Cranial Ontogeny of the Early Triassic Cynodont Thrinaxodon liorhinus

Ontogeny of the Early Triassic Cynodont Thrinaxodon liorhinus (Therapsida): Cranial Morphology

Authors:

Jasinoski et al

Abstract:

The cranial morphology of 68 Thrinaxodon liorhinus specimens, ranging in size from 30 – 96mm in basal skull length, is investigated using both qualitative and quantitative analyses. From this comprehensive survey, we determined that nine cranial features, including five in the temporal region, separated the sample into four ontogenetic stages. A bivariate analysis of 60 specimens indicated that the skull generally increased in size isometrically, with the exception of four regions. The orbit had negative allometry, a result consistent with other ontogenetic studies of tetrapods, whereas the length of the snout, palate, and temporal region showed positive allometry. The last trend had strong positive allometry indicating that during ontogeny the length of the sagittal crest increased at a much faster rate than the rest of the skull. The large number of changes in the temporal region of the skull of Thrinaxodon may indicate a greater development of the posterior fibres of the temporalis musculature from an early ontogenetic stage. For example, the posterior sagittal crest developed much earlier in ontogeny than the anterior crest that formed in adults, and bone was deposited dorsally creating a unified posterior sagittal crest rather than having a suture that spanned the entire depth of the skull roof. In combination with the isometric height of the zygomatic arch and the almost complete absence of the zygomatic arch angulation, these ontogenetic changes suggest that there was greater development of the temporalis relative to the masseter muscles, indicating a strong posterodorsal movement of the mandible in Thrinaxodon.

Anoxia, High Temperatures Effected Benthic Biotic Recovery During Scythian Triassic

High temperature and low oxygen perturbations drive contrasting benthic recovery dynamics following the end-Permian mass extinction

Authors:

Pietsch et al

Abstract:

The end-Permian mass extinction event was the greatest loss of biodiversity ever experienced on the planet. The event is thought to have been triggered by the initiation of the volcanic eruptions of the Siberian Traps. The five million year recovery interval that followed the extinction event was strongly influenced by the environmental effects of sustained volcanic eruptions including extreme temperature events and persistent global and regional oxygen minimum zones. The effects of these environmental perturbations on the paleoecological recovery of the benthic marine fauna were studied in two depositional units from the Southwestern United States representing two substages during the Early Triassic recovery. The Smithian Sinbad Limestone was influenced by high sea surface temperatures and contains a relatively high diversity fauna that exhibits extremely small body size. Gastropods that lived in this environment were size-limited, a possible result of metabolic stress due to extreme temperatures. These microgastropods were able to become dominate components of the benthic fauna by occupying niche space vacated by other taxa that were excluded by high temperatures. The Spathian Virgin Limestone shows evidence for low oxygen conditions. The resulting low diversity benthic fauna had a more ecologically complex community structure than the Smithian Sinbad Limestone including the occupation of epifaunal tiering space by crinoids. As the prevalence of aerobic facies increased through time, diversity, body size, and the complexity of faunal interactions also increased suggesting that low oxygen conditions were the limiting factor for the benthic recovery in that region. The differences in diversity and community structure between the two units highlight the importance of environmental and temporal differences in driving the recovery patterns of the benthic fauna following the extinction event. High temperature events and low oxygen conditions restricted the benthic fauna in different ways but both contributed to the delay in recovery from the end-Permian mass extinction.

Scythian Triassic Lystrosaurus Bonebed Origins and Implications

Origin and palaeoenvironmental significance of Lystrosaurus bonebeds in the earliest Triassic Karoo Basin, South Africa

Authors:

1. Pia A. Viglietti (a)
2. Roger M.H. Smith (b)
3. John S. Compton (a)

Affiliations:

a. Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch, 7701, (+ 27733760065)

b. Iziko South African Museum of Cape Town, P.O. Box 61, Cape Town, 8000 South Africa

Abstract:

