Hydroclimate variability in the Madagascar and Southeast African summer monsoons at the Mid- to Late-Holocene transition

The 4.2 ka event at the Mid- to Late-Holocene transition is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. The event is most clearly manifested in the Mediterranean and Middle East as a regional dry anomaly beginning abruptly at 4.26 kyr BP and extending until 3.97 kyr BP. Yet the impacts of this regional drought are often extended to other regions and sometimes globally. In particular, the nature and spatial extent of the 4.2 ka event in the tropics have not been established. Here, we present a new stalagmite stable isotope record from Anjohikely, northwest Madagascar. Growing between 5.22 and 2.00 kyr BP, stalagmite AK1 shows a hiatus between 4.31 and 3.93 kyr BP (±40 and ± 35 yrs), replicating a hiatus in another stalagmite from nearby Anjohibe, and therefore indicating a significant drying at the Mid- to Late-Holocene transition. This result is the opposite to wet conditions at the 8.2 ka event, suggesting fundamentally different forcing mechanisms. Dry conditions are also recorded in sediment cores in Lake Malawi, Lake Masoko and the Tatos Basin on Mauritius, also in the southeast African monsoon domain. However, no notable event is recorded at the northern (equatorial East Africa) and eastern (Rodrigues) peripheries of the monsoon domain, while a wet event is recorded in sediment cores at Lake Muzi and Mkhuze Delta to the south. The spatial pattern is largely consistent with the modern rainfall anomaly pattern associated a with weak Mozambique Channel Trough and a northerly austral summer Intertropical Convergence Zone position. Within age error, the observed peak climate anomalies overlap with the 4.2 ka event. However regional hydrological change consistently begins earlier than a 4.26 kyr BP event onset. Gradual hydrological change frequently begins around 4.5 kyr BP, raising doubt as to whether any coherent regional hydrological change is merely coincident with the 4.2 ka event or part of a global climatic anomaly

Southern Hemisphere controls on ITCZ variability in southwest Madagascar over the past 117,000 years

Migration of the inter-tropical convergence zone, driven by changes in seasonal insolation and high northern latitude temperatures, is the primary control on tropical rainfall on geologic timescales. We test this paradigm using the timing of growth of stalagmites from southwest Madagascar to infer the timing of expansion of the ITCZ to the south at its southern limit. Over the past 117 ky, speleothems grew in the study area primarily when two conditions are met: summer insolation greater than the mean and relatively high Southern Hemisphere temperatures as indicted by maxima in Antarctic ice core oxygen isotope ratios. We observe little influence of Northern Hemisphere, millennial scale temperature variability on the pluvial periods. Further, we observe periods during which the ITCZ simultaneously expands or contracts in both hemispheres. Because Antarctic isotope maxima are periods of increased atmospheric CO2, our results have implications for how tropical rainfall in the Southern Hemisphere might respond to global warming

Tropical Indian Ocean basin hydroclimate at the Mid- to Late-Holocene transition and the double drying hypothesis

The spatial pattern of Holocene climate anomalies is crucial to determining the mechanisms of change, distinguishing between unforced and forced climate variability, and understanding potential impacts on past and future human societies. The 4.2 ka event is often regarded as one of the largest and best documented abrupt climate disturbances of the Holocene. Yet outside the data-rich Northern Hemisphere mid-latitudes, the global pattern of climate anomalies is uncertain. In this study we investigate the spatial and temporal variability of the tropical Indian Ocean hydroclimate at the Mid- to Late-Holocene transition. We conducted Monte-Carlo principal component analysis, considering full age uncertainty, on ten high-resolution, precisely dated paleohydroclimate records from around the tropical Indian Ocean basin, all growing continuously or almost continuously between 5 and 3 kyr BP. The results indicate the dominant mode of variability in the region was a drying between 3.97 kyr BP (±0.08 kyr standard error) and 3.76 kyr BP (±0.07 kyr standard error) with dry conditions lasting for an additional 300 years in some records, and a permanent change in others. This drying in PC1, which we interpret as a proxy of summer monsoon variability, fits with a previously recognised tropic wide change in hydroclimate around 4.0 kyr BP. An abrupt event from 4.2 to 3.9 kyr BP is seen locally in individual records but lacks regional coherence. A lack of apparent 4.2 ka event in tropical Indian Ocean hydroclimate has ramifications for climate variability in the Indus valley, and for the Harappan civilization. Through a comparison of existing Indian subcontinent paleoclimate records, upstream climatic variability in the Indian Summer Monsoon and winter Westerly Disturbances source regions, and modern climatology, we present the “Double Drying hypothesis”. A winter rainfall drying between 4.2 and 3.9 kyr BP was followed by a summer rainfall drying between 3.97 kyr BP and at least 3.4 kyr BP. The Double Drying hypothesis provides more detailed climatic context for the Harappan civilization, resolves the cropping paradox, and fits the spatial-temporal pattern of urban abandonment. The consequences for the new Mid- to Late-Holocene Global Boundary Stratotype Section and Point in a stalagmite from Meghalaya are explored

Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction.

The relative importance of climate and humans in the disappearance of the Malagasy megafauna remains under debate. Data from southwestern Madagascar imply aridifcation contributed substantially to the late Holocene decline of the megafauna (the Aridifcation Hypothesis). Evidence for aridifcation includes carbon isotopes from tree rings, lacustrine charcoal concentrations and pollen assemblages, and changes in fossil vertebrate assemblages indicative of a local loss of pluvial conditions. In contrast, speleothem records from northwestern Madagascar suggest that megafaunal decline and habitat change resulted primarily from human activity including agropastoralism (the Subsistence Shift Hypothesis). Could there have been contrasting mechanisms of decline in different parts of Madagascar? Or are we lacking the precisely dated, high resolution records needed to fully understand the complex processes behind megafaunal decline? Reconciling these contrasting hypotheses requires additional climate records from southwestern Madagascar. We recovered a stalagmite (AF2) from Asafora Cave in the spiny thicket ecoregion, ~10 km from the southwest coast and just southeast of the Velondriake Marine Reserve. U-series and 14C dating of samples taken from the core of this stalagmite provide a highly precise chronology of the changes in hydroclimate and vegetation in this region over the past 3000 years. Speleothem stable oxygen and carbon isotope analyses provide insight into past rainfall variability and vegetation changes respectively. We compare these records with those for a stalagmite (AB2) from Anjohibe Cave in northwestern Madagascar. Lastly, odds ratio analyses of radiocarbon dates for extinct and extant subfossils allow us to describe and compare the temporal trajectories of megafaunal decline in the southwest and the northwest. Combined, these analyses allow us to test the Aridifcation Hypothesis for megafaunal extinction. The trajectories of megafaunal decline differed in northwestern and southwestern Madagascar. In the southwest, unlike the northwest, there is no evidence of decoupling of speleothem stable carbon and oxygen isotopes. Instead, habitat changes in the southwest were largely related to variation in hydroclimate (including a prolonged drought). The megafaunal collapse here occurred in tandem with the drought, and agropastoralism likely contributed to that demise only after the megafauna had already suffered drought-related population reduction. Our results offer some support for the Aridifcation Hypothesis, but with three caveats: frst, that there was no island-wide aridifcation; second, that aridifcation likely impacted megafaunal decline only in the driest parts of Madagascar; and third, that aridifcation was not the sole factor promoting Comparing the paleoclimates of northwestern and southwestern Madagascar during the late Holocene: Implications for the role of climate in megafaunal extinction Faina et al.: Comparing the paleoclimates of northwestern and southwestern Madagascar 109 megafaunal decline even in the dry southwest. A number of megafaunal species survived the prolonged drought of the first millennium, and then likely succumbed to the activities of agropastoralists

Azendohsaurus madagaskarensis.

