Thomas H. Rich

The evolution of tribospheny and the antiquity of mammalian clades

The evolution of tribosphenic molars is a key innovation in the history of Mammalia. Tribospheny allows for both shearing and grinding occlusal functions. Marsupials and placentals are advanced tribosphenic mammals (i.e., Theria) that show additional modifications of the tribosphenic dentition including loss of the distal metacristid and development of double-rank postvallum/prevallid shear. The recent discovery of Eomaia [Nature 416 (2002) 816], regarded as the oldest eutherian mammal, implies that the marsupial-placental split is at least 125 million years old. The conventional scenario for the evolution of tribosphenic and therian mammals hypothesizes that each group evolved once, in the northern hemisphere, and is based on a predominantly Laurasian fossil record. With the recent discovery of the oldest tribosphenic mammal (Ambondro) from the Mesozoic of Gondwana, Flynn et al. [Nature 401 (1999) 57] suggested that tribospheny evolved in Gondwana rather than in Laurasia. Luo et al. [Nature 409 (2001) 53; Acta Palaeontol. Pol. 47 (2002) 1] argued for independent origins of tribospheny in northern (Boreosphenida) and southern (Australosphenida) hemisphere clades, with the latter including Ambondro, ausktribosphenids, and monotremes. Here, we present cladistic evidence for a single origin of tribosphenic molars. Further, Ambondro may be a stem eutherian, making the split between marsupials and placentals at least 167 m.y. old. To test this hypothesis, we used the relaxed molecular clock approach of Thorne/Kishino with amino acid data sets for BRCA1 [J. Mammal. Evol. 8 (2001) 239] and the IGF2 receptor [Mammal. Genome 12 (2001) 513]. Point estimates for the marsupial-placental split were 182-190 million years based on BRCA1 and 185-187 million years based on the IGF2 receptor. These estimates are fully compatible with the results of our cladistic analyses.

The oldest platypus and its bearing on divergence timing of the platypus and echidna clades

Monotremes have left a poor fossil record, and paleontology has been virtually mute during two decades of discussion about molecular clock estimates of the timing of divergence between the platypus and echidna clades. We describe evidence from high-resolution x-ray computed tomography indicating that Teinolophos, an Early Cretaceous fossil from Australias Flat Rocks locality (121–112.5 Ma), lies within the crown clade Monotremata, as a basal platypus. Strict molecular clock estimates of the divergence between platypus and echidnas range from 17 to 80 Ma, but Teinolophos suggests that the two monotreme clades were already distinct in the Early Cretaceous, and that their divergence may predate even the oldest strict molecular estimates by at least 50%. We generated relaxed molecular clock models using three different data sets, but only one yielded a date overlapping with the age of Teinolophos. Morphology suggests that Teinolophos is a platypus in both phylogenetic and ecological aspects, and tends to contradict the popular view of rapid Cenozoic monotreme diversification. Whereas the monotreme fossil record is still sparse and open to interpretation, the new data are consistent with much slower ecological, morphological, and taxonomic diversification rates for monotremes than in their sister taxon, the therian mammals. This alternative view of a deep geological history for monotremes suggests that rate heterogeneities may have affected mammalian evolution in such a way as to defeat strict molecular clock models and to challenge even relaxed molecular clock models when applied to mammalian history at a deep temporal scale.

A large Cretaceous theropod from Patagonia, Argentina, and the evolution of carcharodontosaurids.

The Cretaceous Carcharodontosauridae is the latest clade of carnosaurs, including the largest predatory dinosaurs yet recorded. Albeit spectacular for their size, the skeletal anatomy of these theropods remains poorly-known, and their diversity was until recently restricted to two Cenomanian species: the highly derived Giganotosaurus carolinii, from southern South America, and the incompletely known Carcharodontosaurus saharicus, from northern Africa. Here we describe an older and basal member of the group, Tyrannotitan chubutensis gen. et sp. nov., from Aptian strata of Patagonia, Argentina. The new taxon gives new insights into the systematics and evolution of carcharodontosaurids and offers a better understanding of the evolution of Southern theropod faunas. We suggest that carcharodontosaurids radiated in Gondwana sharing with spinosaurids the role of top-predators until their extinction in Cenomanian–Turonian times. During this interval, the diplodocoid sauropods and giant titanosaurians went extinct (probably as part of a global-scale crisis), and the smaller abelisaurid theropods took dominance, reigning until the end of the Cretaceous. Electronic Supplementary Material is available.

