Evidence Pareiasaurs had a Lifestyle Very Different From Modern Megafauna

Bone microstructure of pareiasaurs (Parareptilia) from the Karoo Basin, South Africa: implications for growth strategies and lifestyle habits

Authors:

Canoville et al

Abstract:

Numerous morphological studies have been carried out on pareiasaurs; yet their taxonomy and biology remain incompletely understood. Earlier works have suggested that these herbivorous parareptiles had a short juvenile period as compared to the duration of adulthood. Several studies further suggested an (semi-) aquatic lifestyle for these animals, but more recent investigations have proposed a rather terrestrial habitat.

Bone paleohistology is regarded as a powerful tool to assess aspects of tetrapod paleobiology, but few studies have been conducted on pareiasaurs. The present study assesses intra and inter-specific histovariability of pareiasaurs and provides fresh insights into their paleobiology, thereby permitting a re-evaluation of earlier hypotheses. Our sample comprises various skeletal elements and several specimens covering most of the taxonomic and stratigraphic spectrum of South African pareiasaurs, including large and basal forms from the Middle Permian, as well as smaller and more derived forms from the Late Permian.

Our results concerning size of elements and histological tissues show that for pareiasaurs, element size is not a good indicator of ontogenetic age, and furthermore, suggest that the specific diversity of the Middle Permian pareiasaurs may have been underestimated. The bone histology of these animals shows that they experienced a relatively rapid growth early in ontogeny. The periosteal growth later slowed down, but seems to have been protracted for several years during adulthood. Pareiasaur bone microanatomy is unusual for continental tetrapods, in having spongious stylopod diaphyses and thin compact cortices. Rigorous paleoecological interpretations are thus limited since no modern analogue exists for these animals.

How the Skulls of Early Triassic Basal Cynodont Galesaurus Grew

Cranial Ontogeny of the Early Triassic Basal Cynodont Galesaurus planiceps

Authors:

Jasinoski et al

Abstract:

Ontogenetic changes in the skull and mandible of thirty-one specimens of Galesaurus planiceps, a basal non-mammaliaform cynodont from the Early Triassic of South Africa, are documented. The qualitative survey indicated eight changes in the craniomandibular apparatus occurred during growth, dividing the sample into three ontogenetic stages: juvenile, subadult, and adult. Changes in the temporal region, zygomatic arch, occiput, and mandible occurred during the transition from the subadult to adult stage at a basal skull length of 90 mm. At least four morphological and allometric differences divided the adult specimens into two morphs, indicating the presence of sexual dimorphism in Galesaurus. Differences include extensive lateral flaring of the zygomatic arches in the ‘male' morph resulting in a more anterior orientation of the orbits, and a narrower snout in the ‘female'. This is the first record of sexual dimorphism in a basal cynodont, and the first time it is quantitatively documented in a non-mammaliaform cynodont. An ontogenetic comparison between Galesaurus and the more derived basal cynodont Thrinaxodon revealed differences in the timing and extent of sagittal crest development. In Galesaurus, the posterior sagittal crest, located behind the parietal foramen, developed relatively later in ontogeny, and the anterior sagittal crest rarely formed suggesting the anterior fibres of the temporalis were less developed than in Thrinaxodon. In contrast, craniomandibular features related to the masseters became more developed during the ontogeny of Galesaurus. The development of the adductor musculature appears to be one of the main factors influencing skull growth in these basal non-mammaliaform cynodonts.