126 pages : illustrations, map ; 26 cm.During the Triassic, archosauromorphs became one of the first groups of diapsid reptiles to diversify in terms of body size and morphological disparity in both terrestrial and marine ecosystems across Pangaea. This seemingly rapid divergence, and the numerous unique body plans stemming from it, concomitantly has confounded reconstructions of archosauromorph relationships. Teasing apart homology from homoplasy of anatomical characters in this broad suite of body types remains an enormous challenge with the current sample of taxa. Here, we present the postcranial anatomy of Azendohsaurus madagaskarensis, an early archosauromorph from ?Middle to Upper Triassic strata of Madagascar. Azendohsaurus madagaskarensis is known from nearly the entire skeleton in an ontogenetically variable sample. The holotype locality consists of a monotypic bone bed; preservation ranges from complete but disarticulated bones to articulated sections of the skeleton. Azendohsaurus madagaskarensis embodies an aberrant constellation of archosauromorph features, including an elongated neck, a short, stocky tail, robust limbs, and unexpectedly short digits terminating in large recurved unguals on the manus and pes. Together with the cranium, the postcrania reveal A. madagaskarensis to be another representative of a growing coterie of highly apomorphic and bizarre Triassic archosauromorphs. At the same time, recovery and description of the full anatomy of A. madagaskarensis helps to identify a monophyletic grouping of specialized taxa that includes the North American Late Triassic-aged archosauromorphs Trilophosaurus, Spinosuchus, and Teraterpeton, Indian Pamelaria, and Moroccan Azendohsaurus laaroussii. Moreover, information derived from the skeleton of A. madagaskarensis solidifies the systematic position of these taxa among other archosauromorphs. Using the most comprehensively sampled phylogenetic analysis of early archosauromorphs, we found the clade encompassing the aforementioned taxa as the nearest outgroup of Prolacerta broomi + Archosauriformes. The newly recognized clade containing Azendohsaurus, Trilophosaurus, Spinosuchus, Pamelaria, and Teraterpeton demonstrates high morphological disparity even within a closely related group of archosauromorphs, underscores the polyphyly of protorosaurs (5 prolacertiforms), and suggests that most major divergences within this group occurred in the Triassic. Furthermore, our results indicate that craniodental character states ascribed to a herbivorous diet were much more pervasive across Triassic Archosauromorpha than previously conjectured

Analyse biomécanique masticatrice chez des traversodontidés eucynodontes du Trias de Madagascar

L’analyse de la biomécanique de la mandibule ne peut s’effectuer sans la compréhension du degré de développement et du type d’arrangement des muscles adducteurs. Chez les cynodontes non mammaliens, le muscle temporal et le masséter superficiel jouent un rôle primordial dans les mouvements de la mâchoire inférieure lors de la transformation alimentaire. Ils constituent ainsi les éléments clés de cette analyse. Des études antérieures sur des cynodontes non mammaliens dont Dadadon isaloi Flynn, Parrish, Rakotosamimanana, Simpson &amp; Wyss, 1999 et Menadon besairiei Flynn, Parrish, Rakotosamimanana, Simpson &amp; Wyss, 1999, ont démontré la réalisation, par ces animaux, d’une occlusion dynamique complexe des dents postcanines, inférieures et supérieures. La considération du point de morsure comme second point d’appui au même titre que l’articulation crânio-mandibulaire est le fondement même du système à double leviers. Cette méthodologie a permis de : 1) quantifier la résistance opposée par la nourriture au niveau du site occlusal ; et 2) mettre en évidence l’existence d’une charge verticale nette positive, de nature compressive agissant au niveau de l’articulation crânio-mandibulaire chez Dadadon isaloi et Menadon besairiei durant l’interactivité des muscles élévateurs au cours de la mastication.Mandible biomechanics analysis cannot take place without the understanding of the development degree and the arrangement pattern of the adductor muscles. In the non mammalian cynodonts, the temporal and the superficial masseter muscles play a primordial role in the lower jaw motion during the food processing. They constitute the key elements of this analysis. Previous studies on non mammalian cynodonts including Dadadon isaloi Flynn, Parrish, Rakotosamimanana, Simpson &amp; Wyss, 1999 and Menadon besairiei Flynn, Parrish, Rakotosamimanana, Simpson &amp; Wyss, 1999 demonstrated the realization, by these animals, of a complex dynamic occlusion of the lower and superior postcanine teeth. The consideration of the bite point as second occlusal fulcrum equal in status to the cranio-mandibular joint is the basis of the bifulcral model. This methodology allows: 1) to quantify the resistance opposed by food at the level of the occlusal site; and 2) to highlight a positive net vertical load, of compressive nature, acting to the level of the cranio-mandibular joint during the interactivity of the elevator muscles, i.e. during mastication.</p

A new extreme longirostrine temnospondyl from the Triassic of Madagascar: phylogenetic and palaeobiogeographical implications for trematosaurids

