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[dinosaur] Bergqvistherium + tetrapod origin + archosaur shoulder evolution + more






Ben Creisler

Some recent non-dino papers that may be of interest (some with free pdfs):



Free pdf:

Bergqvistherium primigenia gen. & sp. nov.Â



Leonardo M. Carneiro (2018)
A new protodidelphid (Mammalia, Marsupialia, Didelphimorphia) from the Itaboraà Basin and its implications for the evolution of the Protodidelphidae.Â
Anais da Academia Brasileira de CiÃncias (advance online publication)

Free pdf:


The Protodidelphidae was a group of marsupials that lived in Gondwana from the early to middle Eocene. Among South American faunas, the Itaboraà Basin calls attention by the presence of four genera and six species. Herein is described Bergqvistherium primigenia gen. et sp. nov., a new protodidelphid from the Itaboraà Basin - lower Eocene. This taxon differs from other protodidelphids in the smaller size, developed entocristid, less brachyo-bunoid adaptations, and entoconid more mesial than the hypoconid. These characters are recovered as plesiomorphies of the Protodidelphidae, supporting Bergqvistherium as an early-divergent lineage of this group. The Protodidelphidae fauna of the Itaboraà Basin is represented by less specialized "basal" taxa, such as Bergqvistherium and Periprotodidelphis; and more specialized apical taxa, such as Guggenheimia, Protodidelphis, and Carolocoutoia. This result indicates that the diversification of apical protodidelphids probably was a result of a relatively short-time event, occurring during the early Eocene. This evolutionary event can be directly correlated to the increase in the temperatures and the extension of tropical forests resulted by the Paleocene-Eocene Thermal Maximum during the Itaboraiense time span. The study supported a Late Cretaceous origin for the Protodidelphidae, which agrees with molecular studies for the Didelphimorphia.


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Free pdf:

Per E. Ahlberg (2018)Â
Follow the footprints and mind the gaps: a new look at the origin of tetrapods.
Earth and Environmental Science Transactions of the Royal Society of Edinburgh (advance online publication)Â


The hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods.


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Free pdf:

Melanie J. Hopkins, David W. Bapst, Carl Simpson and Rachel C. M. Warnock (2018)Â
The inseparability of sampling and time and its influence on attempts to unify the molecular and fossil records.
Paleobiology (advance online publication)


The two major approaches to studying macroevolution in deep time are the fossil record and reconstructed relationships among extant taxa from molecular data. Results based on one approach sometimes conflict with those based on the other, with inconsistencies often attributed to inherent flaws of one (or the other) data source. Any contradiction between the molecular and fossil records represents a failure of our ability to understand the imperfections of our data, as both are limited reflections of the same evolutionary history. We therefore need to develop conceptual and mathematical models that jointly explain our observations in both records. Fortunately, the different limitations of each record provide an opportunity to test or calibrate the other, and new methodological developments leverage both records simultaneously. However, we must reckon with the distinct relationships between sampling and time in the fossil record and molecular phylogenies. These differences impact our recognition of baselines and the analytical incorporation of age estimate uncertainty.

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David B. Baier, Brigid M. Garrity, Sabine Moritz & Ryan M. Carney (2018)
Alligator (Alligator mississippiensis) sternal and shoulder girdle mobility increase stride length during high walks.
Journal of Experimental Biology 2018: jeb.186791 (advance online publication)
doi: 10.1242/jeb.186791Â


Crocodilians have played a significant role in evolutionary studies of archosaurs. Given that several major shifts in forelimb function occur within Archosauria, forelimb morphologies of living crocodilians are of particular importance in assessing locomotor evolutionary scenarios. A previous X-ray investigation of walking alligators revealed substantial movement of the shoulder girdle, but since the sternal cartilages do not show up in X-ray, the source of the mobility could not be conclusively determined. Scapulocoracoid movement was interpreted to indicate independent sliding of each coracoid at the sternocoracoid joint; however, rotations of the sternum could also produce similar displacement of the scapulocoracoids. Here, we present new data employing marker-based XROMM (X-ray Reconstruction of Moving Morphology), wherein simultaneous biplanar X-ray video and surgically implanted radio-opaque markers permit precise measurement of the vertebral axis, sternum, and coracoid in walking alligators. We found that movements of the sternum and sternocoracoid joint both contribute to shoulder girdle mobility and stride length, and that the sternocoracoid contribution was less than previously estimated. On average, the joint contributions to stride length (measured with reference to a point on the distal radius, thus excluding wrist motion) are as follows: thoracic vertebral rotation 6.2Â3.7%, sternal rotation 11.1Â2.5%, sternocoracoid joint 10.1Â5.2%, glenohumeral joint 40.1Â7.8%, and elbow 31.1Â4.2%. To our knowledge this is the first evidence of sternal movement relative to the vertebral column (presumably via rib joints) contributing to stride length in tetrapods.


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Robert L. Cieri, Sabine Moritz, John G. Capano & Elizabeth L. Brainerd (2018)
Breathing with floating ribs: XROMM analysis of lung ventilation in savannah monitor lizards
Journal of Experimental Biology 2018 : jeb.189449 (advance online publication)
doi: 10.1242/jeb.189449Â


The structures and functions of the vertebrate lung and trunk are linked through the act of ventilation, but the connections between these structures and functions are poorly understood. We used XROMM to measure rib kinematics during lung ventilation in three savannah monitor lizards, Varanus exanthematicus. All of the dorsal ribs, including the floating ribs, contributed to ventilation; the magnitude and kinematic pattern showed no detectable cranial-to-caudal gradient. The true ribs acted as two rigid bodies connected by flexible cartilage, with the vertebral rib and ventromedial shaft of each sternal rib remaining rigid and the cartilage between them forming a flexible intracostal joint. Rib rotations can be decomposed into bucket handle rotation around a dorsoventral axis, pump handle rotation around a mediolateral axis, and caliper motion around a craniocaudal axis. Dorsal rib motion was dominated by roughly equal contributions of bucket and pump rotation in two individuals and by bucket rotation in the third individual. The recruitment of floating ribs during ventilation in monitors is strikingly different from the situation in iguanas, where only the first few true ribs contribute to breathing. This difference may be related to the design of the pulmonary system and life history traits in these two species. Motion of the floating ribs may maximize ventilation of the caudally and ventrolaterally-positioned compliant saccular chambers in the lungs of varanids, while restriction of ventilation to a few true ribs may maximize crypsis in iguanas.

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