Amphibian Origins - Emerging onto a Tangled Bank

Australia: The Land Where Time Began

Amphibian Origins - Emerging onto a Tangled Bank

According to the author¹ research on the transition from fish to amphibian concentrates overwhelmingly on changes occurring across the phylogenetic tree, though the phylogenetic transition from water to land must have been mirrored by transitions occurring within individual life spans as the shores and banks began to be explored. Evidence of just such a life-history transformation has been reported on by Callier et al. (Callier, Clack & Ahlberg, 2009). The humeri of 2 tetrapods from East Greenland, Ichthyostega and Acanthostega, by the authors (Callier, Clack & Ahlberg, 2009).

The results of their study indicate that in both the tetrapods and related "fish", the forelimbs of tetrapods and the lobed fins of the fish, the humerus is a single bone linking the appendage to the body. The humerus is a complicated bone, being covered with bumps and ridges that are the sites of the origin and insertions of the muscles. The biomechanical signature of the transition from fins for swimming to weight-bearing limbs for walking is recorded by the humeri, as they are integral to the pectoral appendages.

In their study Callier et al. examined 9 Ichthyostega humeri from 6 individuals and 4 Acanthostega humeri from 3 species of Acanthostega. When the bones were arranged according to size, as well as other probable proxies of maturity, divergent developmental trajectories were identified in Ichthyostega and Acanthostega. In their study they encountered all the usual problems associated with reconstructing a growth series from fossils, as well as the specimens of Ichthyostega having been recovered from multiple locations and were from a variety of species, though it is argued by the authors (Callier, Clack & Ahlberg, 2009) that the observed shifts represent a developmental signal rather than an artefact.

The pectoral process, a raised area that marks the insertion point of the pectoral muscle, is on the ventral, lower surface, of the humerus in Ichthyostega and Acanthostega, as well as other tetrapods. In tetrapods this feature is located at the conventional position on the leading edge of the humerus in all Acanthostega, but only in the most mature of the Ichthyostega, though in smaller, less mature Ichthyostega the process is located close to the centre of the bone.

Callier et al. argue that within the Ichthyostega development the change mirrors the evolutionary patterns across phylogeny, on the basis of their new interpretations of humeri in "fish" that are closely related to tetrapods. Earlier identifications (Andrews & Westoll, 1970) of a pectoral process in osteolepiforms (lobe-finned fish with a close relationship to tetrapods) are rejected by Callier et al. and they provide a fresh perspective on an isolated humerus from the Devonian (ANSP 21350) which is believed to be from an animal that is just outside the tetrapod group. Callier et al. suggest there is pectoral process on this specimen in the same position, the centre of the bone, as is seen in the smallest Ichthyostega. Callier et al. identify a potential precursor of the pectoral process, that is also centrally located, on specimens of Tiktaalik, that is related more closely to ANSP 21350 plus tetrapods than are osteolepiforms. This clearly implies that the geometry of the pectoral process, as observed in younger Ichthyostega, resembles that present in the most immediately related "fish" tetrapods, though the derived marginal position, that is a characteristic of tetrapods, is present only in older Ichthyostega.

Rearrangement of muscle insertions, as well as corresponding functional changes, is indicated by developmental migration of the pectoral process. Humeral retraction and pronation, movements that are associated with movement on land, appears to have been facilitated by the geometry present in large Ichthyostega. During growth of Ichthyostega the sculpturing of the humerus is complemented by transformations of the structure of the shoulder girdle (Callier, Clack & Ahlberg, 2009). Signal points to older Ichthyostega individuals spending more time on land than do younger individuals is inferred in this case by Callier et al., who concluded they have captured the oldest known evidence seen in fossils of a developmental transition from water to land.

A diverse range of locomotor styles have been reconstructed for early tetrapods and related forms, and these functional inferences are the latest addition. These include:

Panderichthys: terrestrial wriggling (Vorobyeva & Kuznetsov, 1992)

Tiktaalik: push-ups (Shubin, Daeschler & Jenkins, 2006)

Ichthyostega: Inchworm crawling (Ahlberg, Clack & Blom, 2005)

Acanthostega: paddling (Coates & Clack, 1990)

These qualitative studies require quantitative studies to evaluate these qualitative scenarios, and comparative research drawing on living animals, as well as fossils to provide a template for future work (Markey & Marshall, 2007; Canoville & Laurin, 2009).

Callier et al., highlight features common to the humeri of all Ichthyostega, though they appear primitive when relative to Acanthostega and other tetrapods. This observation hints at an inversion of the current consensus that Ichthyostega is closer to living tetrapods than is Acanthostega (Ruta, Coates & Quicke, 2003). The conventional branching order is recovered by the most complete analyses by Callier et al., in spite of the new insights on the anatomy of the humerus, according to which Ichthyostega is above Acanthostega. When certain fossils are excluded from consideration is the only time when the 2 switch positions, and support for this arrangement, that is the orthodox one, is always equivocal.