<p>Trematosaurids form a very large and remarkable clade of Triassic tetrapods (Temnospondyli: Stereospondyli) with a worldwide geographical distribution. Compared with specimens from Europe, Australia or North America, they remain relatively scarce in African rocks, where they are mainly known in the Early Triassic of Madagascar and South Africa. Longirostrine trematosaurids were only known from Madagascar, represented by the genus <i>Wantzosaurus</i>. However, we describe herein a new species of the longirostrine trematosaurid <i>Aphaneramma</i>, <i>Aphaneramma gavialimimus</i> sp. nov., from the Olenekian (Lower Triassic) of Madagascar. This genus was previously known from the Early Triassic of Europe and Asia. Based on a new nearly complete skull, the new species is characterized by a premaxilla-nasal suture anteriorly directed, not contacting the nostrils; choanae completely included within the palatines; the ventral opening of the orbits in the anterior part of the interpterygoid vacuities; a very elongated nasal covering more than 50% of the prenarial length; and an anteriorly widened cultriform process. <i>Aphaneramma gavialimimus</i> sp. nov., with a skull length of about 40 cm, may be one of the largest known trematosaurids. Its inclusion in a new phylogenetic analysis confirms its close affinities with the North American genus <i>Cosgriffius</i>, and clarifies the relationships of trematosaurids in general and lonchorhynchines in particular. The new species also increases the palaeobiodiversity of marine trematosaurs in Gondwana and allows discussing their apparently rapid cosmopolitanism just after the great Permian–Triassic mass extinction.</p> <p><a href="http://zoobank.org/urn:lsid:zoobank.org:pub:6992EB6B-5708-4226-860D-47982CFB7F22" target="_blank">http://zoobank.org/urn:lsid:zoobank.org:pub:6992EB6B-5708-4226-860D-47982CFB7F22</a></p

Mambachiton fiandohana Nesbiư & Patellos & Kammerer & Ranivoharimanana & Andre & Wyss & Flynn 2023, gen.et sp. nov.