Chubutemys, a New Eucryptodiran Turtle from the Early Cretaceous of Argentina, and the Relationships of the Meiolaniidae

Abstract Chubutemys copelloi is the oldest nonmarine cryptodire from South America represented by a skull. The skull and associated postcranial fragments are from the Aptian Cerro Costano Member of the Cerro Barcino Formation of Chubut, Argentina. Chubutemys has a processus trochlearis oticum, showing that it is a cryptodire, and an enclosed canalis caroticus internus extending to the posterior margin of the pterygoid, showing that it is a eucryptodire. The skull of Chubutemys is similar to that of other primitive eucryptodires, particularly Dracochelys, but also to Hangaiemys, Judithemys, Sinemys, and Ordosemys. Chubutemys differs from all these, however, in possessing a solidly roofed skull, formed by long, wide parietals, rather than a posterior emargination. Chubutemys also differs from these taxa in having no cheek emargination. A phylogenetic analysis using PAUP* analyzed 104 parsimony-informative characters resolving into one most parsimonious cladogram of 224 steps, a consistency index of 0.55, an...

Evidence for Low Temperatures and Biologic Diversity in Cretaceous High Latitudes of Australia

A diverse terrestial biota inhabited polar latitudes during the Cretacous, 105 to 130 Ma (million years ago), along what is now the southeast coast of Australia This biota, from rocks in the Otway and Strzelecki groups, cnsisted of more than 150 taxa of vertebrates, invertebrates, and plants. Oxygen isotope ratios in diagenetic calcite suggest that mean annual temperatures were most likely less than 5�C, and rings present in the fossil araucarian-podocarp-ginko woods indicate saonality. Southeastern Austalia, thus, seems to have had a cool, seasonal, nontropical climate. Dinosaurs that have been recovered are up to five species and three genera of hypsilophodontids, all of which were endemic, and three species of theropods. The occurrence of Allosaurus sp. and labyrinthodont amphibians, which had become extinct elsewhere in the Jurassic, indicate that isolation may have allowed extended surival of these taxa in Australia. In that dinosaurs coped with high latitude for at least 65 million years [Valaginian to Albian time in Australia and Campanian to Maastrictian time (80 to 65 Ma) in Alaska] suggests that cold and darkness may not have been prime factors bringing about the extinction of dinosaurs and some other groups at the Cretaceous-Tertiary boundary, unless they were prolonged.

Theropod fauna from southern Australia indicates high polar diversity and climate-driven dinosaur provinciality.

The Early Cretaceous fauna of Victoria, Australia, provides unique data on the composition of high latitude southern hemisphere dinosaurs. We describe and review theropod dinosaur postcranial remains from the Aptian–Albian Otway and Strzelecki groups, based on at least 37 isolated bones, and more than 90 teeth from the Flat Rocks locality. Several specimens of medium- and large-bodied individuals (estimated up to ∼8.5 metres long) represent allosauroids. Tyrannosauroids are represented by elements indicating medium body sizes (∼3 metres long), likely including the holotype femur of Timimus hermani, and a single cervical vertebra represents a juvenile spinosaurid. Single specimens representing medium- and small-bodied theropods may be referrable to Ceratosauria, Ornithomimosauria, a basal coelurosaur, and at least three taxa within Maniraptora. Thus, nine theropod taxa may have been present. Alternatively, four distinct dorsal vertebrae indicate a minimum of four taxa. However, because most taxa are known from single bones, it is likely that small-bodied theropod diversity remains underestimated. The high abundance of allosauroids and basal coelurosaurs (including tyrannosauroids and possibly ornithomimosaurs), and the relative rarity of ceratosaurs, is strikingly dissimilar to penecontemporaneous dinosaur faunas of Africa and South America, which represent an arid, lower-latitude biome. Similarities between dinosaur faunas of Victoria and the northern continents concern the proportional representatation of higher clades, and may result from the prevailing temperate–polar climate of Australia, especially at high latitudes in Victoria, which is similar to the predominant warm–temperate climate of Laurasia, but distinct from the arid climate zone that covered extensive areas of Gondwana. Most dinosaur groups probably attained a near-cosmopolitan distribution in the Jurassic, prior to fragmentation of the Pangaean supercontinent, and some aspects of the hallmark ‘Gondwanan’ fauna of South America and Africa may therefore reflect climate-driven provinciality, not vicariant evolution driven by continental fragmentation. However, vicariance may still be detected at lower phylogenetic levels.

