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Background  Many neritic to nearshore species of marine adapted turtle from the Late Cretaceous of North America are thought to represent the stem lineage of Cheloniidae but due to fragmentary holotypes, low total specimen counts, and resultantly incomplete morphological character suites, are routinely placed either within or outside of crown group Chelonioidea leaving their precise cladistic affinities uncertain. Despite this systematic ambiguity, the referral of these species to either the stem of Cheloniidae or Chelonioidea belies the critical importance of these taxa in any investigation into the origins of extant marine turtles. The adequate incorporation of these species into phylogenetic studies requires the formal description of relatively complete specimens, particularly those possessing associated cranial and post-cranial material.    Methods  Remarkably complete fossil specimens of several adult and juvenile marine turtles from the Mooreville Chalk and Eutaw Formations (Alabama, USA) are formally described and assigned to  Prionochelys matutina . This material provides new information into the anatomy, ontogeny, and cladistic affinities of the species. A phylogenetic hypothesis for Late Cretaceous marine turtles is then generated through the consilience of stratigraphic, morphological, and molecular data.    Results  Phylogenetic analysis places  Prionochelys matutina  on the stem of Cheloniidae as a member of a monophyletic clade with other putative pan-cheloniids, including  Ctenochelys stenoporus ,  Ctenochelys acris ,  Peritresius martini , and  Peritresius ornatus . The members of this clade possess incipient secondary palates, pronounced carapacial and plastral fontanelles at all stages of development, and are characterized by the presence of superficial ossifications at the apices of the neural keel elevations along the dorsal midline of the carapace.    Discussion  The epithecal osteoderms dorsal to the neural series (epineurals) found in Ctenochelyidae are unique among turtles. The presence of epineurals in ctenochelyid turtles shows that epithecal ossifications arose independently in both leatherback (Dermochelyidae) and hard-shelled (Cheloniidae) marine turtles. Whether or not the epineurals of Ctenochelyidae are homologous with the dermal ossicles comprising the carapace of  Dermochelys coriacea  remains untested however, histological thin sectioning of dermochelyid and ctenochelyd epithecal elements may reveal meaningful information in future studies.

peerj

<i>Prionochelys matutina</i>Zangerl, 1953 (Testudines: Pan-Cheloniidae) from the Late Cretaceous of the United States and the evolution of epithecal ossifications in marine turtles

Prionochelys matutinaZangerl, 1953 (Testudines: Pan-Cheloniidae) from the Late Cretaceous of the United States and the evolution of epithecal ossifications in marine turtles