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Sarahsaurus and Lufengosaurus huenei have the same vertebral formula as M. Ignavusaurus possesses 14 dorsal vertebrae, one dorsosacral vertebra and two sacral vertebrae. Only Cv4, Cv8 and Cv9 are visible in lateral and ventral views, the others remaining encased in matrix. In Cv4, the centrum is exposed and has an anteroposterior length to dorsoventral height ratio of approximately 7. This is almost as elongate as in M. This is, however, much more elongate than those of Adeopapposaurus ratio of 5.
A distinct ventral keel that extends along the midline of the ventral centrum surface is visible in all cervicals but Cv7. The proximal mediolateral width to proximodistal length ratio is of 0. Adeopapposaurus has similar proportions to M. The first phalanx of the first digit also appears to be proportionally more gracile in Ngwevu than in M. The latter has a robust protuberance on the plantar surface of the proximal end, with a proximal dorsoventral height to proximodistal length ratio of 0.
Ngwevu does not have this accentuated feature and has a ratio of 0.
This is difficult to compare in other massospondylids. There are no notable differences between the pes of Ngwevu and that of M. A section was taken from the middle of the right humeral midshaft. A relatively thin cortex surrounds an open medullary cavity Fig. Several large resorption cavities line the medullary cavity Fig. Scattered secondary osteons are present in the inner cortex, but do not form dense Haversian bone. Regions of the inner and mid-cortex are diagenetically altered and numerous cracks give the inner cortex a fragmentary appearance. However, small patches of interstitial matrix can still be seen and comprise parallel-fibred bone with evenly distributed globular and flattened osteocyte lacunae.
The preserved vascular canals in the mid-cortex are elongate and circumferentially oriented although this is difficult to confirm due to diagenesis and numerous cracks. The interstitial matrix of the mid-cortex is mainly parallel-fibred bone with some scattered patches of woven-fibred bone identified by the presence of plentiful, disorganized and globular osteocyte lacunae Fig.
There are no secondary osteons present in the mid-cortex.
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The outer cortex is missing its anterior portion. The osteocyte lacunae become progressively more flattened and organized towards the sub-periosteal surface, although patches of woven bone matrix can still be seen. However, the bone tissue is predominantly a mixture of lamellar and parallel-fibred bone tissues. There is a decrease in vascularization from the mid-cortex to the outer cortex.
The vascular arrangement varies between circumferentially-oriented and longitudinally-oriented primary osteons with short anastomoses. The cracks make it difficult to determine an accurate number of lines of arrested growth LAGs , but six LAGs could be confidently identified in the humerus. These correspond to periodic, but temporary cessations in growth. An external fundamental system EFS , consisting of avascular lamellar bone with multiple, closely-spaced LAGs at the sub-periosteal surface and that would indicate the attainment of maximum size, was not observed Fig.
There is a section of pathological bone tissue in the outer cortex, on either side of the missing anterior cortical surface.
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This tissue comprises very densely packed, disorganized and globular osteocyte lacunae with primary osteons arranged either radially or in a reticular network Fig. It appears to be localized and could not be located in the left humerus. Another section was taken from the middle of the left femoral midshaft. The medullary cavity is open and lined with resorption cavities along its anterior and posterolateral margins Fig. These resorption cavities decrease in size and become scattered towards the mid-cortex.
A thick band between the inner and mid-cortex is diagenetically altered Fig. Some scattered primary osteons can be seen in the inner cortex, with a higher number in the posterolateral corner Fig. Very few secondary osteons can be seen in the inner cortex, apart from a few isolated ones in the anterior and lateral portions of the section. The interstitial matrix of the inner cortex is mainly parallel-fibred with some patches of woven bone. It is therefore a mix of parallel fibred and fibrolamellar bone. The vascular canals in the inner cortex form a plexiform arrangement.
The mid-cortex is also a mix of parallel-fibred and woven bone, although there appears to be an increase in woven bone patches compared to the inner cortex. Annuli, which indicate a temporary decrease in growth rate, consist of lamellar bone and interrupt the faster growing bone tissue.
The vascular canals alternate between plexiform and laminar arrangements.