Polar dinosaur bone histology

ABSTRACT We report on the bone microstructure of a hypsilophodont and an ornithomimosaur from the Early Cretaceous, Otway Group of Dinosaur Cove in south-eastern Australia, which at the time lay well within the Antarctic Circle. Although subjected to the same environmental conditions, the dinosaurs exhibit different bone histology. The hypsilophodontid shows a continuous rate of bone deposition, while the ornithomimosaur has a cyclical pattern of bone formation. We interpret these varying patterns of bone microstructure as a reflection of different growth strategies of these dinosaurs.

Dinosaur remains from the Lower Cretaceous of the Chubut Group, Argentina

Abstract Some fragmentary dinosaur remains from the uppermost Hauterivian–Barremian La Paloma Member of the Cerro Barcino Formation of northern Chubut, Argentina are described. Together with the fauna from the La Amarga Formation of Neuquen, Argentina, this material represent the oldest known Cretaceous dinosaur fauna from South America. It includes remains of a probable titanosaurian sauropod and an abelisaurian, probable abelisaurid, theropod. This is the oldest record of titanosaurs from South America and the oldest record of abelisaurids globally. The presence of both small-bodied noasaurids in the La Amarga Formation and medium-sized–large abelisaurids in the Cerro Barcino Formation, in the middle Lower Cretaceous of Argentina, indicates that abelisaurian diversification began well before the final fragmentation of Gondwana. Whereas this explains the wide distribution of abelisaurs in the Late Cretaceous, reasons other than vicariance must be invoked for their apparent absence in the ‘middle’ Cretaceous of Africa.

A new triconodontan (Mammalia) from the Jurassic of China

ABSTRACT Klamelia zhaopengi gen. et sp. nov., a new triconodontan from the Middle or Late Jurassic of northwestern China, has close affinities with Gobiconodon borissiaki Trofimov, 1978, and Guchinodon hoburensis Trofimov, 1978, from the Early Cretaceous of Mongolia. All three species are characterised by a foreshortened anterior region of the mandible and a reduced number of antemolars as compared with all other amphilestid triconodontans. They are therefore here united as the new subfamily Gobiconodontinae. Another member of the Gobiconodontinae may be an undescribed amphilestid from the Early Cretaceous Cloverly Formation of Montana, U.S.A. If so, the presently known geographic and stratigraphic distribution of these forms suggests they were confined to the Middle or Late Jurassic and Early Cretaceous of eastern Asia and western North America.

A Southern Tyrant Reptile

Fossil evidence indicates that tyrannosaurs also inhabited the southern continents in the late Early Cretaceous period. Tyrannosaurids monopolized the apex predator niche in latest Cretaceous Laurasia. Unfortunately, the preceding 100-million-year tyrannosauroid lineage is poorly documented, and its fossil record is restricted to the northern continents. We report an Australian tyrannosauroid, represented by a pubis from the late Early Cretaceous of Victoria. This demonstrates that these extraordinarily successful predators were not restricted to Laurasia. The advanced morphology and small size of the specimen shows that tyrannosauroids with the characteristic short arms and robust skulls probably had a global distribution in the Early Cretaceous. Thus, a potentially cosmopolitan grade of small tyrannosauroids with a tyrannosaurid-like body plan preceded the Late Cretaceous rise of the colossal tyrannosaurids.