Anisian Triassic Dinosauriform Asilisaurus kongwe Just Complicated Dinosauriform Ontogeny

The femoral ontogeny and long bone histology of the Middle Triassic (?late Anisian) dinosauriform Asilisaurus kongwe and implications for the growth of early dinosaurs

Authors:

Griffin et al

Abstract:

The ontogeny of early-diverging dinosauromorphs is poorly understood because few ontogenetic series from the same species-level taxon are known and what is available has not been extensively documented. The large numbers of skeletal elements of the silesaurid Asilisaurus kongwe recently recovered from Tanzania provide an opportunity to examine the ontogenetic trajectory of the earliest known member of Ornithodira and one of the closest relatives to Dinosauria. We examined the ontogeny of the femur and the histology of a series of long bone elements. We observed bone scar variation in a series of femora (n = 27) of different lengths (73.8–177.2 mm). We hypothesize that most femora follow a similar developmental trajectory; however, we observed sequence polymorphism in the order of appearance and shape of bone scars, and we quantified this polymorphism using ontogenetic sequence analysis (OSA). Additionally, five femora, three tibiae, a fibula, and a humerus were thin-sectioned to examine osteological tissues. No lines of arrested growth (LAGs) are present in any specimen, and there is little histological information about the ontogenetic stage of femora, although none have slowed or ceased growth. The woven-fibered bone present in the cortex of elements sectioned is similar to that of the earliest dinosaurs. This sequence polymorphism provides an alternate hypothesis for the robust/gracile dichotomy found in early dinosaurs often interpreted as sexual dimorphism. The shared femoral features found in Asilisaurus and early dinosaurs suggest that this ontogenetic pattern is plesiomorphic for Dinosauria, and that size is a poor predictor of maturity in early dinosauriforms.

Disentangling Arrhinoceratops brachyops' Skull Ontogeny & how it Relates to Other Ceratopsids

Skull ontogeny in Arrhinoceratops brachyops (Ornithischia: Ceratopsidae) and other horned dinosaurs

Authors:

Mallon et al

Abstract:

Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in the fossil record, yet information on growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we describe the partial skull of a juvenile chasmosaurine, attributed to Arrhinoceratops brachyops, within the context of more mature specimens of this species, to better understand the ontogenetic transformations therein. We show that as A. brachyops matured, the postorbital horncores became longer and shifted from a posterior to an anterior inclination, the delta-shaped frill epiossifications became lower and fused to the underlying frill, and the face became more elongate. In these respects, A. brachyops closely resembled Triceratops, suggesting that these ontogenetic changes may have been common to all long-horned chasmosaurines. However, an event-paired cladistic analysis of Chasmosaurinae using a standardized matrix of 24 developmental characters reveals that the relative timing of ontogenetic events in Arrhinoceratops was more like that of Chasmosaurus, particularly in the relatively late reduction in scalloping around the frill margins. Thus, the ontogenetic similarities between Arrhinoceratops and Triceratops appear to be plesiomorphic, partly related to the retention of the elongate postorbital horncores, which are primitive for Ceratopsidae. This study elucidates the otherwise contentious evolutionary relationships of Arrhinoceratops, and highlights the importance of ontogenetic data for resolving phylogenies when morphological data from adults alone are inadequate.

Paleohistology of Temnospondyls Micropholis stowi and Lydekkerina huxleyi

Paleohistology of Micropholis stowi (Dissorophoidea) and Lydekkerina huxleyi (Lydekkerinidae) humeri from the Karoo Basin of South Africa, and implications for bone microstructure evolution in temnospondyl amphibians

Author:

McHugh

Abstract:

Temnospondyl amphibians are a large and diverse group of early tetrapods, whose paleohistology has been incompletely studied. Here, humeri of Micropholis stowi and Lydekkerina huxleyi from the Karoo Basin of South Africa (Katberg Formation) were thin–sectioned for paleohistological analysis. Diaphyseal bone histology of both taxa exhibits a convergence to fibrolamellar tissue and an absence of lines of arrested growth; additionally, medullary cavities free of trabeculae support terrestrial lifestyles in both Micropholis and Lydekkerina. The presence of azonal tissue in Micropholis is unlike that of other dissorophoids or extant caudatans, suggesting an adaptation to local conditions in the Early Triassic of the Karoo Basin, as well as a complicated and incompletely studied pattern of histological evolution in dissorophoids. Additionally, the propodial histology of these and 12 other taxa were assessed through different broad–scale phylogenetic hypotheses for Temnospondyli. Results reveal convergence towards sustained, non–cyclical growth and an absence of lines of arrested growth in the diaphyses of Early Triassic temnospondyls. The optimization of histological traits on to existing phylogenetic hypotheses is equally parsimonious between the different topologies. Homoplasy among histological characters suggests that evolutionary history in this group is overshadowed by developmental plasticity in bone microstructure, potentially due to environmental and biomechanical constraints. However, the interpretation of these data is limited by small sample size, and increased sampling is required to validate the patterns revealed in this study.