Mambachiton fiandohana gen.et sp. nov. Etymology: Genus name is a combination of the Malagasy mamba, meaning crocodile, and Ancient Greek χιτών (khiton), which can refer to a suit of armour. A secondary reference to the molluscan chiton is also intended, based on the evocative superficial similarity in armour morphology between Mambachiton fiandohana and polyplacophorans. Species name is the Malagasy word meaning source or beginning, in reference to this taxon’s phylogenetic position near the crocodile–bird split, the node to which Archosauria is definitionally linked. Holotype — UA 8-25-97-132, an articulated series of anterior to posterior cervical vertebrae and osteoderms, anterior cervical vertebra neural arch fragment, cervical rib, and disarticulated osteoderms (Fig. 1A). A right partial postfrontal was directly associated with the holotype. Referred specimen— FMNH PR 5065, middle trunk vertebrae, posterior trunk vertebrae, sacral vertebra one, anterior caudal vertebra, right scapulocoracoid, less scapulocoracoid, right ilium, and right proximal portion of femur (Fig. 1B). All of the referred elements were found near the holotype in the same horizon over a number of field seasons. We consider it highly probable that the holotype and the referred bones pertain to a single individual, based on consistency of size, consistency of character states for a taxon near the base of Archosauria, similar preservation, and the fact that none of the elements are duplicated with the holotype or each other. However, considering the lack of direct association with the cervical series and several years over which these elements were collected in the field, we refrain from including them in the holotype. Our hypothesis that all of these bones belong to Mambachiton fiandohana can only be tested with the discovery of another specimen showing association of these and diagnostic cervical elements; for now, justification for referring these non-holotype elements to Mambachiton fiandohana is given in the description. Locality and age— The holotype and referred elements of Mambachiton fiandohana were collected within the ‘basal Isalo II’ beds/ Makay Formation, in a coarse-grained grey sandstone at Locality M-13, east of Sakaraha in the southern Morondava Basin of south-western Madagascar (precise locality information on file at the AMNH, FMNH, and UA). Other taxa recovered from the holotype’s locality include the traversodontids Menadon besairiei and Dadadon isaloi (Flynn et al. 1999, 2000), the rhynchosaur Isalorhynchus genovefae (Flynn et al. 1999), the lagerpetid Kongonaphon kely (Kammerer et al. 2020), and the other reptile remains described below. The age of the ‘basal Isalo II’ deposits is Ladinian–Carnian (Mid-to-Upper Triassic) based on the correlations and caveats provided in Flynn et al. (2000) and Kammerer et al. (2020). Diagnosis —The holotype of Mambachiton fiandohana differs from all other archosauriforms by the presence of the following combination of character states (asterisks indicate autapomorphies): *small tuber present at the dorsal margin of the prezygadiapophyseal lamina on the lateral side of the prezygapophysis in the cervical vertebrae; epipophyses absent on the dorsal surface of the postzygapophysis; laterally expanded dorsal portion of the neural spine; tapering anterior process of the cervical osteoderms articulating with a distinct groove on the ventral surface of the preceding osteoderm; smooth, unsculptured osteoderms; *high number of osteoderms per cervical vertebra (five to eight, depending on position); staggered arrangement of osteoderms across the midline. Potential further diagnostic character states for this new taxon discernable from the referred material include: neural spines of the trunk vertebrae and first sacral vertebra laterally expanded dorsally; weakly developed hyposphene–hypantrum articulations between the trunk vertebrae; lateral articulation surface of sacral rib anteriorly and posteriorly constricted, resulting in an ‘I’-shape; coracoids with short postglenoid processes; distal end of the scapula expands anteroposteriorly more than the proximal end; rugose tuber occurs just distal to the glenoid of the scapula; ilium with notch on the articular surface for reception of the ischium. Ontogenetic assessment— No histological sectioning was performed, but a number of co-ossifications throughout the skeleton suggest that the holotype and referred materials of Mambachiton fiandohana werenearskeletalmaturityattimeofdeath (see: Griffin et al. 2020). The neurocentral sutures are completely co-ossified throughout the preserved portions of the cervical series (the first preserved neural arch is broken) and the trunk, sacral, and anterior caudal vertebrae are completely or partially co-ossified at the neurocentral sutures; only one side of the neurocentral suture is fully co-ossified in the posterior trunk vertebra. Co-ossification of the scapulocoracoids is complex; anteriorly it is complete, but a clear suture occurs posteriorly on the less element. No co-ossification occurs in this region on the right element. Nomenclatural acts— This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life Science Identifiers (LSIDs) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix zoobank.org/. The LSIDs for this publication are as follows: urn:lsid:zoobank. org:pub: 5CD3A1E5-2882-4004-8875-FCD534CF88AF. Mambachiton: urn:lsid:zoobank.org:act: A15DFD4D-8C6D-4940-B708-464540433830. Mambachiton fiandohana: urn:lsid:zoobank.org:act: 630DA554- FF99-4046-BBAE-42391392FB00. Description