Evolution of ecospace occupancy by Mesozoic marine tetrapods

Ecology and morphology are different, and yet in comparative studies of fossil vertebrates the two are often conflated. The macroevolution of Mesozoic marine tetrapods has been explored in terms of morphological disparity, but less commonly using ecological‐functional categories. Here we use ecospace modelling to quantify ecological disparity across all Mesozoic marine tetrapods. We document the explosive radiation of marine tetrapod groups in the Triassic and their rapid attainment of high ecological disparity. Late Triassic extinctions led to a marked decline in ecological disparity, and the recovery of ecospace and ecological disparity was sluggish in the Early Jurassic. High levels of ecological disparity were again achieved by the Late Jurassic and maintained during the Cretaceous, when the ecospace became saturated by the Late Cretaceous. Sauropterygians, turtles and ichthyosauromorphs were the largest contributors to ecological disparity. Throughout the Mesozoic, we find that established groups remained ecologically conservative and did not explore occupied or vacant niches. Several parts of the ecospace remained vacant for long spans of time. Newly evolved, radiating taxa almost exclusively explored unoccupied ecospace, suggesting that abiotic releases are needed to empty niches and initiate diversification. In the balance of evolutionary drivers in Mesozoic marine tetrapods, abiotic factors were key to initiating diversification events, but biotic factors dominated the subsequent generation of ecological diversity.     

Remodelling of skeletal tissues bone and structural specialisations in an elasmosaurid (Sauropterygia: Plesiosauroidea) from the Upper Cretaceous of Patagonia,Argentina

Elasmosauridae were cosmopolitan Late Cretaceous plesiosaurs with conspicuous morphological diversity. Within this group, vertebral morphology is a criterion for estimating relative age in plesiosaur. On the other hand, the microstructure of plesiosaur bone is considered as indicative of ontogenetic stage. However, knowledge about ontogenetic tissue transformation in different elements of the skeleton is poorly known. Resorption and remodelling of skeletal tissues are required for development and growth, mechanical adaptation, repair and mineral homeostasis of the vertebrate skeleton. This contribution analyses different postcranial elements of a Late Cretaceous elasmosaurid from Patagonia. Characterisation of bone microstructure indicates the presence of compact bone inner organisation in an adult derived plesiosaur from the Cretaceous and that the distribution of bone specialisations depicts conspicuous variations within a single skeleton depending on the skeletal element considered. Bone compactness or degree of remodelling in elasmosaurids is not necessarily correlated with the ontogenetic age of the animal or to costal versus pelagic lifestyles. The available data are still scarce, but we propose a topic of discussion: perhaps the degree of remodelling and compactness also may be related to the activity level and increased mechanical load in different skeletal elements.     

Understanding selection for long necks in different taxa

There has been recent discussion about the evolutionary pressures underlying the long necks of extant giraffes and extinct sauropod dinosaurs. Here we summarise these debates and place them in a wider taxonomic context. We consider the evolution of long necks across a wide range of (both living and extinct) taxa and ask whether there has been a common selective factor or whether each case has a separate explanation. We conclude that in most cases long necks can be explained in terms of foraging requirements, and that alternative explanations in terms of sexual selection, thermoregulation and predation pressure are not as well supported. Specifically, in giraffe, tortoises, and perhaps sauropods there is likely to have been selection for high browsing. It the last case there may also have been selection for reaching otherwise inaccessible aquatic plants or for increasing the energetic efficiency of low browsing. For camels, wading birds and ratites, original selection was likely for increased leg length, with correlated selection for a longer neck to allow feeding and drinking at or near substrate level. For fish‐eating long‐necked birds and plesiosaurs a small head at the end of a long neck allows fast acceleration of the mouth to allow capture of elusive prey. A swan's long neck allows access to benthic vegetation, for vultures the long neck allows reaching deep into a carcass. Geese may be an unusual case where anti‐predator vigilance is important, but so may be energetically efficient low browsing. The one group for which we feel unable to draw firm conclusions are the pterosaurs, this is in keeping with the current uncertainty about the biology of this group. Despite foraging emerging as a dominant theme in selection for long necks, for almost every taxonomic group we have identified useful empirical work that would increase understanding of the selective costs and benefits of a long neck.     

A New Marine Reptile (Sauropterygia) from New Zealand: Further Evidence for A Late Cretaceous Austral Radiation of Cryptoclidid Plesiosaurs

Kaiwhekea katiki gen. et sp. nov. represents the first described cryptoclidid plesiosaurian from New Zealand. It is one of the largest cryptoclidids known, at a length of over 6.5 m, and represents the third reported genus of austral Late Cretaceous cryptoclidids. Kaiwhekea katiki is from siltstones of the Katiki Formation, upper Haumurian Stage (Cenomanian–Maastrichtian; c. 69–70 Ma) of coastal Otago, South Island, New Zealand. In the Late Cretaceous, the locality lay close to the polar circle. The holotype and only known specimen is an articulated skeleton with skull, preserved mostly as natural molds, but which lacks the forelimbs and pectoral girdle. The skull is relatively large and possesses several distinct characters, including a substantial, deep, jugal. There are about 43 upper and 42 lower teeth in each jaw quadrant; all are homodont, slim, and slightly recurved, lacking prominent ornament. Kaiwhekea probably took single soft-bodied prey. Based on cranial structure, it clearly belongs with the Cryptoclididae, but is not certainly close to the southern Late Cretaceous cryptoclidids Morturneria (Seymour Island, Antarctica) and Aristonectes (Chile, Argentina).     

A NEW ELASMOSAURID PLESIOSAUR FROM THE UPPER CRETACEOUS OF FUKUSHIMA, JAPAN

Abstract:  A new genus and species of an elasmosaurid plesiosaur, Futabasaurus suzukii , is described based on a partial skeleton from the Inoceramus amakusensis Zone (Lower Santonian, Upper Cretaceous) of the Irimazawa Member of the Tamayama Formation, Futaba Group, in Fukushima Prefecture, Japan. The new taxon is characterized by a number of characters such as the wide space between the orbit and external naris, posterior extension of the interclavicle, relatively long humerus and prominent femoral muscle scar. The holotype includes a partial skull and mandible, posterior cervicals to sacrals, ribs, clavicular arch, pelvic girdle and four limbs. The remains are mostly in articulation, and exhibit evidence of predation/scavenging by sharks. The distribution of elasmosaurid species in the circum-Pacific region remains unclear due to the lack of diagnostic materials. The occurrence of F. suzukii is geographically and stratigraphically significant, because it allows species-level comparison; as a diagnosable elasmosaurid specimen, F. suzukii is the first and the oldest from the northern Pacific.     

Ecomorphology of plesiosaur flipper geometry

The Plesiosauria is an extinct group of marine reptiles once common in mesozoic seas. Previous work on plesiosaur hunting styles has suggested that short‐necked, large‐headed animals were pursuit predators, whereas long‐necked, small‐headed animals were ambush predators. This study presents new data on the aspect ratios (ARs) of plesiosaur flippers, and interprets these data via comparison with AR in birds, bats and aircraft. Performance trade‐offs implicit in AR variation are well‐understood in the context of aircraft design, and these trade‐offs have direct ecomorphological analogues in birds and bats. Knowledge of these trade‐offs allows interpretation of variation in plesiosaur AR. By analogy, short‐necked taxa were specialized for manoeuvrability and pursuit, whereas long‐necked taxa were generally specialized for efficiency and cruising. These interpretations agree with previous assessments of maximum swimming speed.