P. Martin Sander

Biology of the sauropod dinosaurs: the evolution of gigantism

The herbivorous sauropod dinosaurs of the Jurassic and Cretaceous periods were the largest terrestrial animals ever, surpassing the largest herbivorous mammals by an order of magnitude in body mass. Several evolutionary lineages among Sauropoda produced giants with body masses in excess of 50 metric tonnes by conservative estimates. With body mass increase driven by the selective advantages of large body size, animal lineages will increase in body size until they reach the limit determined by the interplay of bauplan, biology, and resource availability. There is no evidence, however, that resource availability and global physicochemical parameters were different enough in the Mesozoic to have led to sauropod gigantism.

Longbone histology of the Tendaguru sauropods: implications for growth and biology

Abstract A new sampling technique for fossil bone (coring with a 5/8” bit) was used to sample longbones of all four sauropod genera from the Upper Jurassic Tendaguru beds of Tanzania for paleohistological study. Brachiosaurus and Barosaurus are represented by growth series of humeri and femora, while Dicraeosaurus could be sampled in fewer specimens and only one bone of Janenschia was available. Although all samples are dominated by fibrolamellar bone tissue, taxa can be distinguished by the degree and nature of bone remodeling and the presence and spacing of a peculiar kind of growth line (here termed “polish lines”). In addition, Barosaurus bone revealed two types of histology, tentatively interpreted as sexual morphs. The Tendaguru sauropods show a common growth pattern in which growth is determinate but sexual maturity is achieved well before maximum size is reached. For Brachiosaurus and Barosaurus, size at sexual maturity can be estimated and was reached at about 40% and 70% maximum size, respectively. Quantification of growth is possible in Janenschia using polish lines: the specimen studied reached sexual maturity at ≥11 years, attained maximum size at ≥26 years, and died at ≥38 years.

Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur

Sauropod dinosaurs were the largest animals ever to inhabit the land, with truly gigantic forms in at least three lineages. Small species with an adult body mass less than five tonnes are very rare, and small sauropod bones generally represent juveniles. Here we describe a new diminutive species of basal macronarian sauropod, Europasaurus holgeri gen. et sp. nov., and on the basis of bone histology we show it to have been a dwarf species. The fossils, including excellent skull material, come from Kimmeridgian marine beds of northern Germany, and record more than 11 individuals of sauropods 1.7 to 6.2 m in total body length. Morphological overlap between partial skeletons and isolated bones links all material to the same new taxon. Cortical histology of femora and tibiae indicates that size differences within the specimens are due to different ontogenetic stages, from juveniles to fully grown individuals. The little dinosaurs must have lived on one of the large islands around the Lower Saxony basin. Comparison with the long-bone histology of large-bodied sauropods suggests that the island dwarf species evolved through a decrease in growth rate from its larger ancestor.

HISTOLOGY OF ANKYLOSAUR OSTEODERMS: IMPLICATIONS FOR SYSTEMATICS AND FUNCTION

Abstract Here, we provide a comparative survey of the histology of postcranial dermal-armor osteoderms of ankylosaurs, including material of polacanthids (Polacanthus foxii, Gastonia sp.), ankylosaurids (e.g., Saichania chulsanensis, Pinacosaurus grangeri, Ankylosauridae indet.), and nodosaurids (e.g., Struthiosaurus austriacus, Nodosauridae indet.). Samples of osteoderm-bearing outgroups (Scelidosaurus harrisonii, phytosaurs, and crocodiles) as well as literature data on Stegosaurus stenops plates and spikes helped to elucidate histological evolution and character polarity. The complex histology of ankylosaur osteoderms seems to be of systematic value, as the sectioned osteoderms can be classified into three distinctive groups. All polacanthid osteoderms share a relatively generalized histology with a thickened, uniform cortex completely surrounding trabecular bone. Nodosaurid osteoderms only have an external cortex with the underlying internal spongiosa being much thicker and forming a rather flat base. Ankylosaurids have very thin osteoderms that were greatly strengthened by the incorporation of structural collagen fiber bundles, similar but not identical to Sharpeys fibers, into the thin primary cortex and, uniquely, the secondary bone tissue. Structural fibers are also abundant in nodosaurids but there are significant differences in arrangement and occurrence, being random in ankylosaurids but highly ordered in nodosaurids. In their cortex, the latter have two sets of 3D-orthogonal fibers rotated at 45° to each other. The combination of the greatly strengthened external cortex covering a thick cushion of cancellous bone thus serves to resist local penetration. Ankylosaurid and nodosaurid osteoderms are thus highly optimized towards a resistant yet light-weight armor.

