The Prozostrodontia includes a group of Late Triassic-Early Cretaceous eucynodonts plus the clade Mammaliaformes, in which Mammalia is nested. Analysing their growth patterns is thus important for ...understanding the evolution of mammalian life histories. Obtaining material for osteohistological analysis is difficult due to the rare and delicate nature of most of the prozostrodontian taxa, much of which comprises mostly of crania or sometimes even only teeth. Here we present a rare opportunity to observe the osteohistology of several postcranial elements of the basal prozostrodontid
, the tritheledontid
, and the brasilodontids
and
from the Late Triassic of Brazil (Santa Maria Supersequence).
and
reveal similar growth patterns of rapid early growth with annual interruptions later in ontogeny. These interruptions are associated with wide zones of slow growing bone tissue.
and
exhibit a mixture of woven-fibered bone tissue and slower growing parallel-fibered and lamellar bone. The slower growing bone tissues are present even during early ontogeny. The relatively slower growth in
and
may be related to their small body size compared to
and
These brasilodontids also exhibit osteohistological similarities with the Late Triassic/Early Jurassic mammaliaform
and the Late Cretaceous multituberculate mammals
and
This may be due to similar small body sizes, but may also reflect their close phylogenetic affinities as
and
are the closest relatives to Mammaliaformes. However, when compared with similar-sized extant placental mammals, they may have grown more slowly to adult size as their osteohistology shows it took more than one year for growth to attenuate. Thus, although they exhibit rapid juvenile growth, the small derived, brasilodontid prozostrodontians still exhibit an extended growth period compared to similar-sized extant mammals.
The southern part of the Karoo Basin of South Africa contains a near complete stratigraphic record of the Permo-Triassic boundary (PTB). Isotope- and magneto-stratigraphy confirm that these ...predominantly fluvial strata are approximately the same age as zircon-dated marine PTB sections (252Mya). By August 2013, our team had found 579 in situ vertebrate fossils, mostly of the clade Therapsida, in PTB exposures at three widely separated locations in the southern Karoo Basin. Biostratigraphic ranges of the various taxa found in each of the sections reveal three separate phases of die-off within the same roughly 75 metre-thick stratigraphic interval displaying the same sequence of sedimentary facies interpreted as indicative of climatic drying, increased seasonality and the onset of an unpredictable monsoon-type rainfall regime. The three phases of an inferred ecologically-stepped mass extinction are: Phase 1 (45m–30m below PTB datum) brought on by lowered watertables, which led to loss of shallow rooting groundcover in the more elevated proximal floodplain areas and the disappearance of the smaller groundcover-grazing herbivorous dicynodonts and their attendant small carnivores. Phase 2 (20–0m below PTB datum), is the main extinction that occurs in massive maroon/grey mudrock culminating in an event bed of laminated reddish-brown siltstone/mudstone couplets. This facies reflects progressively unreliable rainfall leading to vegetation loss in proximal and distal floodplain areas. The larger tree-browsing herbivores and their attendant carnivores are confined to watercourses before finally disappearing. Phase 3 (25–30m above PTB datum) occurs in massive maroon siltstone facies with evidence of climatic aridity including the accumulation of mummified carcasses buried by windblown dust. All of the surviving Permian taxa disappear within 30m of the PTB.
Temporal resolution based on accretion rates and pedogenic maturity of each stratigraphic section reveals that Phase 1 and Phase 2 die-offs lasted 21000 and 33000years separated by a short period of 7000years where no disappearances are recorded and this was followed by 50000years of stasis for the final extinction phase, lasting only 8000years, that removed all the Permian survivor taxa,. We propose that the recorded disappearances are real (rather than preferential preservation failure) and that they represent drought-induced die-offs moving progressively up the food chain as the terrestrial ecosystem collapsed; the latter mostly likely caused by volcanogenic greenhouse gas emissions and rapid global warming.
