Ryosuke Motani

Phylogeny of the Ichthyopterygia

ABSTRACT A phylogenetic hypothesis for ichthyopterygians is proposed, based on a cladistic analysis of 105 discrete osteological characters coded for 27 ingroup and five outgroup taxa. The monophyly of Ichthyopterygia is established; at least nine characters support its basal node. The group contains two basal species and a clade formed by the Ichthyosauria and Grippidia. All ichthyosaurs from post-Triassic strata form an unnamed clade, which in turn forms the group Euichthyosauria (new taxon) with four Late Triassic genera. The traditional Shastasauridae is poly-phyletic, with Cymbospondylus forming the sistergroup of a clade containing mixosaurs, shastasaurs, and euichthyo-saurs. In addition, Californosaurus and Toretocnemus are not shastasaurs but euichthyosaurs. Two new generic names, Macgowania and Isfjordosaurus, are proposed.

Large eyeballs in diving ichthyosaurs

The huge eyes of these extinct reptiles may have been useful deep in the ocean.

ON THE EVOLUTION AND HOMOLOGIES OF ICHTHYOPTERYGIAN FOREFINS

ABSTRACT The evolution of forefins within the Ichthyopterygia is re-evaluated based on the largest set of data available up to present, including recently reported Early Triassic forms. Three humeral morphotypes are recognized; one is plesiomorphic, and the other two characterize two of the major clades. The forefin elements of various ichthyopterygians are identified, based on osteogenetic sequences and topological arrangements. Both lines of evidence, ontogenetic and morphological, support a single conclusion that Stenopterygius, and all other ichthyopterygians belonging to the clade that survived into the Jurassic, lack digit I of the forelimb. Digits I and II are probably absent in Shastasaurus and closely related forms. The topology along the primary axis and digital arch is conservative among all ichthyopterygians examined, as expected from the underlying mechanism of limb development.

Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology

Comparison of eye structures between fossils and modern species suggests that Mesozoic archosaurs were active day and night. Variation in daily activity patterns facilitates temporal partitioning of habitat and resources among species. Knowledge of temporal niche partitioning in paleobiological systems has been limited by the difficulty of obtaining reliable information about activity patterns from fossils. On the basis of an analysis of scleral ring and orbit morphology in 33 archosaurs, including dinosaurs and pterosaurs, we show that the eyes of Mesozoic archosaurs were adapted to all major types of diel activity (that is, nocturnal, diurnal, and cathemeral) and provide concrete evidence of temporal niche partitioning in the Mesozoic. Similar to extant amniotes, flyers were predominantly diurnal; terrestrial predators, at least partially, nocturnal; and large herbivores, cathemeral. These similarities suggest that ecology drives the evolution of diel activity patterns.

The Evolution of Marine Reptiles

Reptiles have repeatedly invaded marine environments despite their physiological constraints as air breathers. Marine reptiles were especially successful in the Mesozoic as major predators in the sea. There were more than a dozen groups of marine reptiles in the Mesozoic, of which four had more than 30 genera, namely sauropterygians (including plesiosaurs), ichthyopterygians, mosasaurs, and sea turtles. Medium-sized groups, such as Thalattosauria and Thalattosuchia, had about ten genera, whereas small groups, such as Hupehsuchia and Pleurosauridae, consisted of only two genera or less. Sauropterygia and Ichthyopterygia were the two longest surviving lineages, with 185 and 160 million years of stratigraphic spans, respectively. Mesozoic marine reptiles explored many different swimming styles and diets. Their diet included fish, cephalopods, other vertebrates, and hard-shelled invertebrates, whereas no herbivore is known at this point. Sauropterygians and ichthyopterygians gave rise to cruising forms that probably invaded outer seas. Intermediate forms that led to the cruising species are known in Ichthyopterygia but not as much in Sauropterygia. Discovery of new fossils should eventually reduce the gap in the fossil record.

