Tiarajudens eccentricus and Anomocephalus africanus, two bizarre anomodonts (Synapsida, Therapsida) with dental occlusion from the Permian of Gondwana

Implications of Timanian thrust systems in the Barents Sea and Svalbard on using paleontological constraints for plate tectonics reconstructions

Background

The Svalbard Archipelago is commonly believed to have been located at comparable latitude and, possibly, to have been attached to Laurentia in the early Paleozoic (500-420 Ma) based on trilobite assemblage similarities. Trilobite assemblage differences and lack of mixing between Laurentia-Svalbard and Baltica were further used to propose that these continents were separated by the Iapetus Ocean at that time. However, recent structural correlation of Timanian (650-550 Ma) thrust systems throughout the Barents Sea show that Svalbard was already attached to Baltica in the latest Neoproterozoic and remained so during the Phanerozoic.

Methods

The present study presents a new interpretation of seismic reflection data from the DISKOS database, which were tied to nearby exploration wells. The study uses recently acquired knowledge of the seismic facies of intensely deformed pre-Caledonian rocks and principles of seismic stratigraphy to interpret the data.

Results

The present study reconciles the proximity of Svalbard and Laurentia with the early accretion of Svalbard to Baltica in the latest Neoproterozoic. It also describes the influence of Timanian thrust systems on paleoenvironments and possible effects on trilobite assemblages, e.g., the lack of mixing between those of Laurentia-Svalbard and Baltica.

Conclusions

The identification of elongate, emerged topographic highs in the Barents Sea and Svalbard in the late Neoproterozoic-early Paleozoic suggest that paleontological constraints should be considered with greater care when discussing continent separation since thrust systems may act as major faunal barriers within a single tectonic plate. Other factors to consider when discussing plate separation include paleoclimatic belts.

The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I-Forelimb

This paper is the first in a two-part series that charts the evolution of appendicular musculature along the mammalian stem lineage, drawing upon the exceptional fossil record of extinct synapsids. Here, attention is focused on muscles of the forelimb. Understanding forelimb muscular anatomy in extinct synapsids, and how this changed on the line to mammals, can provide important perspective for interpreting skeletal and functional evolution in this lineage, and how the diversity of forelimb functions in extant mammals arose. This study surveyed the osteological evidence for muscular attachments in extinct mammalian and nonmammalian synapsids, two extinct amniote outgroups, and a large selection of extant mammals, saurians, and salamanders. Observations were integrated into an explicit phylogenetic framework, comprising 73 character-state complexes covering all muscles crossing the shoulder, elbow, and wrist joints. These were coded for 33 operational taxonomic units spanning >330 Ma of tetrapod evolution, and ancestral state reconstruction was used to evaluate the sequence of muscular evolution along the stem lineage from Amniota to Theria. In addition to producing a comprehensive documentation of osteological evidence for muscle attachments in extinct synapsids, this work has clarified homology hypotheses across disparate taxa and helped resolve competing hypotheses of muscular anatomy in extinct species. The evolutionary history of mammalian forelimb musculature was a complex and nonlinear narrative, punctuated by multiple instances of convergence and concentrated phases of anatomical transformation. More broadly, this study highlights the great insight that a fossil-based perspective can provide for understanding the assembly of novel body plans.

At the root of the mammalian mind: The sensory organs, brain and behavior of pre-mammalian synapsids

All modern mammals are descendants of the paraphyletic non-mammaliaform Synapsida, colloquially referred to as the "mammal-like reptiles." It has long been assumed that these mammalian ancestors were essentially reptile-like in their morphology, biology, and behavior, i.e., they had a small brain, displayed simple behavior, and their sensory organs were unrefined compared to those of modern mammals. Recent works have, however, revealed that neurological, sensory, and behavioral traits previously considered typically mammalian, such as whiskers, enhanced olfaction, nocturnality, parental care, and complex social interactions evolved before the origin of Mammaliaformes, among the early-diverging "mammal-like reptiles." In contrast, an enlarged brain did not evolve immediately after the origin of mammaliaforms. As such, in terms of paleoneurology, the last "mammal-like reptiles" were not significantly different from the earliest mammaliaforms. The abundant data and literature published in the last 10 years no longer supports the "three pulses" scenario of synapsid brain evolution proposed by Rowe and colleagues in 2011, but supports the new "outside-in" model of Rodrigues and colleagues proposed in 2018, instead. As Mesozoic reptiles were becoming the dominant taxa within terrestrial ecosystems, synapsids gradually adapted to smaller body sizes and nocturnality. This resulted in a sensory revolution in synapsids as olfaction, audition, and somatosensation compensated for the loss of visual cues. This altered sensory input is aligned with changes in the brain, the most significant of which was an increase in relative brain size.

