The skull and pectoral girdle of a large gar that lived ∼2000 years after the Cretaceous/Paleogene mass extinction event

Résumé fourni par la source

The gars, Lepisosteiformes, include seven extant species divided among two crown radiations, Atractosteus and Lepisosteus, whose Cretaceous split and modern diversity belie a more diverse total group. Here, we detail the anatomy of DMNH EPV.128289, an earliest Paleocene gar recovered from the Fort Union Formation of southwestern North Dakota. The type and only specimen of †Atractosteus grandei, DMNH EPV.128289 is notable both for its size (> 1 m) and its precise stratigraphic placement 18 cm above the Cretaceous/Paleogene (K/Pg) boundary clay, which places the specimen within the first ∼2000 years post-K/Pg extinction. µCT-based analysis and interpretation support a number of novel anatomical conclusions for this specimen and thus an expanded consideration of its phylogenetic position. These phylogenetic analyses confirm previous hypotheses that the specimen represents crown Lepisosteiformes, Pan-Atractosteus, and lies within an unresolved relationship with crown Atractosteus. DMNH EPV.128289 was previously used to erect a new species, †A. grandei. Our study, however, failed to differentially diagnose this specimen from the Green River taxon †Atractosteus simplex, while recognizing the challenges of estimating paleodiversity in a slowly evolving species. Our conclusions support the inference that gars were prominent members of freshwater ecosystems and, in turn, freshwater ecosystems were capable of supporting large-bodied predators within ∼2000 years after the K/Pg extinction.

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