The Kids Were Scarier: Cranial Ornamentation DECREASE With age in Early Triassic Trematosauroid Temnospondyl Benthosuchus?

On the development of surface ornamentation of skull bones in the ontogeny of Early Triassic benthosuchids (Amphibia, Temnospondyli)

Author:

Morkovin

Abstract:

The rate of developmental changes in ornamentation of the dermal skull observed at the submature stages of the trematosauroid Benthosuchus (Amphibia, Temnospondyli) shows a wide range of individual variation divided into two dominant modes. The analysis of this variation based on comparisons within samples of two dermal bones (supratemporal and squamosal) displays that such a bimodality is distinctly expressed at the earliest known growth stage and tends to disappear with age.

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.

How Dinosauriform Silesaurus Grew

Skeletal variation and ontogeny of the Late Triassic Dinosauriform Silesaurus opolensis

Authors:

Piechowski et al

Abstract:

A principal component analysis (PCA) performed for a set of 24 measurements on 33 femora and 15 measurements on 20 ilia of Silesaurus opolensis from the early Late Triassic of Krasiejów, southern Poland, shows that this sample is highly variable but probably monospecific. Most of the morphological variation is concentrated in the muscle attachments and proportions of bones, which significantly change in both size and position during ontogeny. Despite the small sample size, femora of smaller individuals have less flattened shafts and a more sinusoidal appearance. In many large specimens, proximal parts of muscle tendons are ossified at their attachment site on femora and remain attached to the bone in the largest specimens. The specimens with attached ossifications are interpreted as mature females that were statistically larger than proposed males. It is suggested that ossifications developed in females under calcitonin control. The intrapopulation variability of ilia is high, but less dependant on ontogeny. The population represented by a few specimens from the lower level at Krasiejów may represent a different stage in the evolution of the species than that from the upper horizon.

Colobomycter pholeter: an Early Permian Parareptile With a Complicated Tooth Structure and Evolution

Plicidentine in the Early Permian Parareptile Colobomycter pholeter, and Its Phylogenetic and Functional Significance among Coeval Members of the Clade

Authors:

MacDougall et al

Abstract:

Once thought to be an exclusively anamniote characteristic, plicidentine, a pattern of infolding of dentine, is now known to be found in various amniote clades, including Parareptilia. In the absence of detailed analyses of parareptilian dentition, most parareptiles were assumed to lack plicidentine due to the absence of external indicators, such as plications on the tooth base. The clear presence of this dentinal feature in the largest premaxillary and maxillary teeth of Colobomycter pholeter, led us to the present detailed study within the dentition of this unusual parareptile, and those of coeval members of this clade. Our study reveals that there is large variability in the degree of dentine infolding within C. pholeter dentition, as well as within those of closely related parareptiles. This variability ranges from a lack of plications, to very complex anamniote-like plicidentine. Utilizing computed tomography scans in conjunction with histological sections we also demonstrate the utility of computed tomography scans in conducting non-destructive sampling in the identification of plicidentine. Given the variability of plicidentine in this sample of parareptiles, we hypothesize that one function of parareptilian plicidentine is to increase the surface area for attachment tissues, and we suggest that the use of plicidentine as a character in phylogenetic analyses of parareptiles may be misleading.