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•Karoo fossils preserve a terrestrial record of the End-Permian mass extinction.•Extinction and recovery phases defined by 32 vertebrate biostratigraphic ranges.•Fluvial facies reflect rapid climatic drying co-incident with the extinction event.•Three phases of drought-induced die-off occurred over approximately 120000years.•Small herbivores died-off first followed large browsers finally all the surviving taxa.
Living species of mammals, crocodiles and most species of birds exhibit parental care, but evidence of this behaviour is extremely rare in the fossil record. Here, we present a new specimen of ...varanopid 'pelycosaur' from the Middle Permian of South Africa. The specimen is an aggregation, consisting of five articulated individuals preserved in undisturbed, close, lifelike, dorsal-up, subparallel positions, indicating burial in 'life position'. Two size classes are represented. One is 50% larger than the others, is well ossified, has fused neurocentral sutures and is distinguished by a coat of dermal ossifications that covers the neck and shoulder regions. We regard this individual to be an adult. The remaining four skeletons are considered to be juveniles as they are approximately the same size, are poorly ossified, have open neurocentral sutures and lack dermal ossifications. Aggregates of juvenile amniotes are usually siblings. Extant analogues of adult and juvenile groupings suggest that the adult is one of the parents, leading us to regard the aggregation as a family group. The Late Middle Permian age of the varanopid family predates the previously known oldest fossil evidence of parental care in terrestrial vertebrates by 140 Myr.
Lystrosaurus is iconic for surviving the Permo-Triassic Mass Extinction and becoming the most abundant terrestrial vertebrate during the Early Triassic. Previous reports of skeletal remains of the ...Triassic species being found in fossilized burrows hint at a possible reason for its success, but unequivocal evidence showing that Lystrosaurus individuals were the burrow makers was lacking. I present here the first articulated skeleton of Lystrosaurus in a fossilized burrow from the Lower Triassic of the South African Karoo Basin, along with taphonomic evidence indicating that this individual was the burrow maker. The species is identified as L. curvatus, the only Lystrosaurus species recovered from above and below the inferred PermoTriassic extinction horizon. It provides the first evidence of burrowing in a Permian species of Lystrosaurus, suggesting that this behavior was more prevalent than previously thought. Based on its size, the specimen is inferred to be a juvenile, showing that Lystrosaurus was capable of excavating burrows at young ontogenetic stages. The abundance of Lystrosaurus body fossils and similar-sized burrows from Lower Triassic strata suggests that burrowing played a pivotal role in the success of this genus in harsh, unpredictable postextinction conditions. Given the abundance of these burrows throughout the Lower Triassic Lystrosaurus Assemblage Zone, Lystrosaurus may have acted as an ecosystem engineer and refuge provider for other species, which may help to explain the high species diversity in the lowermost Triassic in the Karoo Basin.
The non-dinosaurian dinosauriform silesaurids are the closest relatives of crown-group dinosaurs and are thus, important for understanding the origins of that group. Here, we describe the limb bone ...histology of the Late Triassic silesaurid Sacisaurus agudoensis from the Candelária Sequence of the Santa Maria Supersequence, Rio Grande do Sul state, Brazil. The sampled bones comprise eight femora and one fibula from different individuals. The microscopic analysis of all elements reveals uninterrupted fibrolamellar bone tissue indicating rapid growth. A transition to slower growing peripheral parallel-fibered bone tissue in some individuals indicates a decrease in growth rate, suggesting ontogenetic variation within the sample. The osteohistology of Sacisaurus agudoensis is similar to that of other silesaurids and supports previous hypotheses that rapid growth was attained early in the dinosauromorph lineage. However, silesaurids lack the complex vascular arrangements seen in saurischian dinosaurs. Instead, they exhibit predominantly longitudinally-oriented primary osteons with few or no anastomoses, similar to those of some small early ornithischian dinosaurs. This simpler vascular pattern is common to all silesaurids studied to date and indicates relatively slower growth rates compared to most Dinosauria.