Postffontal A largely complete left postfrontal (UA 8-25-97-132; Fig. 2), found directly associated with the holotype, represents the only cranial material potentially referrable to Mambachiton fiandohana. It is uncertain whether this bone actually pertains to Mambachiton fiandohana, or to an otherwise unknown reptile from the quarry. However, it is clear based on its small size and other features, that this element is not that of a rhynchosaur, the most common archosauromorph from the locality. Yet, we cannot exclude the possibility that it could pertain to a different avemetatarsalian or other, unknown archosauriform. This bone, which forms the posterodorsal portion of the orbit, bears a complex sutural surface with the frontal that angles anterolaterally. The dorsal portion of the orbital margin is raised slightly relative to the rest of the element and is faintly rugose. A fossa on the posteromedial portion indicates that the postfrontal would have participated in the supratemporal fossa, but a contact surface at the posterior edge (either for the postorbital or parietal) indicates that the postfrontal did not participate in the supratemporal fenestra. Cervical series The cervical series (UA 8-25-97-132) consists of six complete vertebrae, the first of which articulates with its neural arch; a fragment of the postzygapophyses of an anterior cervical was also recovered (Figs 3–8). The articulated cervical vertebrae encompass most of the cervical series, but their exact positions within the complete series (e.g. vertebrae 3–9) is unclear. The last element of the series is possibly transitional between the cervical and trunk series, but the parapophysis of this element still lies entirely on the centrum. The centra of the cervical series shorten posteriorly (Table 2) to the fourth preserved vertebra, before lengthening again slightly. The lateral sides of the centra bear a shallow fossa (Figs 3, 4) between the articular facets; the depth of these fossae is constant throughout the series, although taphonomic crushing has distorted them on the less side of the sixth preserved vertebra and on the right side of the fissh (Fig. 8A–F). The amphicoelous centra have circular facets. A distinct but weakly extended ridge on the midline of the ventral surface (=ventral keel) is present on the first three preserved vertebrae (Figs 6, 7E) and on the anterior half of the fourth (Fig. 7K), but is absent on the fissh (Fig. 8E) and sixth. Paramedian ridges parallel the midline ridge on the first complete vertebra (Fig. 6). On the anterior cervical vertebrae, the posterior surfaces of the parapophyses are oval, the long axis oriented anteroposteriorly, whereas in the more posterior cervical vertebrae, the long axis is oriented posterodorsally. The anteroventral position of the parapophysis creates a concave surface medial to the structure, but lateral to the midline, in the first two preserved vertebrae. The parapophyses and diapophyses are separated throughout the series, though these structures converge posteriorly as the centrum shortens. The neural aches preserve a series of distinct laminae and deep fossae as in other long-necked archosaurs (e.g. Arizonasaurus babbitti, Nesbiư 2005; Teleocrater rhadinus, Nesbiư et al., 2018). Laterally, a deep centrodiapophyseal fossa (sensu Wilson et al. 2011) lies medial and ventral to the diapophysis, and is roofed by a posterior centrodiapophyseal lamina (sensu Wilson 1999). A few small laminae of bone are present within this fossa in the more anterior cervical vertebrae (Fig. 7A–F). A shallow parapophyseal centroprezygapophyseal fossa is present in the first preserved vertebra, deepening posteriorly as the result of a more pronounced prezygadiapophyseal lamina. A small tuber is present at the dorsal margin of the prezygadiapophyseal lamina on the lateral side of the prezygapophysis. A postzygapophyseal centrodiapophyseal fossa is absent in the anteriormost preserved neural arch (Figs 3–5), but appears in the second preserved neural arch (first preserved complete vertebra; Fig. 7A, B) and deepens posteriorly on that vertebra as the posterior centrodiapophyseal lamina develops. The postzygapophyseal centrodiapophyseal fossa is a deep pit, with its anterior extent obscured by the posterior centrodiapophyseal lamina in the second completely preserved vertebra; this condition is similar to that of Teleocrater rhadinus (e.g. NMT RB505) and saurischian dinosaurs (Langer and Benton 2006). The prezygapophyses project anterolaterally at an angle about 45 o from the anteroposterior plane, and a large gap separates the articular surfaces at the midline (Figs 3–8). The postzygapophyses are angled about 45 o laterally (from horizontal) in posterior view; a wide gap between the articular surfaces widens anteriorly. No epipophyses are present at the dorsal margin in any of the cervical vertebrae (structures present in most avemetatarsalians), but a slight swelling of the neural spine posteriorly occurs in the homologous position. A fossa between the pre- and postzygapophyses excavates the base of the neural spine. The neural spine is mediolaterally thin and situated over the posterior two-thirds of the centrum. A shallow fossa occurs at the lateral base of the neural spine throughout the preserved series. Most of the neural spines are hidden under osteoderms, but (computed tomography) reconstructions permit a number of observations (Figs 8, 9). Neural spines are expanded laterally at their dorsal margins, while the dorsal margins are nearly