Hidden Pterosaur Diversity in Late Cretaceous Hungary

Does morphology reflect osteohistology-based ontogeny? A case study of Late Cretaceous pterosaur jaw symphyses from Hungary reveals hidden taxonomic diversity

Authors:

Prondvai et al

Abstract:

With a single complete mandible and 56 mandibular symphyseal fragments of various sizes, the Late Cretaceous Hungarian azhdarchid material has been considered one of the most extensive monospecific pterosaur assemblages in the world. Representing a broad size range, these elements have been thought to demonstrate a developmental series of Bakonydraco galaczi. As such, they were ideal to test whether absolute size and/or morphology reliably indicate relative ontogenetic stages in this pterosaur. Forty-five specimens were selected for multivariate morphometrics and classified into four size classes. After acquiring the morphometric data set, we thin-sectioned eight symphyses representing all size groups and classified them into relative ontogenetic stages based on qualitative microstructural inspection prior to quantitative histological analyses. Microstructural characters suggestive of developmental state were then quantified for intra- and interindividual uni- and multivariate analyses to test the correspondence among the results of qualitative and quantitative analyses. In contrast to our expectations, histological features identified the smallest specimen as an adult and not an early juvenile. The substantial size difference between this specimen and other adults, along with its distinct microanatomical and histological features, implies the presence of at least two pterosaur taxa in this symphysis assemblage. This hypothesis is further supported by multivariate morphometrics, which separate the smallest symphyses from all other specimens that form one continuous group. Although the latter group also shows considerable size variability in corresponding ontogenetic stages, this suggests developmental plasticity rather than the presence of even more taxa, and indicates that symphysis size and morphology are poor indicators of skeletal maturity in these animals. Hence, bone histology is an important independent test of the assessment of ontogenetic stage using size and morphology.

Temnospondyls had a Very Adaptable Lifestyle

Life cycles, plasticity and palaeoecology in temnospondyl amphibians

Authors:

Schoch

Abstract:

In the largest early tetrapod clade, the temnospondyls, ontogenies were diverse and quite distinct from the life cycles of extant amphibians. Three well-studied clades exemplify the diversity of these long-extinct ontogenies, here analysed with respect to their bearing on developmental plasticity, reaction norms and evolution. Sclerocephalus readily adjusted by means of developmental evolution to different lake environments. In addition, plasticity (reaction norm) played a significant role, apparent both morphologically and by altered developmental traits. Size increase and extension of the ontogenetic trajectory gave larger predators, a phenomenon also found in the dissorophoid Micromelerpeton. Whereas Sclerocephalus was throughout preying on the same fishes, Micromelerpeton was able to fit into different trophic levels. In the branchiosaurid Apateon, a biphasic life cycle was established, with metamorphosis producing a terrestrial morph in some species; truncation of the ontogenetic trajectory gave a sexually mature larva as an alternative morph (neoteny). Plasticity was high in the larval morphs, permitting neotenes to live as filter feeders or small carnivores. Fine-tuning of development permitted Apateon populations to adjust to specific lake properties and readily change from a filter-feeding to carnivorous mode of life. In the nonmetamorphosing Triassic Gerrothorax, morphology was extremely conserved, but histology reveals much plasticity at the microscopical level, correlating with fluctuating salinity and water energy. In responding to environmental fluctuations by enhanced plasticity, the studied temnospondyls managed to populate lakes inhabitable to other tetrapods and fishes.

Morphology and Ontogeny of a Cambrian Trilobite From China

Morphology and ontogeny of Hunanocephalus ovalis (trilobite) from the Cambrian of South China

Authors:

Dai et al

Abstract:

The juvenile morphology and ontogeny of the Cheiruroidid trilobite Hunanocephalus ovalis Lee, 1963 from the lower Cambrian Shuijingtuo Formation in Hubei Province, South China is presented. The new material comprises a relatively complete meraspid ontogenetic series (degree 0 to 10), which reveals more details on their morphological changes such as the contraction and disappearance of the pronounced posteromedial notch in the pygidium and the addition of the trunk segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. The trunk segmentation schedule of H. ovalis is also discussed, which is similar to the other early Cambrian oryctocephalid trilobites, i.e. as the boundary between the thorax and pygidium migrated posteriorly there is no change in the trunk segment number; the processes of liberation of the thoracic segment and segment insertion into the pygidium are separated from one another, implying that the control of trunk exoskeletal segment appearance and articulation might be decoupled in these trilobites.

