Immaturity in the Latest Devonian: Acanthostega

This blogpost is somewhat outside of the reptile arena,
but marginally pertinent, as you’ll see, nevertheless.

A recent paper by Sanchez et al. 2016
reports that none of the known Acanthostega specimens (Fig. 1) are fully mature, even at six years old. As we learned earlier, these Latest Devonian lobe fins and basal tetrapods were not involved in the transition to land, which occurred in the early Middle Devonian, as documented by footprints. But, as late survivors of that transitional phase, they do give us a good view as to what happened tens of millions of years earlier.

Figure 1. Size comparisons of lobe-find fish, their eggs and embryos. Latimeria at top, Ichthyostega in the middle, Acanthostega at bottom with hypothetical adult with reduced tail fin, egg and hatchling also shown. Figure 1. Size comparisons of lobe-find fish, their eggs and embryos. Latimeria at top, Ichthyostega in the middle, Acanthostega at bottom with hypothetical adult with reduced tail fin, egg and hatchling also shown.

Figure 1. Size comparisons of lobe-find fish, their eggs and embryos. Latimeria at top, Ichthyostega in the middle, Acanthostega at bottom with hypothetical adult with reduced tail fin, egg and hatchling also shown.

The Sanchez abstract reports
“A long early juvenile stage with unossified limb bones, during which [Acanthostega] individuals grew to almost final size, was followed by a slow-growing late juvenile stage with ossified limbs that lasted for at least six years in some individuals. The late onset of limb ossification suggests that the juveniles were exclusively aquatic,”

Hypothetical hatchlings
based on hip dimensions and analogy with Latimeria (Fig. 1) suggest adults were 5-6x longer than juveniles. In Latimeria the ratio is 9x. Coelocanths can live for 48 years. Gestation can be 12-13 months. No information exists for the age of sexual maturity, but it could be as late as 20 years in Latimeria.

So the Sanchez et al. discoveries
seem to make sense in terms of phylogenetic bracketing. Lobefins take a long time to mature and they live a long time thereafter.

References
Sanchez S, Tafforeau  P, Clack JA & Ahlberg PE 2016. Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography. Nature (advance online publication) doi:10.1038/nature19354

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