Ichthyosaurian relationships illuminated by new primitive skeletons from Japan

The Ichthyosauria is a group of reptiles with fish-shaped bodies from the Mesozoic (65–250 million years ago),. Their secondary adaptations to aquatic life have obscured their ancestral features,, and basal ichthyosaurs, which would be expected to retain these ancestral features (plesiomorphies), are poorly represented in the fossil record. As a result, their relationships to other amniotes have been controversial for over 180 years,. New specimens of Utatsusaurus hataii from the Lower Triassic (240 Myr ago) of Japan are the first basal ichthyosaurs to show detailed features for almost the entire skeleton, including previously unknown parts of the skull and pelvic girdle. Computer-assisted retrodeformation of fossil images shows that Utatsusaurus retained features of terrestrial amniotes in both the skull and the postcranial skeleton, such as the connection between the vertebral column and the pelvic girdle. Phylogenetic analyses indicate that ichthyosaurs belong in the Diapsida, but that, unlike the sauropterygians,, they are not included with the Sauria (the crown group containing lizards, crocodiles, birds and Sphenodon). Recent studies have reported that the addition of ichthyosaurs to the amniote data altered the relationships among basal saurians,, but no major clades were affected by the inclusion of ichthyosaurs in our analyses.

Scaling effects in caudal fin propulsion and the speed of ichthyosaurs.

Four unrelated groups of large cruising vertebrates (tunas, whales, lamnid sharks and parvipelvian ichthyosaurs) evolved tuna-shaped (thunniform) body plans. Stringent physical constraints, imposed by the surrounding fluids, are probably responsible for this example of evolutionary convergence. Here I present a mathematical model of swimming kinematics and fluid mechanics that specifies and quantifies such constraints, and test the model with empirical data. The test shows quantitatively that morphology, kinematics, and physiology indeed covary tightly in large cruisers. The model enables calculations of optimal cruising speed from external measurements, and also predicts that wide caudal fin spans, typical of thunniform swimmers, are necessary for large cruisers. This finding is contrary to a popular yet rather teleological view that thunniform tails were selected for their high aspect ratios that increased propulsive efficiency. I also show by calculation that Stenopterygius, a Jurassic ichthyosaur, probably had optimal cruising speeds and basal metabolic rates similar to living tunas.

A basal ichthyosauriform with a short snout from the Lower Triassic of China

The incompleteness of the fossil record obscures the origin of many of the more derived clades of vertebrates. One such group is the Ichthyopterygia, a clade of obligatory marine reptiles that appeared in the Early Triassic epoch, without any known intermediates. Here we describe a basal ichthyosauriform from the upper Lower Triassic (about 248 million years ago) of China, whose primitive skeleton indicates possible amphibious habits. It is smaller than ichthyopterygians and had unusually large flippers that probably allowed limited terrestrial locomotion. It also retained characteristics of terrestrial diapsid reptiles, including a short snout and body trunk. Unlike more-derived ichthyosauriforms, it was probably a suction feeder. The new species supports the sister-group relationships between ichthyosauriforms and Hupehsuchia, the two forming the Ichthyosauromorpha. Basal ichthyosauromorphs are known exclusively from south China, suggesting that the clade originated in the region, which formed a warm and humid tropical archipelago in the Early Triassic. The oldest unequivocal record of a sauropterygian is also from the same stratigraphic unit of the region.

Taxonomy and limb ontogeny of Chaohusaurus geishanensis (Ichthyosauria), with a note on the allometric equation

ABSTRACT Three of the six earliest ichthyosaurs, namely, Chaohusaurus geishanensis, Chensaurus chaoxianensis, and Chensaurus faciles, occur in the Lower Triassic (Spathian) of the Chaohu area, Anhui Province, China. A reexamination of the three holotypes and two referred specimens indicates that the three species share derived features; however, the characters originally used to distinguish among the three are either growth-related or absent. Measurements of the specimens suggest that they represent a growth series of a single species, Chaohusaurus geishanensis Young and Dong 1972. The forefin of this species shows a strongly positive allometry, leading to the unusually large forefin in the largest specimen (the holotype). The use of the standard allometric equation may be problematic when two structures compared start their developments asynchronically. However, the alternative equation of Schmalhausen is also problematic, and therefore more scrutiny is required. The lunate fin elements, which commonly o...

First record of Placodontoidea (Reptilia, Sauropterygia, Placodontia) from the Eastern Tethys

DA-YONG JIANG, RYOSUKE MOTANI, WEI-CHENG HAO, OLIVIER RIEPPEL, YUAN-LIN SUN, LARS SCHMITZ, and ZUO-YU SUN Department of Geology and Geological Museum, Peking University, Beijing 100871, P. R. China, djiang@pku.edu.cn; Department of Geology, University of California, Davis, California 95616-8605, U.S.A; Department of Geology, The Field Museum, Chicago, Illinois 60605-2496, U.S.A., rieppel@fieldmuseum.org.