The first late cretaceous mawsoniid coelacanth (Sarcopterygii: Actinistia) from North America: Evidence of a lineage of extinct ‘living fossils’

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.

Species delimitation and coexistence in an ancient, depauperate vertebrate clade

“…Although multiple coelacanths have been described from the same geological units dating to the early Mesozoic [ 17 ], our site-based approach confirms that three species spanning a large range of body sizes were living in the same lakes and waterways. At the same time, the anatomy of all three species at Firestone and Granton Quarry are remarkably similar,these species all show similar fusiform body plans and differ extensively only in the ornamentation of their opercles, number of foramina in their lower jaws, the size and number of their premaxillary teeth, the form of their scalation, and the form and counts of their fins [ 71 , 72 ] The observation of high size disparity coupled with low skeletal differentiation in the Lockatong coelacanth fauna supports the observation that the prevailing pattern in this clade is morphological conservatism [ 13 , 15 , 18 , 83 , 87 ],but see [ 17 ].…”

“…1 ). Silhouettes of coelacanths (except Latimeria and † Foreyia ) drawn after Schaeffer [ 11 , 18 , 27 , 74 ] …”

“…Placing † Diplurus among coelacanths has been difficult. Alternative approaches to phylogenetic reconstruction have allied this genus with both the Latimeriidae and the extinct coelacanth clade †Mawsoniidae, which was the dominant lineage during much of the Mesozoic [ 15 , 17 , 18 , 83 ]. Although we still found conflicts between the relationships of † Diplurus newarki , † Diplurus longicaudatus , and † D. enigmaticus to other coelacanths resolved in parsimony and Bayesian frameworks (cf.…”

“…The existence of depauperate, evolutionarily stagnant lineages has been a matter of great interest since this pattern was first recognized (e.g., [ 12 , 13 , 21 , 47 , 82 ]. One central point of contention has been whether continuously low species diversity across time scales of tens of millions of years in many of these clades is driven by genuinely low speciation rates or simply the incompleteness of the fossil record (e.g., [ 12 , 13 , 18 , 57 , 77 ]. In the case of depauperons, rigorous species delimitation is therefore all the more essential, as these clades might show a lower level of morphological disparity across species boundaries if taxic depauperacy is coupled with reduced evolvability.…”

“…Despite their historical notoriety as a species-poor, morphologically conservative lineage, the fossil record of coelacanths has shown that this clade diversified into a wide variety of morphologies in the ancient past [ 17 , 29 , 49 ]. Fossil coelacanths show bursts of species diversity during the Devonian and Triassic [ 83 ] and achieved a high degree of body size variation ranging from diminutive species less than 5 cm long to 6 + m giants representing some of the largest freshwater fishes [ 18 ].…”