Friday, September 16, 2022

[Paleontology • 2022] Opisthiamimus gregori • A nearly Complete Skeleton of A New Eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the Evolution and Diversity of Rhynchocephalia (Reptilia: Lepidosauria)

Opisthiamimus gregori
DeMar, Jones & Carrano, 2022

Artwork by Julius Csotonyi 

We describe a new, small-bodied rhynchocephalian reptile, Opisthiamimus gregori gen. et sp. nov., from the Upper Jurassic Morrison Formation of Wyoming, USA. Whereas many fossil rhynchocephalians are based on isolated incomplete jaws, the holotype of O. gregori includes most of the skull and postcranium and therefore represents one of the most complete specimens of Rhynchocephalia known from North America. We used micro-computed tomography to examine its skeletal anatomy in detail and to develop a three-dimensional reconstruction of the skull. The skull of O. gregori is similar to that of several non-neosphenodontian rhynchocephalians such as Planocephalosaurus (e.g. large orbits) and Clevosaurus (e.g. parietal parasagittal crests) yet exhibits a suite of other features related to the proal shearing mechanism that becomes increasingly elaborated among more phylogenetically nested taxa such as Sphenodon (e.g. lateral palatine tooth row parallels maxillary tooth row along its entire length, pyramidal dentary teeth with mesial shearing crests). The postcranial skeleton of O. gregori exhibits characteristics typical of a terrestrial rhynchocephalian. Our phylogenetic analyses use a substantially updated data set of 118 characters and 46 taxa, and both maximum parsimony and Bayesian frameworks. Results place O. gregori inside Eusphenodontia but outside Neosphenodontia, and therefore in a key position for contributing to character polarity for more deeply nested clades such as Clevosauridae, Sphenodontidae and Pleurosauridae. We also erect Leptorhynchia taxon nov., composed primarily of aquatically adapted taxa (e.g. Pleurosaurus, Sapheosaurus), which is supported by both cranial and postcranial characters. Because O. gregori is not particularly closely related to the other named Morrison rhynchocephalians (e.g. Opisthias rarus), it increases both the alpha and beta taxonomic diversities within the formation. Similarly, major differences in body size and inferred diet of the Morrison taxa imply considerable concomitant palaeoecological diversity just prior to a major global decline in rhynchocephalian diversity around the close of the Jurassic.
Keywords: Rhynchocephalia, Eusphenodontia, Late Jurassic, Morrison Formation, phylogeny, diversity

Systematic palaeontology

Lepidosauria Haeckel, 1866 (sensu Evans 1984)
Rhynchocephalia Günther, 1867 (sensu Gauthier et al. 1988)

Sphenodontia Williston, 1925
Eusphenodontia Herrera-Flores, Stubbs, Elsler & Benton 2018

Opisthiamimus gen. nov.

Type and only species: Opisthiamimus gregori sp. nov.

Derivation of name: From Opisthias (a genus of the clade Rhynchocephalia) and the Greek ‘mimos’ (μῖμος, meaning mimic; latinized to mimus), referring to the morphological similarities of the dentary and dentary teeth between this taxon and Opisthias.

Derivation of name: The species epithet ‘gregori’ recognizes Joseph Gregor, a dedicated Smithsonian volunteer who skillfully prepared the holotype and referred specimens.

Diagnosis: Opisthiamimus gregori gen. et sp. nov. is a small-bodied (snout-vent length ∼85 mm) member of Rhynchocephalia based on the presence of four unambiguous synapomorphies: (1) jugal dorsal process extends posteriorly 33% or more of the anteroposterior length of the lower temporal fenestra; (2) lower temporal bar bowed laterally beyond the limit of the adductor chamber; (3) dentary posterior process elongate, reaching articular glenoid level; and (4) splenial absent. Opisthiamimus gregori is a member of Sphenodontia based on the presence of three unambiguous synapomorphies: (1) lacrimal absent; (2) distal marginal tooth attachment acrodont; and (3) weak overlap or imbrication between adjacent distal maxillary teeth. Opisthiamimus gregori is a member of Acrosphenodontia in possessing all 14 unambiguous character states of that major clade: (1) parietal parasagittal crest poorly developed; (2) dentary mentonian process poorly developed; (3) mandibular symphysis oval; (4) anterodorsal tip of dentary with conical, anterodorsally projected successional tooth that does not project beyond anterior margin angle of dentary; (5) mesial notch above dentary symphysis confluent with medial surface of subdental ridge/postsymphyseal lamina; (6) dentary symphysis not split by Meckelian canal; (7) pre-coronoid portion of dentary moderate in length and height; (8) coronoid process of dentary moderately tall; (9) mesial marginal tooth attachment acrodont; (10) distal maxillary teeth with small flanges; (11) mesiodistal length of distal dentary teeth greater than apicobasal height; (12 and 13) distal dentary teeth with a mesiolabial and mesiolingual crest; and (14) pterygoid with two tooth rows. Opisthiamimus gregori belongs to Eusphenodontia based on the presence of a single unambiguous synapomorphy: marginal dentition with conspicuous wear facets. Opisthiamimus gregori is the sister taxon to the Clevosauridae + Neosphenodontia in having a shorter fourth metacarpal relative to the third.

Opisthiamimus gregori 
Insect prey: Morrisonnepa jurassica.
Dinosaur in background guarding egg clutch: Allosaurus jimmadseni.
Yellow leaf litter in foreground: Ginkgoites. sp. Tall shrubs in background: Nilssonia sp. Trees in background: Araucaria sp.

David G. DeMar Jr., Marc E. H. Jones and Matthew T. Carrano. 2022. A nearly Complete Skeleton of A New Eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the Evolution and Diversity of Rhynchocephalia (Reptilia: Lepidosauria). Journal of Systematic Palaeontology. 2093139. DOI: 10.1080/14772019.2022.2093139