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Abstract Osteoglossid bonytongues (arapaimas, arowanas, and relatives) are extant tropical freshwater fishes with a relatively abundant and diverse fossil record. Most osteoglossid fossils come from a 25-million-year interval in the early Palaeogene, when these fishes were distributed worldwide in both freshwater and marine environments. Despite their biogeographic and palaeoecological relevance, and a relative abundance of well-preserved material, the evolutionary relationships between these Palaeogene forms and extant bonytongues remain unclear. Here we describe a new genus of bonytongue from early Eocene marine deposits of Morocco, represented by an articulated, three-dimensionally preserved skull with associated pectoral girdle. This taxon is characterized by an elongated snout, contrasting with the short jaws usually found in marine representatives of the clade. A revision of morphological characters in bonytongues allows us to place this new genus, together with other marine and freshwater Eocene taxa, within crown osteoglossids and closely related to extant arapaimines. The discovery of the new Moroccan taxon hints at a previously underestimated eco-morphological diversity of marine bonytongues, highlighting the diverse trophic niches that these fishes occupied in early Palaeogene seas. 

The separation of closely related terrestrial or freshwater species by vast marine barriers represents a biogeographical riddle. Such cases can provide evidence for vicariance, a process whereby ancient geological events like continental rifting divided ancestral geographical ranges. With an evolutionary history extending tens of millions of years, freshwater ecology, and distribution encompassing widely separated southern landmasses, osteoglossid bonytongue fishes are a textbook case of vicariance attributed to Mesozoic fragmentation of the Gondwanan supercontinent. Largely overlooked fossils complicate the clean narrative invoked for extant species by recording occurrences on additional continents and in marine settings. Here, we present a new total-evidence phylogenetic hypothesis for bonytongue fishes combined with quantitative models of range evolution and show that the last common ancestor of extant osteoglossids was likely marine, and that the group colonized freshwater settings at least four times when both extant and extinct lineages are considered. The correspondence between extant osteoglossid relationships and patterns of continental fragmentation therefore represents a striking example of biogeographical pseudocongruence. Contrary to arguments against vicariance hypotheses that rely only on temporal or phylogenetic evidence, these results provide direct palaeontological support for enhanced dispersal ability early in the history of a group with widely separated distributions in the modern day. 

In contrast to the rich collections of articulated spiny-rayed fishes from early Late Cretaceous and Eocene Lagerstätten, similar skeletal remains are sparse in Maastrichtian–Paleocene strata. Here we coin this poorly understood span “Patterson’s Gap” and review known articulated skeletons from it, summarizing available information on their phylogenetic affinities, age, and environmental context. Roughly fifty percent of taxa in both the Maastrichtian and Paleocene come from Europe and North America, with percomorphs representing around 60% of the skeletal acanthomorph taxa in each interval. This is higher than the only pre-Maastrichtian assemblage with a reasonable sample of percomorphs, but lower than most Eocene and younger sites. Fossils from Patterson’s Gap show a steady accumulation of the principal lineages of spiny-rayed fishes. Material from Paleocene or older strata provides evidence for most of the roughly 20 major acanthomorph divisions recovered by molecular studies. Many fossils from Patterson’s Gap remain undescribed and unnamed, and almost none have been included within formal phylogenetic analyses. Revision of existing material, combined with additional fieldwork, should be a priority for future efforts seeking to clarify this murky but significant interval in the evolutionary history of a major vertebrate radiation.