Ichthyostega: Becoming more fish-like, not less… part 3

Long hailed as a transitional taxon
linking lobe-fin fish to tetrapods with fingers and toes, Ichthyostega (Fig. 1) nests in the large reptile tree (LRT, 1419 taxa) phylogenetically after the appearance of limbs with fingers and toes.

Ichthyostega has a neck,
a derived trait not found in fish — despite flattened hind limbs incapable of supporting weight and better suited to paddling. In the LRT the tibia and fibula are not only atypical in Ichthyostega, but derived relative to the more traditional crus of more primitive taxa (see below).

Plus
Ichthyostega had a sacral ‘hump’ not seen in other basal tetrapods. What did it anchor? A dorsal fin? Mighty tail muscles? In dorsal view, Ichthyostega had the tapered shape of a speedy dolphin.

Figure 1. Ichthyostega after CT scanning (grey tones) here colored with light green 'skin' added and pectoral girdle pushed posteriorly to expose the neck.

Figure 1. Ichthyostega after CT scanning (grey tones) here colored with light green ‘skin’ added and pectoral girdle pushed posteriorly to expose the neck. Note the narrow interclavicle, short torso and giant pectoral girdle, distinct from most basal tetrapods and stem tetrapods.

Tradition dictates
the extra toes in Ichthyostega are primitive traits, transitional between lobe fins and the standard five toes per foot per tetrapod.

That hypothesis is falsified
by more primitive Pholidogaster and Greererpeton, which have no more than five toes. A similar-looking relative, Pederpes, also has five toes. A sister taxon in the LRT, Proterogyrinus, likewise has only five toes—and a traditional crus.

Figure 1. Subset of the LRT focusing on the basal (anamniote) Tetrapoda. The last week has been spent reexamining data and letting the taxa sort themselves out again. Please report any untenable relationships.

Figure 1. Subset of the LRT focusing on the basal (anamniote) Tetrapoda. Note the nesting of Ichthyostega after taxa with five toes and a typical hind limb.

So maybe the extra toes in Ichthyostega
are just that, extra toes that developed upon it’s return to the water, perhaps speeding through, rather than plodding along like a salamander.

So… contra tradition,
Ichthyostega is one of the first tetrapods to return to the water, perhaps having never left the water completely. We first examined this ‘secondarily more aquatic’ hypothesis earlier here and here.


References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Jarvik E 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland 114: 1-90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.

wiki/Ichthyostega

 

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