Ichthyosauria: their diversity, distribution, and phylogeny

KurzfassungDie Ichthyosaurier waren die am stärksten an das Wasserleben angepaßten mesozoischen Reptilien. Die ältesten Ichthyosauier aus der oberen Untertrias (Spath) zeigen schon einen besonderen Satz von Merkmalen (sehr große Augen, verlängerte Schnauze, tief amphicoele Wirbel, zu Flossen umgebildete Beine), die mit ihrer vollaquatischen Lebensweise zusammenhängen, und sie waren vermutlich schon nicht mehr zum Landgang fähig. Die Schlüsselinovation der Ichthyosaurier war das Lebendgebären, das im Fossilbericht seit dem Ende des Anis belegt ist. Wichtige Evolutionstrends im Bewegungsapparat sind die zunehmende Modifikation des Flossenskelettes zu einem Mosaik von gleichartigen, abgeflachten Knochen und der Wechsel vom anguiliformen Schwimmen zum thunniformen Schwimmen bei den jurassischen und späteren Formen. Diese zeichnen sich durch eine Verkürzung des Körpers und die Entwicklung einer halbmondförmigen Schwanzflosse aus.Fast von Anfang an hatten die Ichthyosaurier eine weltweite Verbreitung, die sie auch bis zu ihrem Aussterben im Cenoman beibehielten. Die Vielfalt der Ichthyosaurier ist in der Mitteltrias am größten, als es sowohl rein piscivore Formen gab, als auch solche mit heterodontem Gebiß und mit Knackgebiß. Im Jura ist die Diversität im Lias am höchsten, in der Kreide reduziert sie sich aber auf eine Gattung (Platypterygius).Obwohl Schädelmerkmale die Ichthyosaurier als Diapsiden ausweisen, ist ihre genaue Stellung innerhalb dieser Gruppe unklar. Eine computergestützte kladistische Analyse der gut bekannten Gattungen erhellt dagegen die Verwandschaftsbeziehungen innerhalb der Gruppe: Die basalsten Formen sindGrippia undUtatsusaurus, gefolgt von den Mixosauridae (Mixosaurus undPhalarodon). Die Shastasauriden (Cymbospondylus, Shonisaurus, Besanosaurus) sind die fortschrittlichsten Triasformen und stellen die Schwestergruppe aller posttriassischer Ichthyosaurier dar. Letztere sind eindeutig monophyletisch und werden hier mit dem Namen Neoichthyosauria belegt.AbstractThe Ichthyosauria is the group of Mesozoic marine reptiles that was most highly adapted to the aquatic environment. The first ichthyosaurs from the upper Lower Triassic (Spathian) already show a suite of unique characters (very large eyes, elongate snout, deeply amphicoelous vertebrae, limb modified to fins) correlated with a fully aquatic existence and probably were unable to leave the water. The key evolutionary innovation was vivipary, giving birth to live young, which is documented by the fossil record since the end of the Anisian. Major evolutionary trends in the locomotor apparatus are the increasing modification of the fin skeleton to a mosaic of bones and the change from anguiliform swimming in the earliest forms to thunniform swimming in the Jurassic and later forms, as evidenced by the shortening of the body and the evolution of a semilunate tail fin.Almost from the beginning, ichthyosaurs had a cosmopolitan distribution which was retained until their extinction in the Cenomanian. Ichthyosaurian diversity is greatest in the Middle Triassic with piscivorous, heterodont, and durophagous forms. Jurassic diversity is greatest in the Liassic, declining to one genus (Platypterygius) in the Cretaceous.Although skull characters indicate that ichthyosaurs were diapsids, their exact position within Diapsida is unclear. A cladistic analysis of the well known genera clarifies relationships within the Ichthyosauria. Most basal areGrippia andUtatsusaurus, followed by the Mixosauridae (Mixosaurus andPhalarodon). The Shastasauridae (Cymbospondylus, Shonisaurus, Besanosaurus) are the most advanced Triassic forms and represent the sistergroup of all post-Triassic ichthyosaurs. These are clearly monophyletic and are termed here the Neoichthyosauria.

Shell bone histology indicates terrestrial palaeoecology of basal turtles

The palaeoecology of basal turtles from the Late Triassic was classically viewed as being semi-aquatic, similar to the lifestyle of modern snapping turtles. Lately, this view was questioned based on limb bone proportions, and a terrestrial palaeoecology was suggested for the turtle stem. Here, we present independent shell bone microstructural evidence for a terrestrial habitat of the oldest and basal most well-known turtles, i.e. the Upper Triassic Proterochersis robusta and Proganochelys quenstedti. Comparison of their shell bone histology with that of extant turtles preferring either aquatic habitats or terrestrial habitats clearly reveals congruence with terrestrial turtle taxa. Similarities in the shell bones of these turtles are a diploe structure with well-developed external and internal cortices, weak vascularization of the compact bone layers and a dense nature of the interior cancellous bone with overall short trabeculae. On the other hand, ‘aquatic’ turtles tend to reduce cortical bone layers, while increasing overall vascularization of the bone tissue. In contrast to the study of limb bone proportions, the present study is independent from the uncommon preservation of appendicular skeletal elements in fossil turtles, enabling the palaeoecological study of a much broader range of incompletely known turtle taxa in the fossil record.

Developmental plasticity in the life history of a prosauropod dinosaur.