The Brazilian Pareiasaur Revisited

Provelosaurus americanus is the only known representative of the Pareiasauria in the Americas. This mid-size pareiasaur from the Rio do Rasto Formation of southern Brazil has been traditionally considered to be related to smaller forms from the South African Karoo known as the “dwarf pareiasaurs” of Lopingian age. P. americanus, however, co-existed with dinocephalians, which indicates a Guadalupian age. New fossils provide a nearly complete osteological account that forms the basis for a revised diagnosis and a test of phylogenetic relationships of P. americanus. Our results offer further support to the hypothesis that the Brazilian pareiasaur is the sister taxon of the Karoo “dwarf pareiasaurs,” being the earliest member of this group and one of the oldest pareiasaurs known so far. This is reinforced by a radiometric dating of the Morro Pelado Member of the Rio do Rasto Formation. In addition, the association of four individuals of various ontogenetic stages at the type locality supports some degree of social behavior in P. americanus.

The evolution of the synapsid tusk: insights from dicynodont therapsid tusk histology

The mammalian tusk is a unique and extreme morphotype among modern vertebrate dentitions. Tusks-defined here as ever-growing incisors or canines composed of dentine-evolved independently multiple times within mammals yet have not evolved in other extant vertebrates. This suggests that there is a feature specific to mammals that facilitates the evolution of this specialized dentition. To investigate what may underpin the evolution of tusks, we histologically sampled the tusks of dicynodont therapsids: the earliest iteration of tusk evolution and the only non-mammalian synapsid clade to have acquired such a dentition. We studied the tissue composition, attachment tissues, development and replacement in 10 dicynodont taxa and show multiple developmental pathways for the adult dentitions of dicynodont tusks and tusk-like caniniforms. In a phylogenetic context, these developmental pathways reveal an evolutionary scenario for the acquisition of an ever-growing tusk-an event that occurred convergently, but only in derived members of our sample. We propose that the evolution of an ever-growing dentition, such as a tusk, is predicated on the evolution of significantly reduced tooth replacement and a permanent soft-tissue attachment. Both of these features are fixed in the dentitions of crown-group mammals, which helps to explain why tusks are restricted to this clade among extant vertebrates.

A new Heterodontosaurus specimen elucidates the unique ventilatory macroevolution of ornithischian dinosaurs

Ornithischian dinosaurs were ecologically prominent herbivores of the Mesozoic Era that achieved a global distribution by the onset of the Cretaceous. The ornithischian body plan is aberrant relative to other ornithodiran clades, and crucial details of their early evolution remain obscure. We present a new, fully articulated skeleton of the early branching ornithischian Heterodontosaurus tucki. Phase-contrast enhanced synchrotron data of this new specimen reveal a suite of novel postcranial features unknown in any other ornithischian, with implications for the early evolution of the group. These features include a large, anteriorly projecting sternum; bizarre, paddle-shaped sternal ribs; and a full gastral basket – the first recovered in Ornithischia. These unusual anatomical traits provide key information on the evolution of the ornithischian body plan and suggest functional shifts in the ventilatory apparatus occurred close to the base of the clade. We complement these anatomical data with a quantitative analysis of ornithischian pelvic architecture, which allows us to make a specific, stepwise hypothesis for their ventilatory evolution.

The fossilised skeletons of long extinct dinosaurs are more than just stones. By comparing these remains to their living relatives such as birds and crocodiles, palaeontologists can reveal how dinosaurs grew, moved, ate and socialised. Previous research indicates that dinosaurs were likely warm-blooded and also more active than modern reptiles. This means they would have required breathing mechanisms capable of supplying enough oxygen to allow these elevated activity levels.

So far, much of our insight into dinosaur breathing biology has been biased towards dinosaur species more closely related to modern birds, such as Tyrannosaurus rex, as well as the long-necked sauropods. The group of herbivorous dinosaurs known as ornithischians, which include animals with head ornamentation, spikes and heavy body armour, like that found in Triceratops and Stegosaurus, have often been overlooked. As a result, there are still significant gaps in ornithischian biology, especially in understanding how they breathed.

Radermacher et al. used high-powered X-rays to study a new specimen of the most primitive ornithischian dinosaur, Heterodontosaurus tucki, and discovered that this South African dinosaur has bones researchers did not know existed in this species. These include bones that are part of the breathing system of extant reptiles and birds, including toothpick-shaped bones called gastralia, paired sternal bones and sternal ribs shaped like tennis rackets.