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The outer cortex comprises parallel-fibred and lamellar bone Fig. The vascular arrangement is mostly laminar although some areas form a plexiform network as well. There is also a decrease in vascularisation towards the sub-periosteal surface. Several annuli comprising slowly forming lamellar bone are observed throughout the cortex. They are more visible in CPL light in the mid and outer cortex Fig. Lines of arrested growth are present in the outer cortex and are more easily identified in CPL light.
They are associated with annuli of lamellar bone. They become more closely spaced towards the sub-periosteal surface forming double LAGs in places, indicating a decrease in growth rate Fig. In total 10 growth marks were counted throughout the cortex. No EFS was observed in the femur. When compared to adult M. Due to slight ontogenetic variation, only the character scores common to the two M.
Characters that could only be scored in one M. Character scores for M. The strict consensus tree of these MPTs is presented in Fig. All of the trees place Ngwevu within a monophyletic Massospondylidae that also includes Sarahsaurus aurifontanalis , Ignavusaurus rachelis , Leyesaurus marayensis , Adeopapposaurus mognai, M. This clade is supported by one postcranial unambiguous synapomorphy: lateral margins of the pubic apron concave in lateral view character , state 1.
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It is also supported by 11 ambiguous cranial synapomorphies and one ambiguous postcranial synapomorphy. The postcranial synapomorphy is the relative elongation of the cervical centra with the length of at least cervical 4 or 5 exceeding four times the anterior centrum height character , state 2. In all of the MPTs and in the strict consensus tree, Ngwevu and Lufengosaurus huenei are sister taxa. This relationship is supported by seven ambiguous cranial synapomorphies and one ambiguous postcranial synapomorphy.
A more inclusive monophyletic group including the latter clade, Coloradisaurus and M. In the strict consensus tree, the two adult M. These two M. In the first half of the 20th century, the genus Massospondylus was subjected to intensive splitting Seeley, ; Broom, ; Haughton, but the focused revision of Cooper synonymized many of these species with M. It is therefore imperative that great caution be taken when establishing any new massospondylid taxon. Here, we examine other possible explanations for the morphological differences documented above, including ontogeny, sexual dimorphism and distortion.
In general, two types of taphonomic deformation are prevalent in the fossil record, brittle and plastic deformation, and definitive criteria have been laid out for each Lautenschlager, When brittle deformation occurs, the specimen will display cracks, breaks and fragmentation Lautenschlager, and some degree of disarticulation could also be expected.
The specimen is also well articulated with minimal disruption to the skeleton. Plastic deformation can be identified by the loss of bilateral symmetry without disarticulation, for example the deformation of the orbit into an oval rather than circular shape that maintains bone-on-bone contacts around its periphery. CT scans reveal that the orbital sub-circularity is due to the ventral displacement of the frontals, which gives the orbit a slightly dorsoventrally compressed appearance. There is no other evidence of such distortion in any other part of the skeleton and it seems unlikely that the skull could suffer plastic deformation while other nearby elements, such as the Cv, were unaffected.
Although taphonomic deformation could be responsible for some of the overall orientation differences observed in the skull notably the posteroventral sloping of the posterior portion of the skull , the shape variation of individual bones, the skull proportions, and the robustness of the bones are not explained by taphonomic deformation.
In order for taphonomy to be wholly responsible for the differences observed, deformation would have to be in mutually exclusive directions, which is unlikely. A well-documented ontogenetic series is known for M. Such major, fluctuating changes in cranial proportions through ontogeny seem highly unlikely. This study investigated the ontogeny of vestibular canal shape and size in M. In extinct taxa, sexual dimorphism is often difficult to confirm due to the lack of preservation of these elements, ambiguities in sexing individual skeletons and inadequate sample sizes.
In fact, recent studies using statistical methods, such as unimodality and normality tests, on morphological data found no evidence for sexual dimorphism in any of the nine dinosaur species examined. These species included theropods Coelophysis bauri , Coelophysis rhodesiensis , Allosaurus fragilis , Tyrannosaurus rex , sauropodomorphs Plateosaurus spp.