Ontogeny of Cambrian Olivooidae

The developmental cycles of early Cambrian Olivooidae fam. nov. (?Cycloneuralia) from the Yangtze Platform (China)

Authors:

Steiner et al

Abstract:

Eggs, embryos, and hatchlings of olivooids are common and distinctive components of the earliest Cambrian small shelly fossil assemblage (Anabarites trisulcatus–Protohertzina anabarica Zone) at the northern and northwestern edge of the Yangtze Platform. Embryos are preserved in 3D as phosphatized permineralizations with superficial cellular structures. Nanofocus X-ray computed tomography reveals that diagenetic structures characterize the interior of most phosphatized eggs and embryos. The Olivooidae fam. nov. is herein distinguished from the possibly related hexaconulariids. Statistical size investigations and morphological studies indicate that at least three, but possibly five or more species of olivooids occur in the early Cambrian of South China. Fossilized smooth egg stages reflect a high biodiversity, encompassing 8–10 or more biological species. Eggs and embryos of Olivooides multisulcatus, Olivooides mirabilis and Quadrapyrgites quadratacris are represented by distinct and characteristic size clusters. The size ranges of embryonic parts of hatchlings roughly correspond to the size clusters of the embryos. From reconstruction of the developmental series of Olivooides multisulcatus it appears that the cleavage mode of olivooids was transitional between superficial and total cleavage. Late embryos and hatchlings of Olivooides multisulcatus and Olivooides mirabilis are characterized by pentaradial symmetry, whereas tubes of Quadrapyrgites quadratacris exhibit tetraradial symmetry. Tubular hatchlings of olivooids may represent larval or adult stages, or perhaps, nonfeeding larvae. Although the biological affinity of olivooids is weakely constrained, we hypothesize that they belong within an extinct clade of the Cycloneuralia. Their assignment to the Cnidaria is considered unlikely.

Evolution of Teeth in Marsupials and Their Metatherian Relatives

Heterochrony, dental ontogenetic diversity, and the circumvention of constraints in marsupial mammals and extinct relatives

Authors:

Forasiepi et al

Abstract:

In marsupial mammals and their extinct relatives—collectively, metatherians—only the last premolar is replaced, but the timing of dental eruption is variable within the group. Our knowledge of fossils metatherians is limited, but is critical to understanding several aspects of the evolution and morphological diversification of this clade. We analyzed the sequence of eruption of 76 specimens of metatherians, including Sparassodonta, an extinct clade of specialized carnivores from South America. In Sparassodonta (1) the P3/p3 erupt simultaneously, in common with some didelphids (in other didelphids, p3 erupts before P3, whereas in the remaining didelphids, some peramelids, one caenolestid, and Pucadelphys this order is reversed); (2) the upper and lower molars at the same locus erupt more in synchrony than in other carnivorous metatherians in which the lower molars clearly precede the upper equivalents; (3) the upper canine in thylacosmilids and proborhyaenids is hypselodont; (4) species with similar molar morphologies have different morphologies of the deciduous premolars, suggesting diverse diets among the juveniles of different taxa; (5) deciduous teeth are functional for a long period of time, with thylacosmilids even retaining a functional DP3 in the permanent dentition. The retention of the DP3 and the hypertrophied and hypselodont upper canine of thylacosmilids represent clear heterochronic shifts. Specializations in the timing of dental eruption and in the deciduous tooth shape of sparassodonts are evolutionary mechanisms that circumvent constraints imposed by the metatherian replacement pattern and increase morphological disparity during ontogeny.

