apparatus in small-bodied early Mesozoic diapsids, and a suite of morphologies, functionally related to a powerful bite, unknown in any small-bodied diapsid.”

Figure 2. Marmoretta, a basal rhynchocephalian in the lineage of pleurosaurs

Figure 2. Marmoretta, a basal rhynchocephalian in the lineage of pleurosaurs. Note the variety in the size of the supratemporal (upper) fenestrae, a variety that expands with Colobops.

Unfortunately,
their phylogenetic analysis (Fig. 3) did not include the basal sphenodontid, Marmoretta, more similar to Colobops in the large reptile tree (LRT, 1085 taxa; subset Fig. 4) than any other tested taxon. They are also the same size.

Figure 3. Cladogram from Pritchard et al. failed to include a long list of basal sphenodontians, including Marmoretta, the sister to Colobops in the LRT. Note the shuffling of lepidosauromorph and archosauromorphs in this cladogram, lacking any broad resemblance to the LRT tree topology.

Figure 3. Cladogram from Pritchard et al. failed to include a long list of basal sphenodontians, including Marmoretta, the sister to Colobops in the LRT. Note the shuffling of lepidosauromorph and archosauromorphs in this cladogram, lacking any broad resemblance to the LRT tree topology. Pritchard et al. assume that diapsids are monophyletic, which dooms their analysis. There is so much taxon exclusion here.

Marmoretta oxoniensis (Evans 1991, Waldman and Evans 1994) Middle/Late Jurassic, ~2.5 cm skull length, orginally considered a sister of kuehneosaursdrepanosaurs and lepidosaurs. Here Marmoretta was derived from a sister to Megachirella and PalaegamaMarmoretta was basal to Gephyrosaurus and the rest of the Sphenodontia = Rhynchochephalia. Two specimens are known (Fig. 2) with distinct proportions in the skull roof (frontal and parietal, see above). Note the variety in the supratemporal fenestrae in these closely related tiny flat-headed taxa, including Colobops.

By the way,
the Wikipedia page on Marmoretta likewise suffers from taxon exclusion.

Figure 5. Cladogram of the Sphenodontia includes Colobops and rhynchosaurs.

Figure 4. Cladogram of the Sphenodontia includes Colobops and rhynchosaurs.

Pritchard et al. assumed the monophyly of the Diapsida
which doomed their cladogram to a shuffling of disparate morphologies and by-default nestings (Fig. 3). Several years ago the LRT split the Archosauromorpha from the Lepidosauromorpha at the origin of the Reptilia, and so revealed that the diapsid skull architecture evolved at least twice.

Pritchard et al. nested Colobops
at the base of the Rhynchosauria due to taxon exclusion. In the LRT (subset Fig. 4) rhynchosaurs and Colobops are separated by a long list of taxa. The authors reported, “Two additional steps produce topologies in which C. noviportensis occupies some positions with pan-Archosauria and a position nested within Sphenodontia, a clade that converged anatomically on rhynchosaurs in numerous skull characters.”

If only
Pritchard et al. had used more taxa (or the LRT) they would have known that sphenodontids did not converge with rhynchosaurs, they were basal to rhynchosaurs. The authors report, “Colobops noviportensis represents a combination of morphological traits unknown in extant amniotes, and thus a morphology that would not have been reconstructed in a macroevolutionary analysis based exclusively on extant species.” I don’t see the extant tuatara, Sphenodon. in their taxon list.

Colobops lacks teeth
and lacks alveoli as well. The authors report, “The best insights into the feeding of C. noviportensis come from the general shape of the adductor chamber. In C. noviportensis, the post-temporal process of the parietal is oriented laterally, as in Sphenodontia and Rhynchosauridae, rather than posterolaterally as in most pan-lepidosaurs and pan-archosaurs.” See how they were just peeking in at the insights revealed by the LRT? Yet they followed tradition and previously published phylogenetic analyses beset with problems from the start.

The adductor chambers for jaw muscles in Colobops
are indeed quite large. And the postorbital process that invades the supratemporal fenestra is unique (at present). Sister sphenondontids do not have such a large supratemporal fenestra until Sphenodon. Note that one of the Marmoretta specimens (Fig. 2) had developed a parietal crest, also for the enlargement of the jaw muscles. So they were trying various ways to do this.

Based on the similar sizes of the marmorettid skulls
the skull of Colobops probably represents an adult.

The authors report
“Within individual species, overall skull size appears to correlate strongly with the relative breadth of the adductor chamber; juveniles recapitulate the transition from Permian Diapsida to crown-group with a small supratemporal fossa with small proportionally modest embayments on the parietal giving way to proportionally larger fossae and deeper parietal embayments.” Good to know. Irrelevant in this case.

I’m happy to see these authors have colorize key bones
throughout their paper. That’s the best way to illustrate them.

The final takeaway:
No matter how many co-authors you have with PhDs… no matter how many diagrams you show… no matter how many irrelevant taxa you include… no matter if you have firsthand access to the specimen… no matter if you are published in Nature… if you exclude the most closely related taxa, you’re going to let bloggers report your most basic errors. The LRT is online in order to be freely used. Use it. It’s a good starting point for any new taxon because it minimizes the opportunity for taxon exclusion by including so many taxa.

References
Evans SE 1991. A new lizard−like reptile (Diapsida: Lepidosauromorpha) from the Middle Jurassic of Oxfordshire. Zoological Journal of the Linnean Society 103:391-412.
Pritchard AC and Nesbitt SJ. 2017. A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida. Royal Society Open Science 4, 170499
Pritchard AC, Gauthier JA, Hanson M, Bever GS and Bhullar B-AS 2018. A tiny Triassic saurian from Connecticut and the early evolution of the diapsid feeding apparatus. Nature Communications open access DOI: 10.1038/s41467-018-03508-1
Waldman M and Evans SE 1994. Lepidosauromorph reptiles from the Middle Jurassic of Skye. Zoological Journal of the Linnean Society 112:135-150.

wiki/Marmoretta

 

 

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