Long-bone histology indicates that the most common early dinosaur, the prosauropod Plateosaurus engelhardti from the Upper Triassic of Central Europe, had variable life histories. Although Plateosaurus grew at the fast rates typical for dinosaurs, as indicated by fibrolamellar bone, qualitative (growth stop) and quantitative (growth-mark counts) features of its histology are poorly correlated with body size. Individual life histories of P. engelhardti were influenced by environmental factors, as in modern ectothermic reptiles, but not in mammals, birds, or other dinosaurs.

The norian Plateosaurus bonebeds of central Europe and their taphonomy

Abstract Plateosaurus is probably the single most common dinosaur in the Late Triassic, known from over 40 localities in the Norian Knollenmergel mudstones of central Europe. In addition, it is also one of the best known Triassic dinosaurs due to the discovery of many complete skeletons. These complete skeletons as well as the majority of all other material are actually derived from only a handful of localities, three of which are compared here to one another in detail. Frick (northern Switzerland), Trossingen (southwestern Germany), and Halberstadt (central Germany) are recognized as the result of the same taphonomic processes and can all be described with the term “Plateosaurus bonebed”. This type of bonebed is characterized by its large size, measured in thousands of square meters, the large number of incomplete and complete Plateosaurus skeletons, and the lack of other vertebrate fossils, with the exception of the primitive turtle Proganochelys. The uniformly upright carcass position precludes post mortem transport and strongly argues for in situ death. Shallow depressions in the thick floodplain muds of the Knollenmergel must have formed efficient traps for the heaviest animal at that time, Plateosaurus. The animals became mired and were scavenged upon by small theropods whose shed teeth are always present in Plateosaurus bondebeds. Miring explains the deep position of the feet and hindquarters and, in combination with scavenging and weathering, the bias towoards bones of the posterior body region. The lack of juvenile individuals is due to their lower foot pressure, which was below the threshold for becoming mired.

Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria)

Sauropods were the largest terrestrial tetrapods (>105 kg) in Earths history and grew at rates that rival those of extant mammals. Magyarosaurus dacus, a titanosaurian sauropod from the Upper Cretaceous (Maastrichtian) of Romania, is known exclusively from small individuals (<103 kg) and conflicts with the idea that all sauropods were massive. The diminutive M. dacus was a classical example of island dwarfism (phyletic nanism) in dinosaurs, but a recent study suggested that the small Romanian titanosaurs actually represent juveniles of a larger-bodied taxon. Here we present strong histological evidence that M. dacus was indeed a dwarf (phyletic nanoid). Bone histological analysis of an ontogenetic series of Magyarosaurus limb bones indicates that even the smallest Magyarosaurus specimens exhibit a bone microstructure identical to fully mature or old individuals of other sauropod taxa. Comparison of histologies with large-bodied sauropods suggests that Magyarosaurus had an extremely reduced growth rate, but had retained high basal metabolic rates typical for sauropods. The uniquely decreased growth rate and diminutive body size in Magyarosaurus were adaptations to life on a Cretaceous island and show that sauropod dinosaurs were not exempt from general ecological principles limiting body size.

Postcranial axial skeleton of Europasaurus holgeri (Dinosauria, Sauropoda) from the Upper Jurassic of Germany: implications for sauropod ontogeny and phylogenetic relationships of basal Macronaria

Neosauropods are well represented in the Late Jurassic fossil record, both in Laurasia and Gondwana. Among Macronaria, Europasaurus represents one of the most basal forms of this group. In addition to its systematic importance, Europasaurus is also the first unequivocal dwarf sauropod from which adult and juvenile material is available. Despite the abundance of sauropods in the fossil record, early juvenile specimens are rare, limiting knowledge about sauropod ontogeny. Therefore, the great amount of material of Europasaurus provides an excellent opportunity to improve our knowledge on the early evolution of Macronaria, as well as to shed light on some morphological changes through ontogeny. The postcranial axial skeleton of sauropods is extremely modified with respect to the anatomy observed in its ancestors, the ‘prosauropods’, proving to be one of the most informative regions of the body. Here we provide a detailed description of the axial skeleton of Europasaurus, including adult and juvenile elements, discussing its systematic and ontogenetic importance. We also analyse the phylogenetic position of Europasaurus through a cladistic analysis using TNT, which retrieves this taxon in a basal position among Camarasauromorpha. Additionally, the presence/absence of discrete characters and the comparison of juvenile elements with adult specimens allowed us to recognize different morphological ontogenetic stages (MOS). Whereas early stages lack derived characters (e.g. spinodiapophyseal lamina and prespinal lamina on dorsal vertebrae), all derived characters (including autapomorphies) are present in late immature specimens. Therefore, while late immature specimens provide the same phylogenetic signal as adult specimens of Europasaurus, more immature stages are recovered in a basal position among sauropods. Finally, we apply the MOS to other maturity criteria (e.g. neurocentral closure, sexual maturity) in a search for a wider definition of maturity.