If you never met her,
here’s your second chance, via YouTube videos.
This week marks the passing of Professor Jennifer Clack (1947-2020),
a renown specialist in Devonian tetrapods, especially Acanthostega (Fig. 1). In the above 4-minute YouTube video from 2017, Clack introduces her concept that the first tetrapods, like her discovery of Acanthostega, had more than five manual digits. This is confirmed by Middle Devonian tetrapod tracks (Fig. 3) with more than five digits.
according to the large reptile tree (LRT) which recovers Acanthostega as a terminal taxon, not a transitional one, far from the main line of tetrapod origins. Four digits are found in Panderichthys, Greererpeton and many other basal tetrapods, as we learned earlier here, here and here. More than five digits are found in only a few derived taxa, including the stem reptile, Tulerpeton, far from the origin of digits.
A more complete and technical account
of basal tetrapod traits is provided by Clack in this 20-minute YouTube lecture video from 2016 (above).
It may be that Clack only saw evolutionary progress
without considering the possibility of evolutionary reversal, as happens when taxa return to a more aquatic niche from a less aquatic niche, reducing the importance of their digits and limbs. In the above video, Clack does not provide a phylogenetic analysis, like the LRT (subset Fig. 2) that includes more primitive, but late-surviving basal tetrapods, all of which follow the pattern of a wider than deep torso, as in ancestral fish with embedded arm bones in their lobefins. Rather, she concentrates on individual traits, which while valuable, set her up for ‘Pulling a Larry Martin‘, rather than concentrating efforts on determining a phylogeny that minimizes taxon exclusion and lets the software determine (= mirror) evolutionary events, as the LRT does while minimizing taxon inclusion bias.
Only after a phylogeny is documented and validated
can one then discuss the various traits and their uses by the creature that possessed them.
Lest we forget
the first tetrapod tracks (Fig. 1, Niedźwiedzki et al. 2010) predate fossil tetrapods, including Acanthostega, by 20 to 30 million years, as we looked at here. And even they had more than five toes. Thus the phylogenetic origin of tetrapods goes back even further. The early Devonian must have provided quite a few niches for such rapid evolution to take place.
We need to look more closely at
Trypanognathus (Fig. 4; latest Carboniferous), which is the most primitive, but by far not the earliest, taxon in the LRT to document fingers and limbs, rather than lobe fins. Note the anterior eyes, wide flat skull and body, and primitive sprawling limbs. Can someone count the fingers and toes on this specimen? I find no more than four digits. Some may be hiding here.
We’ve seen the chronology of several fossil finds
at odds with their phylogeny in the LRT (e.g. multituberculates, bats, Gregorius). That keeps it interesting, but only a wide gamut phylogenetic analysis based on traits will deliver a valid tree topology. As time goes by and more discoveries are made the competing hypotheses will someday converge.
And one more thing,
Clack 1994 described Silvanerpeton (Fig. 5, Viséan, 335 mya) first as an anthrcosauroid and later (Ruta and Clack 2006) as a stem tetrapod, all without recovering it as the basalmost reptile, as shown in the LRT. Adding taxa, creating a wider gamut phylogenetic analysis, would have brought even more fame to this well-respected paleontologist.
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M and Ahlberg PE 2010. Tetrapod trackways from the early Middle Devonian period of Poland Nature 463, 43-48. doi:10.1038/nature08623
Ruta M and Clack, JA 2006 A review of Silvanerpeton miripedes, a stem amniote from the Lower Carboniferous of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 97, 31-63.
(make sure to click on the parts 2-4 links therein)