: We provide a redescription of the therocephalian therapsid Olivierosuchus parringtoni based on a new specimen recovered from the Lower Triassic Lystrosaurus Assemblage Zone of South Africa and ...discuss the biostratigraphic implications of Lower Triassic South African therocephalians. The new specimen comprises a skull and articulated anterior portion of the postcranial skeleton. Olivierosuchus parringtoni can be distinguished from its akidnognathid relatives, Promoschorhynchus and Moschorhinus, by the presence of a relatively slender snout. Features that further distinguish Olivierosuchus from Promoschorhynchus include fewer upper postcanines, an obtuse angle of the transverse process of the pterygoid and an oblique alignment of the suborbital fenestra margin of the palatine. Features that further distinguish Olivierosuchus from Moschorhinus include the presence of a sharp rather than blunt crista choanalis, a spatulate posterior portion of the ectopterygoid instead of a narrow shaft, the presence of prominent pterygoid tuberosities and a narrow, elongated tabular. A reappraisal of Lower Triassic therocephalian biostratigraphy reveals that most of these taxa are restricted to the lowermost part of the Lystrosaurus Assemblage Zone revealing a high diversity, whereafter the diversity decreases dramatically in the middle of the zone. However, despite their scarcity in the middle and upper Lystrosaurus Assemblage Zone, therocephalians in the Karoo Basin remain the most diverse therapsid clade in the lowermost Triassic, which suggests that they were able to recover relatively quickly from the end‐Permian extinction event and form an important part of the postextinction earliest Triassic recovery.
The South African non-archosauriform archosauromorph Prolacerta and the archosauriforms Proterosuchus, Erythrosuchus, and Euparkeria were important constituents of the Early to early Middle Triassic ...Karoo ecosystem following the end-Permian mass extinction. We present new data on the osteohistology of these stem archosaurs and provide insight into their paleobiology. Bone tissues of the Early Triassic Prolacerta contain a poorly defined fibro-lamellar complex, with parallel-fibered bone in some regions, whereas the contemporaneous Proterosuchus exhibits rapidly forming uninterrupted fibro-lamellar bone early in its ontogeny, which becomes slow forming lamellar-zonal bone with increasing age. The early Middle Triassic Erythrosuchus deposited highly vascularized, uninterrupted fibro-lamellar bone throughout ontogeny, whereas the growth of the contemporaneous Euparkeria was relatively slow and cyclical. When our data are combined with those of previous studies, preliminary results reveal that Early and Middle Triassic non-crown group archosauromorphs generally exhibit faster growth rates than many of those of the Late Triassic. Early rapid growth and rapid attainment of sexual maturity are consistent with life history expectations for taxa living in the unpredictable conditions following the end-Permian mass extinction. Further research with larger sample sizes will be required to determine the nature of the environmental pressures on these basal archosaurs.
Karst caves are suitable environments for the accumulation and preservation of fossils. Cave deposits are often complex and the environmental conditions within cave sites result from intricate ...interactions between various biological, physical and chemical factors. However, it is not fully understood how the complexity of the environmental conditions of caves influences bone diagenesis. The study of the initial stages of bone diagenesis depends to a large extent on understanding the changes in the bone histology. To contribute to this issue, we examine a set of postmortem changes affecting the bone histology of Quaternary mammals that accumulated naturally in Locus 2, a pitfall site in Cuvieri Cave, located in the tropical region of Brazil. Our analyses show that bones deposited in caves may be subject to a peculiar set of environmental conditions that in tropical regions may prevent the preservation of bone histological structure. The effect of diagenetic processes on the bones differs depending on the taphonomic stage of the bone and the diagenetic alterations appear to have influenced each other. The deposition of bioclasts following the entrapment of individuals in Locus 2 favours the proliferation of bacteria on bones and appears to be important in directing the diagenetic alteration. The hydrological regime of the cave, that is recharge with potential phases of higher humidity, also is important in directing the diagenetic alteration and further decreased the preservation potential of the bone microstructure. The formation of macroscopic and microscopic cracks related to bone weathering in caves shows that the taphonomic processes peculiar to these environments are poorly understood, highlighting the need for more research to be conducted on cave taphonomy.
