Fish expert, John Long 1995 (p. 209) wrote:
“The juvenile skull of a crossopterygian fish, Eusthenopteron (Figs. 1,3) has more features in common with that of an early amphibian Crassigyrinus (Fig. 4), that it’s adult skull would have had.”
Long goes on to explain about paedomorphosis and heterochrony during the transition from fish to tetrapod.
Euthenopteron was a good transitional taxon several years ago. Recently it was replaced in the LRT by a flatter taxon, Cabonnichthys.
Let’s put Long’s 1995 statement to the test
by adding Eusthenopteron ‘junior’ (Schultze 1984) to the large reptile tree (LRT, 1698+ taxa; subset Fig. 5).
Results: The juvenile nested with the adult Eusthenopteron in the LRT, falsifying Long’s statement.
Worthy of note:
The juvenile Eusthenopteron shares several traits with another, often overlooked, small taxon with similar large eyes, Koilops, which nests at the base of a nearby derived node in the LRT (Fig. 5). Based on phylogenetic bracketing, Koilops is also a juvenile. All sister taxa are larger and without juvenile proportions.
So Long’s point about paedomorphosis and heterochrony
was not correct in this case. His ‘matching tetrapod’, Crassigyrinus (Fig. 4), nests several nodes apart from pre-tetrapods in the LRT (off the subset chart in Fig. 5).
Koilops post-crania remains unknown,
but it nests at the base of Elpistostege, Tiktaalik and Spathicepahlus on one branch, Panderichthys + Tetrapoda on the other. So Koilops likely had lobe fins and a straight tail. Perhaps Koilops was a juvenile elpistostegid ready to mature into something larger, with smaller eyes, more like Elpistostege.
From the Schultze 1984 abstract:
“A size series of thirty-five specimens of Eusthenopteron foordi Whiteaves, 1881 , shows isometric and allometric changes. As in Recent fishes, the main difference between small (juvenile) and large (adult) specimens is the relative size of the orbit and of the head. With the exception of the caudal prolongation, all fin positions remain isometric to standard length.”
Contra Long 1995 and all prior basal tetrapod workers, the LRT indicates the transition from fish to tetrapod occurred among flat-head taxa, like Trypanognathus. Crassigyrinus Fig. 4) is a distinctly different stegocephalid with a taller skull, more like those of the more famous traditional transitional taxa, Ichthyostega and Acanthostega. The new fish-to-tetrapod transitional taxa were recovered by simply adding taxa overlooked by prior workers. Taxon exclusion continues to be the number one problem with vertebrate paleontology today, according to results recovered by the LRT. This free, online resource minimizes taxon exclusion.
Not sure if fish expert John Long
would make the same statement today. Let’s hope things have changed in the last 25 years of vertebrate paleontology.
Long JA 1995. The Rise of Fishes. The Johns Hopkins University Press, Baltimore and London 223 pp.
Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform Rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4(1):1–16.