Abstract
Today, the only living genus of coelacanth, Latimeria is represented by two species along the eastern coast of Africa and in Indonesia. This sarcopterygian fish is nicknamed a "living fossil", in particular because of its slow evolution. The large geographical distribution of Latimeria may be a reason for the great resilience to extinction of this lineage, but the lack of fossil records for this genus prevents us from testing this hypothesis. Here we describe isolated bones (right angular, incomplete basisphenoid, fragments of parasphenoid and pterygoid) found in the Cenomanian Woodbine Formation in northeast Texas that are referred to the mawsoniid coelacanth Mawsonia sp. In order to assess the impact of this discovery on the alleged characteristic of "living fossils" in general and of coelacanths in particular: 1) we compared the average time duration of genera of ray-finned fish and coelacanth in the fossil record; 2) we compared the biogeographic signal from Mawsonia with the signal from the rest of the vertebrate assemblage of the Woodbine formation; and 3) we compared these life traits with those of Latimeria. The stratigraphical range of Mawsonia is at least 50 million years. Since Mawsonia was a fresh, brackish water fish with probably a low ability to cross large sea barriers and because most of the continental components of the Woodbine Fm vertebrate assemblage exhibit Laurasian affinities, it is proposed that the Mawsonia’s occurrence in North America is more likely the result of a vicariant event linked to the break-up of Pangea rather than the result of a dispersal from Gondwana. The link between a wide geographic distribution and the resilience to extinction demonstrated here for Mawsonia is a clue that a similar situation existed for Latimeria, which allowed this genus to live for tens of millions of years.
Highlights
The extant coelacanth, Latimeria is the only living genus of a lineage that split from other sarcopterygians over 400 million years ago
We suggest that C. carteri should be included in the genus Ferganoceratodus, as well as the specimen referenced by Harrell & Ehert [124] from the Santonian Eutaw Formation located in the Appalachians in the Late Cretaceous
The Texan discovery of Mawsonia sp. adds an important new component to the Woodbine vertebrate fauna. It is an unexpected Gondwanian representative in this Appalachian assemblage with predominantly Laurasian (European and Asian) affinities. It considerably increases the geographical distribution of this genus, and confirms its occurrence at the beginning of the Late Cretaceous
Summary
The extant coelacanth, Latimeria is the only living genus of a lineage that split from other sarcopterygians over 400 million years ago. Showing only minor morphological difference, the two extant species diverged 30 to 40 million years ago [7, 8]. The recent study of a specimen of L. menadoensis captured in eastern Indonesia indicates a molecular divergence with others populations of the same species 13 million years ago [3], possibly indicating the existence of a third distinct species of Latimeria. Observation in vivo and genetic studies indicate that Latimeria is a sedentary fish [9] These evolutionary traits suggest that the lineage of Latimeria has evolved slowly over a large geographic area. There is good extinction resilience for clades with larger geographic ranges than for those with smaller ranges throughout the Triassic and Jurassic, but this relationship is no longer observed at the end-Triassic mass extinction [15]
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