flat. The neural spines are slightly shorter (measured from their bases) than the centra are long. The neural spines shorten anteroposteriorly as their corresponding centra shorten posteriorly. The anterior edge of the neural spine slopes slightly anterodorsally in the more anterior elements, but not to the steep degree seen in aphanosaurs (Nesbiư et al. 2018). The posterior margin of all the neural spines are nearly vertical. A single isolated rib, missing much of the shass, is preserved anteriorly in the cervical series (Figs 3–6). Its disarticulation prevents its position within the cervical series from being assigned with certainty, but it appears to derive from the posterior portion of the series based on the positions of the capitulum and tuberculum. A sheet of bone on one side of the capitulum and the tuberculum may represent an anterior process, as is present in most archosauromorphs. The capitulum is broken, but a thin web of bone links it to the tuberculum. Cervical osteoderms Osteoderms cover the dorsal margins of the cervicals throughout the preserved series (in the holotype UA 8-25-97-132; Figs 3–5, 9, 10). Twenty articulated osteoderms are preserved on the right side of the holotype and nine on the less (Fig. 9). The long (mediolateral) axes of the osteoderms are deflected ventrolaterally about 60 o to the horizontal in posterior view. Each osteoderm is thick medially and progressively thins laterally as the osteoderms become dorsoventrally compressed. The dorsal surface of the medial edge sometimes preserves a ridge, with a slight depression adjacent to it. Where exposed, the surface of the medial portions of the osteoderms is slightly rugose, but no interdigitating contact between elements is evident. In anterior view, the lateral portion of the osteoderms flexes slightly dorsally. In dorsal view, the lateral margin is rounded, with a greater lateral extent more posteriorly, and the posterior margin varies from rounded to straight; some osteoderm margins bear small projections, but the paưern is inconsistent. CT data show that each osteoderm possesses a round (in cross-section), anteriorly tapering process (Fig. 9B, C, G–L). This process makes up half the length of the medial edge; although it occurs near the midline, it does not form the midline in articulation. No clear dorsal articular surfaces mark where osteoderms overlap (Figs 9, 10). Dorsal surfaces are smooth and unsculptured, but at least seven non-serial osteoderms on the right side bear a small dimple, which differs in size and location between elements. The osteoderms are consistently shaped throughout the column; they become only slightly longer (anteroposteriorly) posteriorly (see analysis below). In Mambachiton, the configuration of osteoderms relative to each other and to their associated vertebra is unique. They are arranged in two rows along the midline, but are staggered across the midline, as in some pseudosuchians (e.g. Prestosuchus chiniquensis; Nesbiư 2011; Nundasuchus songeaensis, Nesbiư et al. 2014). The osteoderms are imbricated, such that the posterior edge of the more anterior osteoderm overlaps the anterior edge of the subsequent one. Additionally, the tapering anterior process articulates with a distinct groove (Fig. 9J, N) on the preceding osteoderm, an arrangement unique among archosauriforms. The number of osteoderms per vertebra exceeds the typical ‘one paramedian pair to vertebra’ ratio typical in Archosauriformes (Nesbiư 2011). Nine osteoderms cover the longest cervical vertebra in Mambachiton, whereas six or seven osteoderms occur per vertebra in the posterior portion of the preserved vertebral series. The number of paramedian osteoderms per vertebra in Mambachiton is the highest known among any archosauriform. Variation- Our principal components analysis (Fig. 11) show some clustering based on relative location of the osteoderms along the cervical column. PC1 shows the most variance along Landmarks 3, and 1 (respectively) which correlates with the angle between the anterior process and the rugose medial flare. PC2 shows the most variance along Landmark 5, 3, and 4 (respectively) which correlates with the medial edge of the osteoderm. The Principal Component 1 vs. Principal Component 2 graph shows a cluster of similar shapes among anterior and posterior osteoderms respectively (Fig. 11). The shapes of more central osteoderms do not cluster as closely, varying more broadly across both principal components. Anterior osteoderms form a cluster between 0.00 and 0.15 along PC1, and 0.05 and -0.05 in PC2. Posterior osteoderms form a cluster between 0.00 and -0.15 along PC1, and show less variability along PC2, in a cluster between 0.00 and -0.05. This indicates that anterior osteoderm shapes typically show more variation along both PC1 and PC2, whereas the shape of the posterior osteoderms tends to vary along only PC1, and are less variable along PC2. Central osteoderms show higher variability along both PC1 and PC2, more so than either the anterior or posterior osteoderms. Central osteoderms form a cluster mainly between 0.05 and -0.10 along PC1, and between 0.05 and -0.05 along PC2. Two major outliers (central osteoderms 5 and B) cause the central osteoderms to show high deviation in shape, occupying more of the graph space along PC1 and PC2. The PCA graph (Fig. 11) shows liưle apparent separation between right and less osteoderm variation, but this may be the result of a relatively smaller sample size for the right elements. Interestingly, mirrored paired osteoderms (taking into