Let’s put tiny eyeballs into Brachydectes

and see what happens…

Here is the problem:
When you see what appears to be a large orbit in what is purported to be a lepospondyl / lissamphibian, you might be wrong, as I was. As it happens, the orbit is not large, but tiny and confluent with a large lateral temporal fenestra.

Brachydectes plays by ‘the rules.’ I just had to read ‘the rules’ more carefully. Even so, Brachydectes also breaks a few rules. For instance, no other amphibians (that I know of) lose the circumorbital ring.

Earlier I overlooked the possibility that Brachydectes had a very tiny eyeball tucked way up in front of that large triangular ‘orbit’, which can now be considered a confluent orbit/lateral temporal fenestra (LTF). This was described 15 years ago by Wellstead 1991. A LTF is unknown in most lepospondyls — except Cacops. The Cacops LTF is posterior to the squamosal. In Brachydectes it is anterior to the squamosal (Fig. 1). In any case, I wondered if re-establishing where the eyeball is would be the key to unlocking the affinities of long, caecilian-like Brachydectes, a very weird tetrapod in several regards. Pardo and Anderson (2016) suggested long-legged, short and squat Batropetes might be a close relative, but they are working on their analysis now and will publish on that later. They also reported amniote-like traits in the braincase. So Brachydectes seemed to brim with possibilities!

Figure 1. Various views of Brachydectes with the eyeball added to emphasize the small orbit and large lateral temporal fenestra.

Figure 1. Various views of Brachydectes with the eyeball added to emphasize the small orbit and large lateral temporal fenestra.

Does the presence of a LTF change the nesting of Brachydectes in the LRT?
No. However, the interest in this strange taxon created a whirl of discovery, including finding data for the tiny four fingers and five toes (Fig. 1) in an effort to further understand these specimens. In the larger specimen (Fig. 2 at right) the tabular and squamosal are fused. In the smaller specimen under CT scan, those bones are not fused.

Figure 2. Starting with a dorsal view of two Brachydectes specimens Wellstead (right) produced one reconstruction. Pardo and Anderson produced another with CT scans. The eyeball is added here to dispel the illusion that the lateral opening is only the orbit, as in most other amphibians. Here a confluent lateral temporal fenestra is present along with the loss of several circumorbital bones.

Figure 2. Starting with a dorsal view of two Brachydectes specimens Wellstead (right) produced one reconstruction. Pardo and Anderson produced another (left) with CT scans. The eyeball is added here to dispel the illusion that the lateral opening is only the orbit, as in most other amphibians. Here a confluent lateral temporal fenestra is present along with the loss of several circumorbital bones.

Organically missing bones in Brachydectes
include the jugal, quadratojugal, postfrontal, postorbital. In most amphibians these bones might fuse, but they never disappear. No other amphibians, that I know of, have a mandibular fenestra. Batropetes does have a similar occiput, but little else is similar. Sister taxa do expose a tiny coronoid and splenial, bones that were ignored in the Pardo and Anderson reconstruction, perhaps with good reason. You’ll also note that the Pardo and Anderson tracing does not quite match the reconstruction. Their CT scan of the skull roof does not match the nasals, frontals, parietals, etc. in their freehand figure. Was this an oversight? All this is important in scoring. Finally, its worth noting that the prefrontal not only contacts and wraps the naris, but it extends behind the orbit, where we usually find the postfrontal. Some of the tracings above have the supraoccipital contacting the parietals and separating or nearly separating the postparietals. Which is correct? And finally, the supratemporal is present on sister taxa, so it would be interesting to find out which bone it fused to. Or did it shrink and disappear?

In any case
Brachydectes nests strongly within the Lepospondylia (Lissamphibia), and it has sister taxa in the LRT. Finding transitional taxa to that odd morphology would be helpful in several regards.

With regard to mistakes made earlier…
Pardo was very professional in his response. By contrast, another reader held my feet to the fire, claiming that everything I presented for the last five years was suspect based on the mistakes i made earlier. Be wary of such comments. There is a certain madness behind them. Rationality gets tossed. Primal emotion holds sway. Based on the number of taxa, times the characters used to describe every taxon, times every suture and shape in every reconstruction, a few to a few hundred mistakes sprinkled throughout a study are statistically insignificant when tens of millions of data points are employed. Even so, bring them all to my attention when you find them, so I can eliminate all such errors. Case in point, I made a list of mistakes based on what I expected to find (= personal bias) on Brachydectes. I thought it had a large orbit. When the mistakes were corrected, most tended to cement the caecilian relationship established when the mistakes were in place.

References
Carroll RL 1967. An Adelogyrinid Lepospondyl Amphibian from the Upper Carboniferous: Canadian Journal of Zoology 45(1):1-16.
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society, Philadelphia, 211 pp.
Cope ED 1868. Synopsis of the extinct Batrachia of North America. Proc Acad Nat Sci 20: 208–221. doi: 10.5962/bhl.title.60482
Jenkins FA and Walsh M 1993. An Early Jurassic caecilian with limbs. Nature 365: 246–250.
Jenkins FA, Walsh DM and Carroll RL 2007. Anatomy of Eocaecilia micropodia, a limbed caecilian of the Early Jurassic. Bulletin of the Museum of Comparative Zoology 158(6): 285-366.
Marjanović D and Laurin M 2013. The origin(s) of extant amphibians: a review with emphasis on the “lepospondyl hypothesis”. Geodiversitas 35 (1): 207-272. http://dx.doi.org/10.5252/g2013n1a8
Pardo JD and Anderson JS 2016. Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from µCT. PLoS ONE 11(8): e0161823. doi:10.1371/journal.pone.0161823. online here.
Wellstead C F 1991. Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondyl amphibians. Bulletin of the American Museum of Natural History 209: 1–90.

wiki/Eocaecilia

 

 

 

 

 

 

2 thoughts on “Let’s put tiny eyeballs into Brachydectes

  1. A LTF is unknown in most lepospondyls — except Cacops. The Cacops LTF is posterior to the squamosal.

    That’s because it’s not a LTF – it’s neither homologous nor analogous to those that most amniotes keep their jaw muscles in. It’s homologous to the spiracle (the first gill slit) and probably functioned as a middle ear for the reception of airborne sound (adult dissorophids were terrestrial). The tabular and the quadrate have grown around it, presumably to support a large eardrum.

    BTW, Cacops is a temnospondyl, not a lepospondyl. Among lepospondyls, however, various embayments of the ventral margin of the skull behind the maxilla occur a few times, and apparently phylogenetically close to Brachydectes.

    Some of the tracings above have the supraoccipital contacting the parietals and separating or nearly separating the postparietals. Which is correct?

    All of them. It’s individual variation.

  2. David, can you send me a temnospondyl that will attract Cacops away from Amphibamus? That’s why I put Eryops in the LRT, but it was not attractive enough. Wiki also reports a Cacopes/Amphibamus alliance, but what do they know…

    Interesting about the spiracle issue…

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