Birkenia and the origin of facial bones

The most important taxa
in the large reptile tree (LRT, 1601 taxa; Fig. A) are the basal forms at each clade and the basalmost forms at the base of the LRT. Trying to understand where we all came from, I’ve been adding fish to the LRT using generalized reptile traits with some success. Novel topologies have appeared due to testing taxa that have not been tested together before.

Now the LRT is adding
a mid-Cambrian lancelet/fish without bones, Metaspriggina, (Simonetta and Insom 1993; Fig. 1) and Silurian jawless fish, like Thelodus and Birkenia elegans (Figs. 2, 3; Traquair 1898; Middle Silurian, 1.5-10cm long) in an effort to make sure the base of the LRT is not biased due to taxon exclusion. Turns out it was the right thing to do (Fig. A).

With that short intro, on to today’s topic
About half of the traits in the LRT come from the skull. It is the one part of the vertebrate body that changes the most in evolution.  Metaspriggina is a Cambrian lancelet with eyes, muscles, guts and gill slits, but no bones. The LRT tests bones, principally. So let’s figure out where and how facial bones first appeared in the vertebrate fossil record. Up to this point, no one, it seems, has put in the effort to do so.

Certain clades of jawless fish enclose themselves
in turtle-like shells (e.g. Arandaspis) with only a scaly or bone armored tail sticking out the back. Those genera are not useful to the present purposes and may never be added to the LRT.

On the other hand, one of the earliest taxa with facial bones
is jawless Birkenia (Figs. 2, 3), a relative to extant lampreys. Here (Fig. 3) the bones are not sutured plates, the sort we expect to see. Rather, in Birkenia areas of parallel, interwoven and concentric splints, whether cartilaginous or bone, form the primordia on which bones are phylogenetically later produced.

Distinctly different,
the squamosal, quadrate and dentary are tiny splints, too (Fig. 3), but they are internal and little different from the other concentric gill bars behind them that remain gill bars before they eventually turn into jaw and throat elements in tetrapods.

Frontal and quadratojugal
bones are not present in finless Birkenia. Those appear in more derived taxa like Gogonasus. In Birkenia lamprey-like gill openings are retained along with the developing, transitional gill bars.

Birkenia is reported to have
a terminal, rather than ventral, sucking mouth. This specimen (Fig. 3) does not have a terminal mouth. Rather it has a ventral permanent opening, like a lancelet does. “Sucking” was not possible. Perhaps other specimens attributed to Birkenia have different mouth morphologies or some oddly crushed specimens (Fig. 2) have been misinterpreted.

Today’s post may represent a novel observation
with regard to the origin of facial bones. If there is a similar earlier paper, let me know and I will cite it for proper credit.

Final note:
The thelodont Thelodus (Fig. 4) appears to have a mandible and teeth similar to, but phylogenetically distinct from all other vertebrates. Those ‘teeth’ are probably derived from cilia, since basal vertebrate teeth are not like these. Crushing may have given this fossil the illusion of a mandible. Or this may represent a convergent appearance of a mandible that is not phylogenetically related to the jaws of other tested vertebrates. Or it may represent an early appearance of the mandible, since pectoral fins are also present here, distinct from Birkenia.

References
Blom H, Märss T and Miller CG 2001. Silurian and earliest Devonian birkeniid anaspids from the Northern Hemisphere. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 92.03 (2001): 263-323.
Simonetta AM and Insom E 1993. New animals from the Burgess Shale (Middle Cambrian)and the possible significance for the understanding of the Bilateria. Bolletino Di Zoologia 60:97–107.
Traquair RH 1898. Report on fossils fishes. Summary of Progress of the Geological Survey of the United Kingdom for 1897: 72-76.

wiki/Birkenia 
wiki/Metaspriggina