Massimo Delfino, Jeremy E. Martin, France de Lapparent de Broin & Thierry Smith (2017)
Evidence for a pre-PETM dispersal of the earliest European crocodyloids
Historical Biology (advance online publication)
Crocodyloid remains from the late Paleocene of Mont de Berru (France) hosted in the collections of the MusÃum National dâHistoire Naturelle (Paris, France) and in the Institut royal des Sciences naturelles de Belgique (Brussels, Belgium) are described for the first time. This material, although fragmentary, can be clearly referred on a morphological basis to Asiatosuchus depressifrons (Blainville, 1855), a species previously reported from several Eocene Belgian localities thanks to abundant material including a nearly complete skeleton. The Paleocene material shares with A. depressifrons the number of alveoli involved in the dentary symphysis, the exclusion of the splenials from the symphysis, and the presence of a distinct depression on the jugal. The fossil remains from Berru represent the oldest European crocodyloid. Along with the alligatoroid Diplocynodon remensis Martin, Smith, de Lapparent de Broin, EscuilliÃ and Delfino, 2014, previously reported from the same locality, the crocodyloid A. depressifrons indicates that these generaÂ reached Europe before the PaleoceneâEocene Thermal Maximum. Although more complete remains from outside Europe are needed to refine phylogenetic hypotheses, according to the currently established fossil record the forerunners of diplocynodontids likely dispersed from North America, whereas those related to Asiatosuchus likely dispersed from Asia.
Francisca Lea & Martin J. Cohn (2017)
Developmental, genetic, and genomic insights into the evolutionary loss of limbs in snakes.
Genesis (advance online publication)
The evolution of snakes involved dramatic modifications to the ancestral lizard body plan. Limb loss and elongation of the trunk are hallmarks of snakes, although convergent evolution of limb-reduced and trunk-elongated forms occurred multiple times in snake-like lizards. Advanced snakes are completely limbless, but intermediate and basal snakes have retained rudiments of hindlimbs and pelvic girdles. Moreover, the snake fossil record indicates that complete legs were re-acquired at least once, suggesting that the potential for limb development was retained in some limb-reduced taxa. Recent work has shown that python embryos initiate development of a transitory distal leg skeleton, including a footplate, and that the limb-specific enhancer of the Sonic hedgehog gene, known as the zone of polarizing activity regulatory sequence (ZRS), underwent gradual degeneration during snake evolution. In this article, we review historical and recent investigations into squamate limblessness, and we discuss how new genomic and functional genetic experiments have improved our understanding of the evolution of limblessness in snakes. Finally, we explore the idea that pleiotropy of cis-regulatory elements may illuminate the convergent genetic changes that occurred in snake-like lizards, and we discuss a number of challenges that remain to be addressed in future studies.
Glenna T. Clifton, Jennifer A. Carr and Andrew A. Biewener (2017)
Comparative hindlimb myology of foot-propelled swimming birds.
Journal of Anatomy (advance online publication)
Several groups of birds have convergently evolved the ability to swim using their feet despite facing trade-offs with walking. However, swimming relative to terrestrial performance varies across these groups. Highly specialized divers, such as loons and grebes, excel at swimming underwater but struggle to stand on land, whereas species that primarily swim on the water surface, such as Mallards, retain the ability to move terrestrially. The identification of skeletal features associated with a swimming style and conserved across independent groups suggests that the hindlimb of foot-propelled swimming birds has adapted to suit the physical challenges of producing propulsive forces underwater. But in addition to skeletal features, how do hindlimb muscles reflect swimming ability and mode? This paper presents the first comparative myology analysis associated with foot-based swimming. Our detailed dissections of 35 specimens representing eight species reveal trends in hindlimb muscle size and attachment location across four independent lineages of extant swimming birds. We expand upon our dissections by compiling data from historical texts and provide a key to any outdated muscle nomenclature used in these sources. Our results show that highly diving birds tuck the femur and proximal tibiotarsus next to the ribcage and under the skin covering the abdomen, streamlining the body. Several hindlimb muscles exhibit dramatic anatomical variation in diving birds, including the flexor cruris lateralis (FCL) and iliofibularis (IF), which reduce in size and shift distally along the tibiotarsus. The femorotibialis medius (FTM) extends along an expanded cnemial crest. The resulting increased moment arms of these muscles likely help stabilize the hip and knee while paddling. Additionally, distal ankle plantarflexors, including the gastrocnemius and digital flexors, are exceptionally large in diving birds in order to power foot propulsion. These patterns exist within distantly related lineages of diving birds and, to a lesser extent, in surface swimmers. Together, our findings verify conserved muscular adaptations to a foot-propelled swimming lifestyle. The association of muscle anatomy with skeletal features and biomechanical movement demands can inform functional interpretation of fossil birds and reveal selective pressures underlying avian diversification.
Karen Sears, Jennifer A. Maier, Alexa Sadier, Daniel Sorensen and Daniel J. Urban (2017)
Timing the developmental origins of mammalian limb diversity.
Genesis (advance online publication)
Mammals have highly diverse limbs that have contributed to their occupation of almost every niche. Researchers have long been investigating the development of these diverse limbs, with the goals of identifying developmental processes and potential biases that shape mammalian limb diversity. To date, researchers have used techniques ranging from the genomic to the anatomic to investigate the developmental processes shaping the limb morphology of mammals from five orders (Marsupialia, Chiroptera, Rodentia, Cetartiodactyla, and Perissodactyla). Results of these studies suggest that the differential _expression_ of genes controlling diverse cellular processes underlies mammalian limb diversity. Results also suggest that the earliest development of the limb tends to be conserved among mammalian species, while later limb development tends to be more variable. This research has established the mammalian limb as a model system for evolutionary developmental biology, and set the stage for more in-depth, cross-disciplinary research into the genetic controls, tissue-level cellular behaviors, and selective pressures that have driven the developmental evolution of mammalian limbs. Ideally, these studies will be performed in a diverse suite of mammalian species within a comparative, phylogenetic framework.