How Skulls and Faces of Centrosaurus Changed as They Matured

Craniofacial ontogeny in Centrosaurus apertus

Authors:

Fredrickson et al

Abstract:

Centrosaurus apertus, a large bodied ceratopsid from the Late Cretaceous of North America, is one of the most common fossils recovered from the Belly River Group. This fossil record shows a wide diversity in morphology and size, with specimens ranging from putative juveniles to fully-grown individuals. The goal of this study was to reconstruct the ontogenetic changes that occur in the craniofacial skeleton of C. apertus through a quantitative cladistic analysis. Forty-seven cranial specimens were independently coded in separate data matrices for 80 hypothetical multistate growth characters and 130 hypothetical binary growth characters. Both analyses yielded the max-limit of 100,000 most parsimonious saved trees and the strict consensus collapsed into large polytomies. In order to reduce conflict resulting from missing data, fragmentary individuals were removed and the analyses were rerun. Among both the complete and the reduced data sets the multistate analyses recovered a shorter tree with a higher consistency index (CI) than the additive binary data sets. The arrangement within the trees shows a progression of specimens with a recurved nasal horn in the least mature individuals, followed by specimens with straight nasal horns in relatively more mature individuals, and finally specimens with procurved nasal horns in the most mature individuals. The most mature individuals are further characterized by the reduction of the cranial horn ornamentations in late growth stages, a trait that similarly occurs in the growth of other dinosaurs. Bone textural changes were found to be sufficient proxies for relative maturity in individuals that have not reached adult size. Additionally, frill length is congruent with relative maturity status and makes an acceptable proxy for ontogenetic status, especially in smaller individuals. In adult-sized individuals, the fusion of the epiparietals and episquamosals and the orientation of the nasal horn are the best indicators of relative maturity. This study recovers no clear evidence for sexually specific display structures or size dimorphism in C. apertus.

Neotony Or Larger Adult Size in Maastrichtian Cretaceous Plesiosaurs From Patagonia?

Reappraisal of Tuarangisaurus? cabazai (Elasmosauridae, Plesiosauria) from the Upper Maastrichtian of northern Patagonia, Argentina

Authors:

O'Gorman et al.

Abstract:

The holotype of Tuarangisaurus? cabazai Gasparini, Salgado and Casadio, 2003 (Elasmosauridae, Plesiosauria) from the Upper Maastrichtian Jagüel Formation (northern Patagonia, Argentina) is redescribed, its taxonomic status is reconsidered, and its phylogenetic relationships analysed. We conclude that it must be referred as an aristonectin elasmosaurid. The complete ilium of an Aristonectinae is described for the first time, and reveals a distinctive proximodistally elongated facet in its distal end. Aristonectins are the most commonly recorded Weddellian elasmosaurids. There is a high proportion of osteologically immature individuals (“juveniles”) within the investigated sample of specimens. Most of these are larger than other juvenile elasmosaurids, and apparently retain juvenile features at larger body sizes.

Torosaurus is NOT Triceratops

Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs

Authors:

Maiorino et al

Abstract:

Background

Recent assessments of morphological changes in the frill during ontogeny hypothesized that the late Maastrichtian horned dinosaur Torosaurus represents the “old adult” of Triceratops, although acceptance of this finding has been disputed on several lines of evidence.

Methodology/Principal Findings

Examining the cranial morphology of 28 skulls in lateral view and 36 squamosals of Nedoceratops hatcheri, Triceratops spp. and Torosaurus spp. by means of landmark-based geometric morphometrics, we compared ontogenetic trajectories among these taxa. Principal Component Analysis and cluster analysis confirmed different cranial morphologies. Torosaurus shape space is well separated from Triceratops, whereas Triceratops horridus and Triceratops prorsus partially overlap within Triceratops shape space. Linear regressions between shape and size suggest different ontogenetic trajectories among these taxa. Results support the “traditional” taxonomic status of Torosaurus. We hypothesize that ontogeny drives cranial morphology with different patterns between Torosaurus and Triceratops.

