Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological Setting of the Origin of the Avian Flight Stroke

The origin of avian flight is a classic macroevolutionary transition with research spanning over a century. Two competing models explaining this locomotory transition have been discussed for decades: ground up versus trees down. Although it is impossible to directly test either of these theories, it is possible to test one of the requirements for the trees-down model, that of an arboreal paravian. We test for arboreality in non-avian theropods and early birds with comparisons to extant avian, mammalian, and reptilian scansors and climbers using a comprehensive set of morphological characters. Non-avian theropods, including the small, feathered deinonychosaurs, and Archaeopteryx, consistently and significantly cluster with fully terrestrial extant mammals and ground-based birds, such as ratites. Basal birds, more advanced than Archaeopteryx, cluster with extant perching ground-foraging birds. Evolutionary trends immediately prior to the origin of birds indicate skeletal adaptations opposite that expected for arboreal climbers. Results reject an arboreal capacity for the avian stem lineage, thus lending no support for the trees-down model. Support for a fully terrestrial ecology and origin of the avian flight stroke has broad implications for the origin of powered flight for this clade. A terrestrial origin for the avian flight stroke challenges the need for an intermediate gliding phase, presents the best resolved series of the evolution of vertebrate powered flight, and may differ fundamentally from the origin of bat and pterosaur flight, whose antecedents have been postulated to have been arboreal and gliding

The largest specimen of <i>Apateon</i> and the life history pathway of neoteny in the Paleozoic temnospondyl family Branchiosauridae

Two distinct developmental trajectories, metamorphosis and neoteny (the retention of larval somatic features in adult animals), have been reported for the small gill-bearing branchiosaurids of the Late Carboniferous and Early Permian of central Europe. Based on a very large specimen of the species <i>Apateon caducus</i> (Ammon, 1889), anatomical features characteristic for the neotenic phenotype of branchiosaurids are described. Large neotenes lack changes that occur during a short phase of transformation into terrestrial adults (metamorphosis), such as ossification of the braincase and palatoquadrate and intercentra, further ossification of the girdles and formation of muscle attachment scars and processes on the limb bones. They also lack a distinct sculpturing of the dermal skull roofing elements with deep polygonal ridges and grooves. Instead, larval somatic features are retained including ossified branchial denticles indicative of open gill slits and accentuated larval-type sculpturing of the dermal skull roof. Large size, high degree of ossification as compared to the larvae, and the presence of uncinate processes on the ribs clearly demonstrate an adult ontogenetic stage. Neotenes remained in the aquatic environment throughout their life and were most likely not capable of effective terrestrial locomotion. The frequency distribution of the two phenotypes in modern salamander populations and the environmental cues that influence the development of them provide a comparative framework for the discussion of the evolution of the two life history pathways in branchiosaurids. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.200800012" target="_blank">10.1002/mmng.200800012</a

New specimen of Cacops woehri indicates differences in the ontogenetic trajectories among cacopine dissorophids

The Lower Permian Dolese locality has produced numerous exquisitely preserved tetrapod fossils representing members of a lower Permian upland fauna. Therein, at least nine taxa of the clade Dissorophoidea, ranging in size from the large predaceous trematopid Acheloma to the miniaturized amphibamid Doleserpeton, highlight the great taxic and ecological diversity of this anamniote clade. Here we describe a large specimen of the dissorophid Cacops woehri, which was previously only known from the juvenile or subadult holotype skull. Another member of the genus Cacops present at the Dolese locality, Cacops morrisi, is also represented by specimens spanning juvenile, subadult, and adult stages, allowing for a comparison of morphological changes taking place in the late phases of the ontogenetic trajectory of cacopine dissorophids. The new find shows that, in contrast to C. morrisi and C. aspidephorus, C. woehri only undergoes relatively subtle changes in skull morphology in late ontogeny and retains the overall more gracile morphology into adult stages. This includes retention of the rather shallow skull shape as well as a pattern of sculpturing consisting of elongate ridges and grooves and a large occipital flange. This suggests somewhat different functional demands in C. woehri than in other known species of Cacops, possibly associated with a different ecology paralleling the great taxic diversity of dissorophoids at the Dolese locality

Ossification sequences and associated ontogenetic changes in the bone histology of pachypleurosaurids from Monte San Giorgio (Switzerland/Italy)

Evolutionary changes in lifestyle (e.g., terrestrial vs. aquatic habits) influence tetrapod limb morphology. Similar evolutionary trends in osteogenetic sequences (order of bone ossification) and bone microstructure are often shared in extant tetrapods with similar habit preferences: such data are sometimes available from extinct taxa. The pachypleurosaurids from Monte San Giorgio (Switzerland, Italy) are Triassic marine reptiles with high quality, complete ontogenetic series. We studied osteogenesis and bone histology in the four species from this locality, comparing these with data from Recent terrestrial lizards and secondarily aquatic reptiles in order to determine if the osteogenetic sequences of pachypleurosaurids were similar to either the hypothesized plesiomorphic condition for terrestrial eureptilians or those of Recent aquatic reptiles. Pachypleurosaurian limb osteogenesis occurs in two steps: (1) developmental sequences of ossification during embryology and/or in early neonates, (2a) additional primary periosteal compaction processes, and (2b) additional primary and secondary endosteal compaction processes during neonate ontogeny. Taphonomic patterns reveal information on the order of the initiation and termination of these steps, which are (A) onset of ossification, (B) onset of additional compaction processes (early phase), and (C) termination of additional compaction processes (final phase). An event pairing analysis found that ossification of forelimb elements in pachypleurosaurids precedes that of the hind limb elements in all osteogenetic stages except for initiation of ossification (A). The order of their early phase compaction processes is similar to the hypothesized plesiomorphic eureptilian condition, whereas their final phase compaction processes varies among pachypleurosaurids, with S. mirigiolensis showing minor heterochronic shifts and N. edwardsii showing many heterochronic shifts relative to the hypothesized ancestral condition. Pachypleurosaurids from Monte San Giorgio increase the number of heterochronic shifts with decreasing stratigraphic age, showing a transition from more ‘terrestrial’ to more ‘aquatic’ osteogenetic sequences in comparison to data on ossification sequences of Recent aquatic reptiles

The Late Permian herbivore Suminia and the early evolution of arboreality in terrestrial vertebrate ecosystems

Vertebrates have repeatedly filled and partitioned the terrestrial ecosystem, and have been able to occupy new, previously unexplored habitats throughout their history on land. The arboreal ecospace is particularly important in vertebrate evolution because it provides new food resources and protection from large ground-dwelling predators. We investigated the skeletal anatomy of the Late Permian (approx. 260 Ma) herbivorous synapsid Suminia getmanovi and performed a morphometric analysis of the phalangeal proportions of a great variety of extant and extinct terrestrial and arboreal tetrapods to discern locomotor function and habitat preference in fossil taxa, with special reference to Suminia. The postcranial anatomy of Suminia provides the earliest skeletal evidence for prehensile abilities and arboreality in vertebrates, as indicated by its elongate limbs, intrinsic phalangeal proportions, a divergent first digit and potentially prehensile tail. The morphometric analysis further suggests a differentiation between grasping and clinging morphotypes among arboreal vertebrates, the former displaying elongated proximal phalanges and the latter showing an elongation of the penultimate phalanges. The fossil assemblage that includes Suminia demonstrates that arboreality and resource partitioning occurred shortly after the initial establishment of the modern type of terrestrial vertebrate ecosystems, with a large number of primary consumers and few top predators