Coeruleodraco: Traditional choristodere mistakes resurface

Occasionally within the Archosauriformes
the antorbital fenestra disappears. That is the case with the clade Choristodera, which Wikipedia describes as “an extinct order of semiaquatic diapsid reptiles. Cladists have placed them between basal diapsids and basal archosauromorphs, but the phylogenetic position of Choristodera is still uncertain.” 

That is so unnecessarily vague.
Just run the analysis. In the large reptile tree chorisotderes are derived from phylogenetically miniaturized proterosuchians like the BPI 2871 specimen and its sister Elachistosuchus.

Figure 1. Coeruleodraco skull as originally interpreted (below) and interpreted here (colors). This is a traditional error. Also note the remnants of an antorbital fenestra in this phylogenetically miniaturized taxon. The maxilla continues posterior to the orbit as in other choristoderes.

Figure 1. Coeruleodraco skull as originally interpreted (below) and interpreted here (colors). This is a traditional error. Also note the remnants of an antorbital fenestra in this phylogenetically miniaturized taxon. The maxilla continues posterior to the orbit as in other choristoderes. Firsthand access does not guarantee better interpretations.

 

Matumoto, Dong, Wang and Evans 2018
bring us a new genus of short-snouted, small choristodere, Coeruleodraco jurassicus (Fig. 1; Late Jurassic). The authors use a ‘by default’ very distant outgroup for their choristodere cladogram: the basal diapsids, Petrolacosaurus and Araeoscelis, because “Outgroup choice is problematic for Choristodera, because the position of the group within Diapsida remains uncertain.” The LRT solved that problem years ago and posted it online. Unfortunatley, the authors did not test the listed outgroup taxa. That’s all they had to do.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975.Note the tiny remnant of the antorbital fenestra in this phylogenetically miniaturized proterosuchid, basal to Choristodera.

A traditional mistake associated with choristoderes
is the mislabeling of the nasals as the prefrontals (Fig. 1). Both Coeruleodraco and outgroup taxa, like the BPI 2870 specimen demonstrate the ascending process of the premaxilla extends beyond the naris. That it becomes detached from the toothy lateral processes in Champsosaurus (Fig. 3) does not turn the premaxilla into a nasal. We looked at that earlier here and once again, it is due to the exclusion of taxa that clarify the issue. Choristodere workers are not looking at these outgroup taxa for guidance or analysis.

Figure 2. Champsosaurus skull with premaxilla in yellow.

Figure 3. Champsosaurus skull with premaxilla in yellow.

The authors also messed up the finger identification.
The original interpretation of the Coeruleodraco manus (Fig. 4) misidentified the lateral and medial digits along with the olecranon and ulna (violet), which extends behind the humerus as in all other tetrapods. DGS revealed the middle phalanges of manual digit 4 behind the others. The apparently short digit 4 becomes the longest digit when reconstructed (Fig. 4). This matches the manus of other choristoderes.

Figure 3. Manus of Coeruleodraco as originally identified and repaired and reconstructed in color.

Figure 4. Manus of Coeruleodraco as originally identified and repaired and reconstructed in color. Note frame that includes middle phalanges of digit 4. Digit 5 also has a semi-buried element.

Yes, I see things in fossils that others don’t see.
These are just a few of the many examples. In science it’s okay to point out where others have missed things, and the only way to convey that data over the Internet is by tracing and publishing photos (Fig. 4). Others are free to confirm or refute.

Firsthand access does not guarantee better interpretations.
It is important to understand what sister taxa are present and what traits they present. Without a good cladogram answers will not arrive. If there is any question, as in Champsosaurus and Coeruleodraco it’s okay to look at sister taxa for guidance.

Choristoderes are archosauriformes
in which the antorbital fenestra is reduced to absent. Others, like the Wikipedia authors, Matsumoto, Dong, Wang and Evans, who look only at a list of traits present or absent in a taxon are “Pulling a Larry Martin.” That’s a common problem that leads to taxon exclusion. The LRT is a science experiment that you can confirm or refute yourself. It’s time to put the choristodere enigma to rest.

References
Matsumoto R, Dong L, Wang Y and Evans SE 2019. The first record of a nearly complete choristodere (Reptilia: Diapsida) from the Upper Jurassic of Hebei Province, People’s Republic of China, Journal of Systematic Palaeontology
DOI:10.1080/14772019.2018.1494220

Thanks to co-author, Dr. S. Evans,
for sending a PDF link to the paper. I sent her a pdf of the LRT noting that it provided outgroups for choristoderes back to Devonian tetrapods, but no reply accompanied the pdf link.

wiki/Coeruleodraco

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.