Early fish development sheds light on limb evolution

Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning

Jonathan E. Jeffery, Glenn W. Storrs, Timothy Holland, Clifford J. Tabin, and Per E. Ahlberg

What data were used? The data were primarily gained from a single fossil specimen (with both pelvic fins) from the Museum of Comparative Zoology (MCZ) at Harvard University. But several have been described and are stored at the MCZ.

Figure 1: Pelvic region of the specimen from the the MCZ. (A) The original specimen and (B) the line drawing of the same specimen to better visualize the specific elements of interest. Here we can see both hind-find are isolated in different colors.

Methods: First the specimen had to be carefully prepared as the bones are still embedded in matrix. Any broken pieces of the fossil were glued back together. The fossils were imaged using a micro computed tomography (CT) scanner, which takes many fine images through the specimen using X-rays. The images can then be compiled and reconstructed in more complex 3D rendering programs. In this study, the authors used Avizo. The fossil specimens (real and digital) were examined thoroughly. These authors used the data collected from their thorough examinations and used it to explore developmental and evolutionary questions.

Results: The new data shows the general tetrapod pattern of a humerus (arm bone that connects to your shoulder) connecting with the forearm bones. In a phylogenetic, or evolutionary, context this provides additional information in the transition from fin limbs to tetrapod (animals with four legs) limbs we can easily recognize today – these are represented in the diagrams above the tree. The developmental comparisons of modern skeletons and allows the researchers to compare modern animal growth to these extinct forms. It is still unclear how the three bones came from the one (refer to the tree figure). The researchers ruled out a known protein can cause duplications of bones because each of these three bones is distinct, rather than having two of the same bone repeated.

Figure 2: Evolutionary history depicting the transition from fin to limb. The fore-fins/limbs are drawn on top and the hind-fins/limbs are on the bottom. Rhizodus is the genus of animal that is described in this paper and is found near the fin side of the tree.

Why is this study important? The three forearm bones in these pelvic limbs was an unexpected result from this study. It is quite different, even from the upper limbs in the same specimen and starkly different from other early finned fish. This study provides new evidence on the transition from fin to true limb. This specimen suggests that the fore-fins suggests that the mechanism of transition from fin to limb happens first in the fore-fins and later in the hind-fins.

The big picture: The fish to tetrapod transition has been well studied and is very important to understanding the evolution of most of terrestrial life. It is really difficult to find these specimens because they only preserve in very specific environments. This specimen is particularly important because it provides some new information that could help scientists reinterpret previously confusing results when the fore-fin/limb looks quite different from the hind-fin/limb in these more transitional forms.

Citation: Jeffery, J.E., Storrs, G.W., Holland, T., Tabin, C.J., and Ahlberg, P.E. 2018. Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning. PNAS, doi: 10.1073/pnas.1810845115

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