Temnospondyl fingers: 1-4? or 2-5?

If you’re going to have a good understanding
of reptile origins, you’re going to need a good topology of pre-reptiles (aka basal tetrapods). So today, tomorrow and the next day, we’ll cover temnospondyls and how they relate to their sisters and to reptiles.

According to Wikipedia
Temnospondyls have been known since the early 19th century, and were initially thought to be reptiles. They were described at various times as batrachiansstegocephalians, and labyrinthodonts, although these names are now rarely used. Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Experts disagree over whether temnospondyls were ancestral to modern amphibians (frogssalamanders, and caecilians), or whether the whole group died out without leaving any descendants.” The large reptile tree (LRT) indicates that temnospondyls are now extinct and not related to lepospondyls (extant amphibians and their kin).

Figure 1. Manus of Sclerocephalus, a well-preserved temnospondyl with four fingers, here labeled 1-4 (traditional) and 2-5 (heretical). The carpus is not as fully ossified as in Eryops, figure 2.

Figure 1.Manus of Sclerocephalus, a well-preserved temnospondyl with four fingers, here labeled 1-4 (traditional) and 2-5 (heretical). The carpus is not as fully ossified as in Eryops, figure 2.

Pertinent to todays discussion
Wikipedia also reports, “most temnospondyls have small limbs with four toes on each front foot and five on each hind foot.” So, the question is: which finger (not toe) is missing from the temnospondyl manus? Traditionally temnospondyl fingers have been labeled 1-4, perhaps based on the hypothesis that the medial digit has two phalanges, as in pentadactyl taxa. That would mean digits 2 and 3 lose one phalanx and digit 4 loses two phalanges — IF you’re starting with a complete pentadactyl limb — but you’re not.

Figure 2. The manus of the temnnospondyl Eryops compared to those of the polydactyl, Acanthostega and the pentadactyl reptilomorphs, Seymouria and Proterogyrinus. Homologous digits and carpal element are colored the same. In Eryops the pollex or digit 1 is absent, leaving only a small bump on Centrale 1. Note distal carpal 3 is smaller than 2 and 4 in Eryops and Seymouria. Digit 4 is the longest in Acanthostega and temnospondyls.

Figure 2. The manus of the temnnospondyl Eryops compared to those of the polydactyl, Acanthostega and the pentadactyl reptilomorphs, Seymouria and Proterogyrinus. Homologous digits and carpal element are colored the same. In Eryops the pollex or digit 1 is absent, leaving only a small bump on Centrale 1. Note distal carpal 3 is smaller than 2 and 4 in Eryops and Seymouria. Digit 4 is the longest in Acanthostega and temnospondyls.

After comparing sister taxa
(and they are few at present) the digits have been relabeled here: 2-5. That means digit 1 is absent in temnospondyls. Thus, digits 2-5 lose one or two phalanges.

In the old days
paleontologists added a hallux to museum mounts and illustrations of Eryops (Fig. 3). So they were thinking the same thing with regard to digit numbers. The correct identification of the fingers is key to scoring these traits correctly in phylogenetic analyses that involve temnospondyls.

Figure 3. Old illustration of Eryops with five fingers. Hallux (in yellow) should not be there.

Figure 3. Old illustration of Eryops with five fingers. Hallux (in yellow) should not be there. See figure 2.

 

References
wiki/Temnospondyli

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3 thoughts on “Temnospondyl fingers: 1-4? or 2-5?

  1. That wasn’t supposed to be a hallux (big toe) or a pollex (thumb) in the old museum mounts and reconstruction drawings. It was supposed to be a prepollex, which is something most frogs actually have, and arguably lungfish as well. I recommend you find out more about all that!

    As for which fingers temnospondyls actually had, that’s anyone’s guess. We haven’t even figured out which ones the extant lissamphibians have, or if they all have the same ones, or if the question even makes sense. There’s some discussion and several references in a paper of mine that you’ve reviewed on this very blog…

      • Unfortunately I can’t do better than “intriguing”. A lot more research needs to be done, especially in development biology of extant animals.

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