account the slight staggering of the less and right osteoderm columns) plot relatively distantly from each other in PCA space (by a similar length), despite being similar to one another in anatomy. In this case, the mirrored pairs are between osteoderms 3 and A, 5 and C, and 6 and D. These variations among the osteoderms are congruent with our morphometric analysis. Anteriorly positioned osteoderms appear more rounded, whereas posterior elements appear more angular and more parallelogram shaped. Central osteoderms lie somewhere between these two end members, though the morphometric analysis results shows no clear paưern that would indicate a gradual transition in shape from one extreme to the other. Rather, anterior and posterior osteoderms show liưle variability, and plot separately within the PCA space. Central osteoderms are much more variable, and can overlap in morphology with either the anterior or posterior osteoderms. Less and right paramedian osteoderms are nearly identical, but not when paired together. Other osteoderms A variety of other osteoderms were recovered with the holotype (UA 8-25-97-132), but their position in the skeleton is unknown. A cluster of three disarticulated, but similarly oriented, osteoderms prepared in ventral view are the most similar to the osteoderms articulated along the cervical vertebral column (Fig. 10). These osteoderms may have belonged to the anterior portion of the neck dorsal to the vertebrae, given that this part of the neck is slightly disarticulated and the morphology of these osteoderms closely matches the more anterior osteoderms in the articulated cervical series. These osteoderms are triangular, and this triangular shape becomes posterolaterally extended anteriorly. These three osteoderms have long and tapered anterior processes near their medial edges; their posterolateral portions broaden, as in the articulated neck osteoderms. A deep, anteroposteriorly oriented groove lies near the medial edge, probably the articulation surface for the long anterior process, reminiscent of the groove on the ventral surface of the cervical osteoderms (observed through CT data) discussed earlier. The medial surface, thickest dorsoventrally, is covered by a system of ridges and grooves, probably marking the midline and contact surface with its antimere osteoderm. A rectangular osteoderm also was found directly associated with the holotype (Fig. 11F, G). We are tentatively assigning it to the holotype given its close proximity to the cervical series and because no other taxa from the quarry are known to bear osteoderms. Nevertheless, we do recognize the possibility that another, currently unknown reptile taxon (e.g. an aetosauriform) could be the source of this single osteoderm. Given the highly divergent shape of this osteoderm relative to of all others known from this locality, its position and anatomical directions are unknown. Accordingly, we employ the anatomical orientation of a typical paramedian aetosaur osteoderm, given their general similarities (Parker 2007, 2008, Desojo et al. 2013). This rectangular osteoderm is about twice as wide as long, with a distinct bend just off the mediolateral centre. The presumed anterior edge of the dorsal surface bears a flat lamina across its entire edge. The dorsal surface is ornamented; small dimples are present near the apex, and the medial and lateral portions bear more elongated grooves and rounded ridges. This isolated osteoderm is consistently thick dorsoventrally throughout its body. Its ventral surface is smooth and concave.Overall, this osteoderm is aetosaur-like in form, the presence of an anterior articulation surface (=anterior bar, anterior lamina) being rather rare among archosauriforms. The ornamentation of this osteoderm also is similar to small-bodied aetosaurs (e.g. Small and Martz 2013). Table 2. Continued Two other fragments found with the holotype also appear to be osteoderms, but this cannot be confirmed definitively (Fig. 10E). Both have compressed, tapered processes that could correspond to the anterior processes of the other osteoderms. Referred vertebrae Four trunk vertebrae (part of FMNH PR 5065) found near the holotype are referred to Mambachiton fiandohana based on similar size, consistency of character states, preservation, and the fact that none of the elements are duplicated with the holotype or each other; we consider it possible, and even likely, that all these vertebrae belong to the same individual as the holotype (Figs 12, 13). Two of these vertebrae are from the middle trunk (Fig. 12), and two from the posterior trunk (Fig. 13), judging from centrum shapes as well as diapophysis and parapophysis shapes. The well-preserved middle trunk vertebrae (Fig. 12) both are essentially complete. The centra are strongly waisted between the anterior and posterior

The earliest-diverging avemetatarsalian: a new osteoderm- bearing taxon from the Triassic (?Earliest Late Triassic) of Madagascar and the composition of avemetatarsalian assemblages prior to the radiation of dinosaurs

Nesbiư, Sterling J., Patellos, Emily, Kammerer, Christian F., Ranivoharimanana, Lovasoa, Andre, Wyss, R., Flynn, John J. (2023): The earliest-diverging avemetatarsalian: a new osteoderm- bearing taxon from the Triassic (?Earliest Late Triassic) of Madagascar and the composition of avemetatarsalian assemblages prior to the radiation of dinosaurs. Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 199 (2): 327-353, DOI: 10.1093/zoolinnean/zlad038, URL: http://dx.doi.org/10.1093/zoolinnean/zlad03