Earth experienced its most devastating extinction event at the end of the Permian period 251 million years ago (Ma). Despite an estimated 75 to 90% loss of species globally in both marine and terrestrial realms across the Permian–Triassic Boundary (PTB), around 20% of the terrestrial tetrapod genera in southwestern Gondwana survived and were immediately joined by a number of immigrant taxa to occupy most of the vacant niches of the earliest Triassic. Preserved in the Karoo Basin of South Africa is an almost continuous stratigraphic record of terrestrial sedimentation through the PTB that hosts a fossil record of ecosystem collapse, survivorship and recovery. The adaptation of the mammal like reptiles (therapsids) of the Lystrosaurus Assemblage Zone to a highly seasonal, potentially drought prone semi-arid earliest Triassic Karoo Basin is associated with changes in modes of fossilisation. Isolated dicynodont skulls and postcranial elements are commonly found in the latest Permian. However, in the earliest Triassic the dicynodonts occur as articulated, curled-up skeletons and multi-individual monotaxic bonebeds. Lack of epiphyses and relatively small skull length confirm that the bonebeds comprise several subadult Lystrosaurus declivis (L. declivis) carcasses. No significant evidence for hydraulic bone concentration along with clusters of ribs in life position point to complete carcasses being present at the site of death, and suggest that animals behaviourally congregated before perishing together. The bonebeds are hosted by an 8 m thick horizon of floodplain mudrocks in the lower Katberg Formation named the Lystrosaurus abundant zone. The bonebed horizon is overlain by sand-filled mud cracks capped by coarse sediments indicative of rapid deposition during waning floods. Stable isotope analyses of pedogenic and early diagenetic calcite nodules in association with the bonebeds yield average δ13C values ranging from − 9.5 to − 5‰ and δ18O values of 13.5 to 16‰, respectively. These isotope values support a seasonally cold, semi-arid climate at high latitude (~ 55ºS). The presence of vertebrate burrow casts on bonebed horizons and evidence of shelter sharing suggests tetrapods were attempting to escape extreme climatic conditions. Aggregation behaviour of subadult Lystrosaurus during unusual cold snaps may best explain the origin of bonebed assemblages.

Dinosaurs Ancestors Evolved Right After the Permian Extinction

The oldest evidence of the dinosaur lineage—fossilized tracks—is described this week in Proceedings of the Royal Society B. Just one or two million years after the massive Permian-Triassic extinction, an animal smaller than a house cat walked across fine mud in what is now Poland. This fossilized trackway places the very closest relatives of dinosaurs on Earth about 250 million years ago—5 to 9 million years earlier than previously described fossilized skeletal material has indicated. The paper also described the 246-million-year-old Sphingopus footprints, the oldest evidence of a bipedal and large-bodied dinosaur.

"We see the closest dinosaur cousins immediately after the worst mass extinction," says Stephen Brusatte, a graduate student affiliated with the Division of Paleontology at the American Museum of Natural History. "The biggest crisis in the history of life also created one of the greatest opportunities in the history of life by emptying the landscape and making it possible for dinosaurs to evolve."

The new paper analyzes three sets of footprints from three different sites in the Holy Cross Mountains of central Poland. The sites, all quarries within a 25-mile radius of each other, are windows into three ecosystems because they represent different times periods. The Stryczowice trackway is the oldest at 250 million years. The Baranów trackway is the most recent at 246 million years of age while the Wióry trackway is sandwiched in time between the others.

Because footprints are only an imprint of a small part of the skeleton, identification of trackmakers is often tricky. Luckily, dinosaurs have a very distinctive gait, especially when compared to their diapsid relatives (the evolutionary group that includes birds, reptiles, and extinct lineages) like crocodiles and lizards. While lizards and crocodiles have a splayed walking style, dinosaurs place their two feet closer together. The footprints at all three Polish sites show this feature as well as indisputable dinosaur-like features, including three prominent central toes and reduced outer two toes, a parallel alignment of these three digits (a bunched foot), and a straight back edge of footprints, additional evidence of a dinosaur-like simple hinged ankle.

Because all of these features are seen in footprints at the oldest site, Brusatte and colleagues conclude that the Stryczowice prints—which are only a few centimeters in length—are the oldest evidence of the dinosaur lineage. These dinosaurs, though, are considered "stem dinosaurs," or the immediate relatives of dinosaurs not part of the slightly more derived clade that technically defines dinosaurs. Also, this animal did walk on all four limbs, an abnormal posture for early dinosaurs and their close relatives, although it appears that its forelimbs were already being reduced to more dinosaur-like proportions since the footprints overstep handprints.

The Baranów and Wióry trackways show changes early in the evolutionary history of dinosaurs. Wióry at 248 million years ago shows slight diversification in the types of tracks, but all tracks remain quadrupedal. Footprints from Baranów at 246 million years ago, however, may be the earliest evidence of moderately large-bodied and bipedal true dinosaurs. These tracks, which are called Sphingopus, are 15 centimeters long.

Less than no time.