In order to distinguish between sexes morphologically, they have to be identified beforehand through other means, such as the presence of eggs, embryos, biomechanical structures and differences in histological data Mallon, Even in cases where an apparently clear sexual difference has been identified the reliability of these features have been questioned. For example, a recent study found that the presence of medullary-like bone in non-avian archosauromorphs that laid soft-shelled eggs or were not yet reproductively mature, challenged the long standing thought that these medullary like tissues are strictly associated with the production of eggshell Prondvai, Ascertaining the sex of a fossil individual is therefore tenuous and ideally requires large sample sizes as well as unambiguous evidence.
Although these differences in size and robusticity could be due to sexual dimorphism, the other morphological differences that are present are unlikely to be the product of sexual dimorphism including shape differences between bones that do not have clear display or mating structures, such as the structure of the braincase and palate. Many of the morphologies that distinguish Ngwevu from M. For example, the well-developed coronoid eminence, mediolaterally wide palate, short snout and fused skull roof bones may have allowed for larger muscle mass and stronger bite force.
In comparison to M. Ngwevu is from the upper Elliot Formation, which preserves numerous contemporaneous basal sauropodomorphs and represents deposition over a period of approximately five million years. Consequently, another hypothesis that should be considered to account for the differences between it and other specimens is anagenetic change, as species do not generally persist for such long intervals Scannella et al. However, in order to confirm anagenesis, it is necessary to have high-resolution stratigraphy in order to identify evolutionary trends Scannella et al.
Based on our phylogenetic hypothesis, which does not recover Ngwevu as a close relative of other upper Elliot sauropodomorphs, anagenesis seems unlikely. However, due to the clear dorsoventral crushing of this skull, further investigation is necessary. Massospondylidae was a biogeographically widespread clade, with members on three continents at times ranging from the Late Triassic to the Early Jurassic. Throughout massospondylid evolutionary history Pangaea remained intact, so perhaps unsurprisingly our phylogenetic analysis does not identify any endemic, geographically restricted subclades.
Our strict consensus tree recovers a clade comprising Coloradisaurus from South America, M. This larger clade is, in turn, the sister group of a clade comprising Ignavusaurus from southern Africa and Sarahsaurus from North America. One minor exception to this is the sister-taxon relationship found between Leyesaurus and Adeopapposaurus , which are both South American. Understanding palaeobiogeographical and evolutionary patterns requires a robust phylogenetic hypothesis.
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In basal sauropodomorphs, this is hindered by the incompleteness of the fossil record, the available sample sizes for many taxa as well as the completeness of character matrices used in analyses. The latter is especially true for Massospondylidae, which is a poorly resolved clade that has been resolved into many different topologies depending on the characters and taxa included in respective analyses.
Although massospondylid remains are fairly numerous, the character matrices used to resolve early branching sauropodomorphs lack sufficient homology hypotheses for robustly resolving higher level relationships. Some of these characters cannot be scored in some taxa due to specimen incompleteness or lack of preparation. This work illustrates the importance of CT scanning in expanding anatomical datasets, and therefore the potential for identifying new, useful characters and character states for further detailed analyses and also for increasing the sample sizes of included specimens.
It is likely that we will only obtain a better understanding of massospondylid ingroup relationships and indeed relationships in some other areas of the sauropodomorph tree if we are able to gather more information in this way from both new and historically collected specimens. Furthermore, in the case of massospondylids, identifications to species-level usually require the discovery of reasonably complete skeletons, as many of these taxa are diagnosed by suites of characters that are distributed throughout the skeleton and that exhibit at least some homoplasy rather than by unambiguous autapomorphies.
It will be necessary, therefore, to re-assess specimens that have already been accessioned in order to identify them more robustly. The historical practice of referring incomplete specimens to specific massospondylid taxa should be abandoned unless there is a firm basis for such referrals see also Barrett et al.
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Using this more rigorous approach to identification will facilitate better understanding of the diversity and biostratigraphy of the Late Triassic—Early Jurassic. Although previously referred to Massospondylus carinatus , the holotype specimen of Ngwevu intloko can be differentiated from other basal sauropodomorphs by a combination of 22 characters. These characters are not the result of ontogeny, sexual dimorphism or distortion. Phylogenetic analysis reveals that Massospondylidae was a diverse, successful clade with members present on three major continents between the Late Triassic and Early Jurassic.
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