Together, these new pieces of anatomy form a complicated chest skeleton with a large range of motion that would have allowed the body to expand during breathing cycles. But this increased motion of the chest was only possible in more primitive ornithischians. More advanced species lost much of the anatomy that made this motion possible. Radermacher et al. show that while the chest was simpler in advanced species, their pelvis was more specialised and likely played a role in breathing as it does in modern crocodiles.

This new discovery could inform the work of biologists who study the respiratory diversity of both living and extinct species. Differences in breathing strategies might be one of the underlying reasons that some lineages of animals go extinct. It could explain why some species do better than others under stressful conditions, like when the climate is warmer or has less oxygen.

The evolution of anteriorly directed molar occlusion in mammals

In non-mammalian synapsids and early mammals, evolutionary transformations in the feeding and hearing apparatuses are posited to have been prerequisites for the radiation of extant mammals. Unlike most vertebrates, including many early synapsids, mammals have precise dental occlusion, a lower jaw composed of one bone, and middle ear ossicles derived from ancestral jaw bones. We illuminate a related functional transition: therian mammals (eutherians and metatherians) evolved anteriorly directed chewing strokes, which are absent in other synapsid lineages. Anteriorly directed jaw movement during occlusion necessitates anteriorly directed muscle force vectors, and we posit that a shift in muscle orientation is reflected in the fossil record by the evolutionary appearance of a posteriorly positioned angular process in cladotherians (therians and their close kin). Anteriorly directed occlusion might have been absent in earlier synapsids because of the presence of attached middle ear elements in the posterior region of the jaw that prohibited the posterior insertion of jaw musculature. These changes to the masticatory apparatus in cladotherians are likely to have permitted the evolution of novel masticatory movements, including grinding in both the anterior and medial directions (e.g. rodents and ungulates, respectively). Thus, this evolutionary transition might have been a crucial prerequisite for the dietary diversification of therians.

Morphology of the temporal skull region in tetrapods: research history, functional explanations, and a new comprehensive classification scheme

The morphology of the temporal region in the tetrapod skull traditionally has been a widely discussed feature of vertebrate anatomy. The evolution of different temporal openings in Amniota (mammals, birds, and reptiles), Lissamphibia (frogs, salamanders, and caecilians), and several extinct tetrapod groups has sparked debates on the phylogenetic, developmental, and functional background of this region in the tetrapod skull. This led most famously to the erection of different amniote taxa based on the number and position of temporal fenestrae in their skulls. However, most of these taxa are no longer recognised to represent natural groupings and the morphology of the temporal region is not necessarily an adequate trait for use in the reconstruction of amniote phylogenies. Yet, new fossil finds, most notably of parareptiles and stem-turtles, as well as modern embryological and biomechanical studies continue to provide new insights into the morphological diversity of the temporal region. Here, we introduce a novel comprehensive classification scheme for the various temporal morphotypes in all Tetrapoda that is independent of phylogeny and previous terminology and may facilitate morphological comparisons in future studies. We then review the history of research on the temporal region in the tetrapod skull. We document how, from the early 19th century with the first recognition of differences in the temporal region to the first proposals of phylogenetic relationships and their assessment over the centuries, the phylogenetic perspective on the temporal region has developed, and we highlight the controversies that still remain. We also compare the different functional and developmental drivers proposed for the observed morphological diversity and how the effects of internal and external factors on the structure of the tetrapod skull have been interpreted.

Histological and developmental insights into the herbivorous dentition of tapinocephalid therapsids

Tapinocephalids were one of the earliest therapsid clades to evolve herbivory. In acquiring derived tooth-to-tooth occlusion by means of an exaggerated heel and talon crown morphology, members of this family have long been considered herbivorous, yet little work has been done to describe their dentition. Given the early occurrence of this clade and their acquisition of a dentition with several derived features, tapinocephalids serve as an important clade in understanding adaptations to herbivory as well as macroevolutionary patterns of dental trait acquisition. Here we describe the histology of tapinocephalid jaws and incisors to assess adaptations to herbivory. Our results yield new dental characters for tapinocephalids including a peculiar enamel structure and reduced enamel deposition on the occlusal surface. These traits are convergent with other specialized herbivorous dentitions like those found in ornithischian dinosaurs and ungulates. Furthermore, these results demonstrate that while acquiring some specializations, tapinocephalids also retained plesiomorphic traits like alternate, continuous replacement. We interpret these findings as an example of how different combinations of traits can facilitate a derived and specialized dentition and then discuss their implications in the acquisition of a mammal-like dentition.

