Evolutionary pathways toward gigantism in sharks and rays

Through elasmobranch (sharks and rays) evolutionary history, gigantism evolved multiple times in phylogenetically distant species, some of which are now extinct. Interestingly, the world's largest elasmobranchs display two specializations found never to overlap: filter feeding and mesothermy. The contrasting lifestyles of elasmobranch giants provide an ideal case study to elucidate the evolutionary pathways leading to gigantism in the oceans. Here, we applied a phylogenetic approach to a global dataset of 459 taxa to study the evolution of elasmobranch gigantism. We found that filter feeders and mesotherms deviate from general relationships between trophic level and body size, and exhibit significantly larger sizes than ectothermic‐macropredators. We confirm that filter feeding arose multiple times during the Paleogene, and suggest the possibility of a single origin of mesothermy in the Cretaceous. Together, our results elucidate two main evolutionary pathways that enable gigantism: mesothermic and filter feeding. These pathways were followed by ancestrally large clades and facilitated extreme sizes through specializations for enhancing prey intake. Although a negligible percentage of ectothermic‐macropredators reach gigantic sizes, these species lack such specializations and are correspondingly constrained to the lower limits of gigantism. Importantly, the very adaptive strategies that enabled the evolution of the largest sharks can also confer high extinction susceptibility

UPPER CENOMANIAN FISHES FROM THE BONARELLI LEVEL (OAE2) OF NORTHEASTERN ITALY

The Bonarelli Level (BL) is a radiolarian-ichthyolithic, organic-rich marker bed that was deposited close to the Cenomanian/Turonian boundary (CTB) representing the sedimentary expression of the global Oceanic Anoxic Event 2 (OAE2). In northeastern Italy this horizon yielded fossil remains documenting a rather diverse ichthyofauna. The assemblage was studied by Sorbini in 1976 based on material from a single locality, Cinto Euganeo. Subsequently, other localities yielding fish remains have been discovered. Our revision also includes fish remains from three new fish-bearing localities, the Carcoselle Quarry, the Valdagno-Schio tunnel and Quero other than those from Bomba Quarry near Cinto Eugeneo. At least 28 taxa were identified, including nine previously not reported from the Bonarelli Level, namely: Scapanorhynchus raphiodon, Cretalamna appendiculata, Archaeolamna kopingensis, ‘Nursallia’ tethysensis, Belonostomus sp., Dixonanogmius dalmatius, ‘Protosphyraena’ stebbingi and the beryciform Hoplopteryx sp. The overall assemblage mostly consists of crossognathiforms, tselfatiiforms and aulopiforms. A comparison of the taxonomic diversity with coeval assemblages evidences a general similarity with nearby western Tethyan fish assemblages and especially with the Jebel Tselfat ichthyofauna, although some of the taxa are exclusively shared with the assemblages of the boreal realm (English Chalk, Westphalia and Saxony). However, additional information would be necessary to more properly define the main global ichthyogeographic patterns during the Cenomanian

A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States

<div><p>ABSTRACT</p><p><i>Eorhincodon casei</i> from Russia and <i>Megachasma comanchensis</i> from the United States are two Cretaceous taxa initially described as putative planktivorous elasmobranchs, but the type specimens of these two taxa were subsequently reinterpreted to represent taphonomically abraded teeth of an odontaspidid, <i>Johnlongia</i> Siverson (Lamniformes: Odontaspididae). Here, we redescribe the type materials of ‘<i>E. casei</i>’ and ‘<i>M. comanchensis</i>’ and describe additional specimens of these species from other Late Cretaceous localities in Russia and the United States. These specimens demonstrate that (1) the two fossil taxa are valid species; (2) they warrant the establishment of a new genus of presumed planktivorous sharks, <i>Pseudomegachasma</i>, gen. nov., to accommodate the two species; and (3) the new genus is sister to <i>Johnlongia</i> and together constitute a new subfamily Johnlonginae, subfam. nov., tentatively placed in the family Odontaspididae sensu stricto. This taxonomic placement indicates that the putative planktivorous clade was derived from a presumed piscivorous form (<i>Johnlongia</i>), with an implication that <i>Pseudomegachasma</i>, gen. nov., evolved a plankton-eating habit independent of the four known planktivorous elasmobranch clades (Rhincodontidae, Megachasmidae, Cetorhinidae, and Mobulidae). It also indicates that planktivorous diets evolved independently at least three times in the order Lamniformes (i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and more significantly, <i>Pseudomegachasma</i>, gen. nov., would represent the oldest known plankton-feeding elasmobranch in the fossil record. The present fossil record suggests that <i>Pseudomegachasma</i>, gen. nov., evolved in a relatively shallow-water environment in Russia in the early Cenomanian or earlier and subsequently migrated to the North American Western Interior Seaway by the mid-Cenomanian.</p><p>http://zoobank.org/urn:lsid:zoobank.org:pub:D5D0400FD438-4A95-8301-DD47991572F6</p><p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP</p></div