Pentasauropus argentinae nov. isp.
Abdelouahed Lagnaoui, Ricardo N. Melchor, Eduardo S. Bellosi, Pablo M. Villegas, Nahuel Espinoza & Aldo M.Umazano (2019)
Middle Triassic Pentasauropus-dominated ichnofauna from western Gondwana: Ichnotaxonomy, palaeoenvironment, biostratigraphy and palaeobiogeography.
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
Highlights
Oldest occurrence (Middle Triassic) of Pentasauropus from a large track assemblage
Establishment of the new ichnospecies Pentasauropus argentinae from the Middle Triassic
Potential Kannemeyeriiform trackmaker for P. argentinae with abducted fore-limbs and semi-abducted hind limbs
The ichnoassemblage indicates an Anisian-Ladinian age for the track-bearing strata of the Cerro de Las Cabras Formation.
Pentasauropus trackmaker probably originated and dispersed from Argentina to others parts of Gondwana and North America.
Abstract
Quadruped trackways of large pentadactyl footprints are reported from the Middle Triassic (Anisian-Ladinian) Cerro de las Cabras Formation of the Cuyo Basin, Mendoza Province, central-western Argentina. The track-bearing strata are interpreted as deposited by sheetfloods in a mixed flat where water was ponded in a playa-lake setting. The vertebrate trackways are assigned to the ichnogenus Pentasauropus, originally described from the Upper Triassic Elliot Formation of South Africa, based on the presence of five equally spaced digit imprints that form an anteriorly convex broad arcuate pattern. A new ichnospecies, Pentasauropus argentinae nov. isp., is erected for the Argentinian material because of the distinct heteropody, inward rotation of the pes and outward rotation of the manus imprints, and the presence of palm/sole traces, and the diagnosis of the ichnogenus is emended. Pes/manus set arrangement and trackway patterns indicate a sprawling limbed trackmaker with an abducted posture for the fore-limbs and at least a semi-abducted posture for the hindlimbs, which suggests that the trackmaker was a kannemeyeriiform dicynodont. Associated archosaur ichnotaxa (Chirotherium barthii, Chirotherium cf. C. rex, Isochirotherium cf. I. coureli) point to a Middle Triassic age for the trackway-bearing strata in agreement with bracketing geochronological data. The rare occurrence of the therapsid footprint Dicynodontipus isp. is also compatible with the inferred age. The moderate abundance and oldest occurrence of Pentasauropus from three areas in Argentina suggest an origin for this ichnogenus in southwestern Gondwana, and presumably this area was a faunal exchange gate between southeastern and southwestern Gondwana and south Gondwana and North America.
Nicholus M. Ledbetter & Ronald M. Bonett (2019)
Terrestriality constrains salamander limb diversification: Implications for the evolution of pentadactyly.
Journal of Evolutionary Biology (advance online publication)
Terrestriality constrains salamander limb diversification: Implications for the evolution of pentadactyly
Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Extant salamanders display three distinct life cycle strategies that range from aquaticâtoâterrestrial (biphasic), to fully aquatic (paedomorphic), to fully terrestrial (direct development). Life cycle variation is associated with changes in body form such as loss of digits, limb reduction, or body elongation. However, the relationships among these traits and life cycle strategy remain unresolved. Here we use a Bayesian modeling approach to test whether life cycle transitions by salamanders have influenced rates, optima, and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted toward smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number, as well as reduced correlations between limb lengths. Our results support the idea that terrestrial plantigrade locomotion constrains limb evolution and, when lifted, leads to higher rates of trait diversification and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestrial life stages provide an important framework for understanding the evolutionary and developmental mechanisms behind major shifts in ecological zones as seen among early tetrapods during their transition from water to land.