Geo-palaeontological data suggest the Vera Formation is composed of two sequences
The Lopingian-Early Jurassic tetrapod track record from southern Gondwana is highlighted
Data suggest correlations between Patagonia and southern Africa
Trackmakers in both realms of southern Gondwana are homogeneously distributed
In recent years, new studies about vertebrate tracks found in the late Permian-Triassic Los Menucos Complex and Early Jurassic Marifil Volcanic Complex (RÃo Negro province, Patagonia, Argentina) have been published. In those studies, the chronostratigraphic information of each track-bearing unit has been discussed and the relationships between the record from Patagonia and southern Africa have been highlighted. With the aim of deepening both subjects, the biochronological and palaeobiogeographical information of the main ichnotaxa found in the lower Mesozoic units of Patagonia, Dicynodontipus, Pentasauropus and Anomoepus-like tracks, have been analysed. Moreover, the updated chronological data from the Vera Formation, Los Menucos Complex, in the Tscherig and Yancaqueo farm areas, and from the Marifil Volcanic Complex near Perdomo farm, have been studied. The entire biochronological, chronological and tectonic evidence suggests that within the Los Menucos Complex there are two sequences: a lower one, Wuchiapingian-Olenekian in age, near the Tscherig farm and bearing Dicynodontipus tracks, and an upper sequence of Anisian-Rhaetian (?Norian) age, in the Yancaqueo farm and bearing Pentasauropus tracks. This suggests that the rocks historically defined as Vera Formation have different ages in different areas and span a longer time interval than previously thought. In the case of the Anomoepus-like tracks found in the Marifil Volcanic Complex, the biochronological and geological data are consistent with an Early Jurassic age. According to ichnological information and geological background we propose a correlation between Patagonia and southern Africa constrained in three main phases: 1. Lopingian to Early Triassic, Vera Formation (Tscherig farm) and Balfour Formation, respectively; 2. Middle to Late Triassic, Vera Formation (Yancaqueo farm) and lower Elliot Formation, respectively; and 3. Early Jurassic, Marifil Volcanic Complex and upper Elliot Formation, respectively. In addition, this correlation is supported by a similarity in palaeoclimatic settings that allow inferring a homogeneous distribution of trackmakers in both realms of southern Gondwana. Finally, the palaeobiogeographic information of the studied ichnotaxa and their putative trackmakers is consistent and shows an extensive distribution, during a geotectonic moment with most of the continents assembled forming Pangaea.
Michael R. Rampino & Shu-Zhong Shen (2019)
The end-Guadalupian (259.8 Ma) biodiversity crisis: the sixth major mass extinction?
Historical Biology (advance online publication)
The modern loss of species diversity has been labelled the 'sixth extinction' subsequent to the five major mass extinctions widely recognised in the Phanerozoic geologic record -- the end-Ordovician (443.8 Ma), the Late Devonian (372.2 Ma), end-Permian (251.9 Ma), end-Triassic (201.4 Ma) and end-Cretaceous (66 Ma) events. Rankings in terms of numbers of genera suffering extinction, and especially in terms of ecological impact, however, put the end-Guadalupian (end-Capitanian) (259.8 Ma) extinction event in the same category with the other major mass extinctions. Thus, there were apparently six major Phanerozoic mass extinctions, and the current loss of species should perhaps be called the 'seventh extinction'.
Alec T. Wilken, Kevin M. Middleton, Kaleb C. Sellers, Ian N. Cost & Casey M. Holliday (2019)
The roles of joint tissues and jaw muscles in palatal biomechanics of the Savannah monitor (Varanus exanthematicus) and their significance for cranial kinesis.