Conclusions/Significance

Torosaurus is a distinct and valid taxon. Whether looking at entire skulls, skulls without the frill, frills alone, or squamosals, Torosaurus has different morphologies and distinct allometric trajectories compared to Triceratops. This new approach confirms the taxonomic status of Torosaurus as well as the comparatively low diversity of ceratopsids at the end of the Maastrichtian in North America.

Carboniferous Lepospondyls did not Undergo Metamorphosis

Morphometric analysis of skeletal growth in the lepospondyls Microbrachis pelikani and Hyloplesion longicostatum (Tetrapoda, Lepospondyli)

Authors:

Jennifer C. Olori

Abstract:

Although morphological evidence alone appears insufficient to resolve the problem of extant amphibian ancestry, comparison of developmental processes and functional correlates of ontogenetic change may provide a basis for evaluating relationships among extinct and extant tetrapods. In the first allometric study of cranial and postcranial growth in any lepospondyl, I investigate skeletal development in the microsaurs Microbrachis pelikani and Hyloplesion longicostatum using traditional measurement-based and geometric morphometric analyses. Regression analyses against both skull and centrum lengths for M. pelikani indicate positive allometric scaling of the interclavicle plate length, pubis, and ilium, and negative scaling of the diameter of the orbit. Preliminary data for H. longicostatum support negative scaling of neural arch height and posterior centrum height. Results from geometric morphometrics suggest slight widening of the cheek in M. pelikani, rather than marked elongation. Overall, cranial and postcranial growth in both microsaurs was primarily isometric and comparison with allometric data from other Paleozoic taxa suggests that isometric growth is an ancestral feature of development in early tetrapods. All regression analyses for M. pelikani and H. longicostatum had a constant linear slope, indicating that ontogenetic trajectories were continuous, with gradual skeletal growth and no shift in feeding or locomotor function during ontogeny. The lack of morphological change suggests that these microsaurs did not undergo an extant amphibian-like metamorphosis that included reorganization of the skeleton. These data support the hypothesis that metamorphosis is a derived mode of development present only in extant amphibians and their closest relatives.

Tooth Replacement in Olenekian Triassic Cynodont Thrinaxodon liorhinus

Ontogeny of the Early Triassic cynodont Thrinaxodon liorhinus (Therapsida): dental morphology and replacement

Authors:

Fernando Abdala, Sandra C. Jasinoski & Vincent Fernandez

Abstract:

Here for the first time we use micro-computed tomography (μCT) scanning techniques to study dental replacement in the Early Triassic cynodont Thrinaxodon liorhinus. We analyzed five specimens ranging 37–87 mm in skull length using μCT scanning, which were supplemented by detailed anatomical analysis of 48 specimens with a basal skull length of 30–96 mm. Our results indicate that lower postcanines are more numerous and present a more complex morphology than the upper postcanines, even in the same individual; only the lower postcanines have more than three sectorial cusps and a cingular collar on the lingual margin. Complexity of the postcanines increases from the smallest individual to specimens with a skull length of 75 mm, but complexity decreases in larger specimens. Our results confirm the alternate replacement of the postcanines and the posterior migration of the postcanine series (including the loss without replacement of the anterior-most postcanines). Observations point to a posterior-to-anterior replacement wave in lower postcanines, but the evidence is not clear-cut for the upper series. The virtual extraction of functional and replacement teeth permitted us to conclude that in most of the cases the upper canines were replaced anteriorly, whereas lower canines were replaced posteriorly. The presence of two simultaneous replacements of the upper canine tooth was observed in two small juveniles, suggesting a higher rate of canine replacement at a younger age. Incisors also had a sequential replacement pattern, and more replacement teeth were present in medium-sized individuals.