•Cave deposits are often very complex.•Environmental conditions inside caves are dictated by several factors.•The influence of this complexity on bone diagenesis is poorly understood.•The histology of bones from Cuvieri Cave were altered by diagenetic processes.•Environmental conditions on tropical caves may be very destructive to bone histology.
The continuous fossil record of therocephalian therapsids (Eutheriodontia) across the Permo-Triassic boundary and their differential survivorship of the end-Permian extinction offer an exceptional ...deep-time perspective on vertebrate life-history evolution during episodes of large-scale ecological perturbation. To examine potential impacts of the extinction on body size evolution (e.g., “Lilliput” effects) and growth patterns, we investigated cranial sizes and limb bone histology in the therocephalian Moschorhinus kitchingi both before and after the end-Permian extinction, facilitated by analysis of thin-sections of 23 limb bones from an ontogenetic sample of ten individuals across the Permo-Triassic boundary. In general, early subadult Moschorhinus displayed propodial cortices with extensive woven- and parallel-fibered bone (PFB) with dense radial and reticular vascularization and a moderately thickened bone wall with few growth marks. The outer cortex of propodials and epipodials showed a transition to PFB and lamellar bone with longitudinally oriented canals in individuals interpreted as late subadults or adults (>80% largest size). Most elements displayed several (3+) growth marks, though growth marks were more faithfully recorded in the epipodials of Permian individuals. Pearson product-moment correlation tests were performed to examine the relationship between size and robusticity on growth proxies (% cortical vascularity, mean primary osteon diameter), but variation in histomorphology could not be explained by size alone. Variation in body size may be affected by differences in juvenile growth rate and duration, which are highly variable in environmentally stressed extant reptile species. Geologic stage was a more consistent predictor of cortical vascularity. We suggest that Permian and Triassic Moschorhinus exhibited differential rates of early skeletal growth, corroborating the hypothesis that increased environmental variability in the earliest Triassic was associated with rapid growth to a minimum body size requirement and, consequently, shortened developmental times.
Fossorial Origin of the Turtle Shell Lyson, Tyler R.; Rubidge, Bruce S.; Scheyer, Torsten M. ...
Current biology,
07/2016, Volume:
26, Issue:
14
Journal Article
Peer reviewed
Open access
The turtle shell is a complex structure that currently serves a largely protective function in this iconically slow-moving group 1. Developmental 2, 3 and fossil 4–7 data indicate that one of the ...first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell 5). Broadened ribs alone provide little protection 8 and confer significant locomotory 9, 10 and respiratory 9, 11 costs. They increase thoracic rigidity 8, which decreases speed of locomotion due to shortened stride length 10, and they inhibit effective costal ventilation 9, 11. New fossil material of the oldest hypothesized stem turtle, Eunotosaurus africanus 12 (260 mya) 13, 14 from the Karoo Basin of South Africa, indicates the initiation of rib broadening was an adaptive response to fossoriality. Similar to extant fossorial taxa 8, the broad ribs of Eunotosaurus provide an intrinsically stable base on which to operate a powerful forelimb digging mechanism. Numerous fossorial correlates 15–17 are expressed throughout Eunotosaurus’ skeleton. Most of these features are widely distributed along the turtle stem and into the crown clade, indicating the common ancestor of Eunotosaurus and modern turtles possessed a body plan significantly influenced by digging. The adaptations related to fossoriality likely facilitated movement of stem turtles into aquatic environments early in the groups’ evolutionary history, and this ecology may have played an important role in stem turtles surviving the Permian/Triassic extinction event.
•Recently discovered stem turtles indicate the shell did not evolve for protection•Adaptation related to digging was the initial impetus in the origin of the shell•Digging adaptations facilitated the movement of turtles into aquatic environments•Fossoriality likely helped stem turtles survive the Permian/Triassic extinction
The origin of the turtle shell is a major evolutionary transition whose initial function was unknown. Lyson et al. present a strongly supported idea that a burrowing ecology and adaptations related to digging favored the initial transformations on the road to the modern turtle shell. Only later was the shell coopted for protection.
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