Journal of Experimental Biology jeb.201459
doi: 10.1242/jeb.201459 https://jeb.biologists.org/content/early/2019/08/30/jeb.201459
Many vertebrates exhibit cranial kinesis, or movement between bones of the skull and mandible other than at the jaw joint. Many kinetic species possess a particular suite of features to accomplish this movement, including flexible cranial joints and protractor musculature. Whereas the musculoskeletal anatomy of these kinetic systems is well understood, how these joints are biomechanically loaded, how different soft tissues affect joint loading and kinetic capacity, and how the protractor musculature loads the skull remain poorly understood. Here we present a Finite Element Model of the savannah monitor, Varanus exanthematicus, a modestly kinetic lizard, to better elucidate the roles of soft tissue in mobile joints and protractor musculature on cranial loading. We describe the 3D resultants of jaw muscles and the histology of palatobasal, otic and jaw joints. We tested the effects of joint tissue types, bite point, and muscle loads to evaluate the biomechanical role of muscles on the palate and braincase. We found the jaw muscles have significant mediolateral components that can impart stability across palatocranial joints. We found articular tissues affect the magnitude of strains experienced around the palatobasal and otic joints. We found that without protractor muscle loading, the palate, quadrate and braincase experience higher strains suggesting this muscle helps insulate the braincase and palatoquadrate from high loads. Finally, we found the cross-sectional properties of the bones of Varanus exanthematicus are well suited for performing under torsional loads. These findings suggest that torsional loading regimes may have played a more important role in the evolution of cranial kinesis in lepidosaurs than previously appreciated.
Novel phenotypes are often linked to major ecological transitions during evolution. Here, we describe for the first time an unusual network of large blood vessels in the head of the sea snake Hydrophis cyanocinctus. MicroCT imaging and histology reveal an intricate modified cephalic vascular network (MCVN) that underlies a broad area of skin between the snout and the roof of the head. It is mostly composed of large veins and sinuses and converges posterodorsally into a large vein (sometimes paired) that penetrates the skull through the parietal bone. Endocranially, this blood vessel leads into the dorsal cerebral sinus, and from there, a pair of large veins depart ventrally to enter the brain. We compare the condition observed in H. cyanocinctus with that of other elapids and discuss the possible functions of this unusual vascular network. Sea snakes have low oxygen partial pressure in their arterial blood that facilitates cutaneous respiration, potentially limiting the availability of oxygen to the brain. We conclude that this novel vascular structure draining directly to the brain is a further elaboration of the sea snakes' cutaneous respiratory anatomy, the most likely function of which is to provide the brain with an additional supply of oxygen.
Claire Dobson, ÂSam Giles, ÂZerina Johanson, ÂJeff Liston & Matt Friedman (2019)
Cranial osteology of the Middle Jurassic (Callovian) Martillichthys renwickae (Neopterygii, Pachycormiformes) with comments on the evolution and ecology of edentulous pachycormiforms.
Our understanding of the ecology and phylogenetic relationships of Pachycormiformes, a group of Mesozoic stem teleosts including the iconic Leedsichthys, has often been hindered by a lack of comprehensive morphological information. MicroâCT scanning of an articulated, although flattened, cranium of the edentulous Martillichthys renwickae from the Middle Jurassic (Callovian) Oxford Clay of the UK reveals previously unknown internal details of the most complete suspensionâfeeding pachycormiform skull known, including the palate, braincase and branchial skeleton. The latter preserves gill rakers with elongate, pointed projections similar to those of Asthenocormus, in contrast to the finer fimbriations associated with Leedsichthys. We also reinterpret some previously described features, including dermal bone patterns of the snout, skull roof and lower jaw, and the morphology of the ventral hyoid arch. These new anatomical data reinforce the phylogenetic placement of Martillichthys as part of the Jurassic clade of edentulous pachycormiforms. The elongate skull geometry of these Jurassic taxa is strikingly similar to that of Ohmdenia, the sister taxon to edentulous pachycormiforms, but contrasts sharply with the morphology of the Late Cretaceous edentulous pachycormiform Bonnerichthys, raising questions over the phylogenetic relationships among these taxa. Most significantly, Martillichthys shows specialized characters with a restricted phylogenetic distribution among suspensionâfeeding pachycormiforms, including the distinctive gill rakers and a greatly extended occipital stalk. Our analysis of Martillichthys supports past interpretations of a close relationship with Asthenocormus, and provides a model for interpreting the less complete remains of other members of this enigmatic group of fishes.