The evolution of living fossils is not unusual. The odd thing about them is that they have ancient origins and are, today, not very diverse groups. Sturgeon are ray-finned fishes in the family Acipenseridae, which are known from almost 200 million year old fossils. Fossils that are recognisably similar to modern sturgeons appear about 100 million years ago. There are 23 (but maybe a few more) modern species in the Acipenseridae, which is probably fairly similar to their historical diversity.
|A sturgeon in the genus Acipenser, possibly A. transmontanus (image Wikipedia).|
One hypothesis is that some groups are more 'evolvable'. These groups are able to change their morphology relatively quickly and are consequently able to form more new species. For this hypothesis to be true, we should see a correlation between the amount of phenotypic change within a group and the amount of speciation. Studies that have looked at this relationship within lineages have failed to detect it.
A new paper by Rabosky and others argues that a fairer test of the hypothesis is to look at the relationship among lineages. They constructed an enormous evolutionary tree containing almost 8,000 species from across the entire spectrum of diversity of ray-finned fishes. They used body size evolution as a proxy for phenotypic change and found overwhelming support for the hypothesis; families with greater diversity in body size typically contained a greater number of species.
One group to buck this trend were the sturgeons. The rate of change in body size in sturgeons is amongst the fastest for all fishes. The baluga, Huso huso, is one of the largest ray-finned fishes, growing to over five and a half meters and 1,000 kilograms, while the dwarf sturgeon, Pseudoscaphirhynchus hermanni, grows to just 27 cm and 50 grams. This considerable difference in body size has arisen almost 5.4 times more rapidly than expected and only five of the 172 families in the analysis diverged faster.
These results are in complete contrast to the popular conception of living fossils as slow changing evolutionary oddballs. Indeed, in this analysis sturgeon are outliers because of the speed of their evolution. Clearly some traits in sturgeon are highly conserved over large periods of time, but we should not assume that this means that all of their traits are highly conserved. Living fossils are evolving and can diverge rapidly.
I've also written about living fossils here and here.