Tetrapod tracks in Permo-Triassic eolian beds of southern Brazil (Paraná Basin)

Tetrapod tracks in eolianites are widespread in the fossil record since the late Paleozoic. Among these ichnofaunas, the ichnogenus Chelichnus is the most representative of the Permian tetrapod ichnological record of eolian deposits of Europe, North America and South America, where the Chelichnus Ichnofacies often occurs. In this contribution, we describe five sets of tracks (one of which is preserved in cross-section), representing the first occurrence of Dicynodontipus and Chelichnus in the “Pirambóia Formation” of southern Brazil. This unit represents a humid desert in southwestern Pangea and its lower and upper contacts lead us to consider its age as Lopingian–Induan. The five sets of tracks studied were compared with several ichnotaxa and body fossils with appendicular elements preserved, allowing us to attribute these tracks to dicynodonts and other indeterminate therapsids. Even though the “Pirambóia Formation” track record is sparse and sub-optimally preserved, it is an important key to better understand the occupation of arid environments by tetrapods across the Permo–Triassic boundary.

Evolution of facial innervation in anomodont therapsids (Synapsida): Insights from X-ray computerized microtomography

Anomodontia was the most successful herbivorous clade of the mammalian stem lineage (non-mammalian synapsids) during the late Permian and Early Triassic. Among anomodonts, Dicynodontia stands apart because of the presence of an osseous beak that shows evidence of the insertion of a cornified sheath, the ramphotheca. In this study, fourteen anomodont specimens were microCT-scanned and their trigeminal canals reconstructed digitally to understand the origin and evolution of trigeminal nerve innervation of the ramphotheca. We show that the pattern of innervation of the anomodont "beak" is more similar to that in chelonians (the nasopalatine branch is enlarged and innervates the premaxillary part of the ramphotheca) than in birds (where the nasopalatine and maxillary branches play minor roles). The nasopalatine branch is noticeably enlarged in the beak-less basal anomodont Patranomodon, suggesting that this could be an anomodont or chainosaur synapomorphy. Our analyses suggest that the presence or absence of tusks and postcanine teeth are often accompanied by corresponding variations of the rami innervating the caniniform process and the alveolar region, respectively. The degree of ossification of the canal for the nasal ramus of the ophthalmic branch also appears to correlate with the presence of a nasal boss. The nasopalatine canal is absent from the premaxilla in the Bidentalia as they uniquely show a large plexus formed by the internal nasal branch of the maxillary canal instead. The elongated shape of this plexus in Lystrosaurus supports the hypothesis that the rostrum evolved as an elongation of the subnarial region of the snout. Finally, the atrophied and variable aspect of the trigeminal canals in Myosaurus supports the hypothesis that this genus had a reduced upper ramphotheca.

Cranial Bosses of Choerosaurus dejageri (Therapsida, Therocephalia): Earliest Evidence of Cranial Display Structures in Eutheriodonts

Choerosaurus dejageri, a non-mammalian eutheriodont therapsid from the South African late Permian (~259 Ma), has conspicuous hemispheric cranial bosses on the maxilla and the mandible. These bosses, the earliest of this nature in a eutheriodont, potentially make C. dejageri a key species for understanding the evolutionary origins of sexually selective behaviours (intraspecific competition, ritualized sexual and intimidation displays) associated with cranial outgrowths at the root of the clade that eventually led to extant mammals. Comparison with the tapinocephalid dinocephalian Moschops capensis, a therapsid in which head butting is strongly supported, shows that the delicate structure of the cranial bosses and the gracile structure of the skull of Choerosaurus would be more suitable for display and low energy combat than vigorous head butting. Thus, despite the fact that Choerosaurus is represented by only one skull (which makes it impossible to address the question of sexual dimorphism), its cranial bosses are better interpreted as structures involved in intraspecific selection, i.e. low-energy fighting or display. Display structures, such as enlarged canines and cranial bosses, are widespread among basal therapsid clades and are also present in the putative basal therapsid Tetraceratops insignis. This suggests that sexual selection may have played a more important role in the distant origin and evolution of mammals earlier than previously thought. Sexual selection may explain the subsequent independent evolution of cranial outgrowths and pachyostosis in different therapsid lineages (Biarmosuchia, Dinocephalia, Gorgonopsia and Dicynodontia).