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Cranial characters associated with the proboscis postnatal-development in Tapirus (Perissodactyla: Tapiridae) and comparisons with other extant and fossil hoofed mammals
Abstract
Among extant hoofed mammals, only elephants and tapirs exhibit a true proboscis. We investigated the development of cranial characters potentially associated with the presence of a short proboscis in tapir species, following a comparative framework with a selection of extant and fossil hoofed mammals that exhibit modified narial structures. The core characters present in all taxa (with or without true short proboscis) develop prenatally in tapirs, and other characters that complement the osteological “proboscis syndrome” develop postnatally and are absent or irregularly distributed in other taxa. The taxa examined here showed varying degrees of cranial modification. On the basis of our comparisons, some fossil taxa are indicated as candidates to have possessed a true short proboscis. A terminal addition sequence of character evolution is suggested to explain the development and convergent evolution of the proboscis in hoofed mammals.
... Macrauchenia patachonica had anatomical peculiarities including extremely retracted nasals resulting in the nostrils located at the top of its skull and posterior to the orbits with no match in living terrestrial mammals, and enlarged cervical vertebrae reminiscent of extant giraffes (MacFadden and Shockey, 1997). The morphology of the skull's anterior part suggests that M. patachonica had a proboscis (MacFadden and Shockey, 1997), although there is an argument that Macraucheniidae lacked a proboscis and perhaps had a more Alces-like prehensile lip (Moyano and Giannini, 2018). ...
... The big muscle attachments have been interpreted as indicating that M. patachonica had a fleshy nose, prolongated as a trunk, which more or less exceeded the mouth and would be longer than the tapir trunk but shorter than the elephant trunk (Lydekker, 1903). Further morphological studies of cranial characters potentially associated with the presence of proboscis in extant and extinct hoofed mammals argue that Macraucheniidae lacked a proboscis and perhaps had a more Alces-like prehensile lip (Moyano and Giannini, 2018). ...
Macrauchenia patachonica Owen, 1838 was among the last and largest litopterns, an extinct order of South American native ungulates. Macrauchenia patachonica had anatomical peculiarities as extremely retracted nasals, enlarged cervical vertebrae, and limb bones proportions without good living analogs that lead to asking about its paleobiology. To quantitatively assess the strange combination of limb bone features in M. patachonica, we constructed an indicator of differences in anatomical adaptations for efficient running between forelimb and hind limb (IDFH). We also made a multivariate analysis using data on osteological ratios of living mammals and two other litopterns. We discuss several biomechanical and paleobiological implications of the striking differences between hind limb and forelimb design in M. patachonica. Our main suggestion is that M. patachonica, during fast locomotion, probably used a posture with the neck in a horizontal position.
... To explain this strange feature, some authors suggested that M. patachonica had a proboscis (Burmeister, 1864;Lydekker, 1903;MacFadden & Shockey, 1997 and references therein). However, morphological studies of cranial characters in extant and extinct hoofed mammals argue that Macraucheniidae could have a more Alces-like prehensile lip (Moyano & Giannini, 2018). Recently it was suggested that M. patachonica had a narial structure related to minimizing the inhalation of dust during migration in arid environments (Blanco et al., 2021). ...
... In addition, many notoungulates possessed euhypsodont teeth, not only among rodent-sized paedotherians and medium-sized mesotheriids but also among the larger toxodontians. The peculiar facial anatomy of macrauchenians defies comparison to any living terrestrial mammal, and their narial anatomy remains uncertain (Forasiepi et al., 2016(Forasiepi et al., , 2021Moyano & Giannini, 2018). (Fariña et al., 1998). ...
The Pleistocene Pampean Megafauna (PPM) may be considered as the assemblage consisting mainly of large mammals (i.e., bodymass ≥ 45 kg) that inhabited what is currently considered the Pampean Region during the Pleistocene and became extinct near the Pleistocene–Holocene transition. This contribution addresses several conceptual issues that may guide future efforts in its reconstruction and to consider potential consequences of its extinction. First, we approach the concept of megafauna by urging its explicit definition and contextualization in every investigation as a means of avoiding conceptual ambiguity. Second, we call attention to the risks of succumbing too readily to the temptation of employing a simplistic approach that assumes that extinct taxa had virtually the same biological requirements as those of their extant counterparts. We claim that within the PPM the abundance of taxa distantly related to or markedly distinct morphologically from their living counterparts poses significant challenges for understanding their paleobiology. Paleobiological interpretations need not be rigidly phylogenetically restricted, and phylogenetically based interpretations require critical assessment before their application. Third, we consider the paleoecology of the PPM from a metabolic perspective: as it was clearly dominated by allegedly hypometabolic megaherbivores (xenarthrans), there is no clear counterpart among living faunas. Fourth, we call attention to the fact that the loss of the PPM may have left in its wake an enduring but little-recognized legacy on the functioning of the contemporary ecosystem of the Pampean Region. Extinction of the PPM opened an enormous ecological chasm in the herbivore guild during the Holocene that persisted for about 6000 years, until it was filled, at least in part, by herds of cattle introduced since the sixteenth century.
The Huayquerías Formation (late Miocene, Huayquerian SALMA) is broadly exposed in west-central Argentina (Mendoza). The target of several major paleontological expeditions in the first half of the 20th century, the Mendozan Huayquerías ("badlands") have recently yielded a significant number of new fossil finds. In this contribution we describe a complete skull (IANIGLA-PV 29) and place it systematically as Huayqueriana cf. H. cristata (Rovereto, 1914) (Litopterna, Macraucheniidae). The specimen shares some nonexclusive features with H. cristata (similar size, rostral border of the orbit almost level with distal border of M3, convergence of maxillary bones at the level of the P3/P4 embrasure, flat snout, very protruding orbits, round outline of premaxillary area in palatal view, and small diastemata between I3/C and C/P1). Other differences (e.g., lack of sagittal crest) may or may not represent intraspecific variation. In addition to other features described here, endocast reconstruction utilizing computer tomography (CT) revealed the presence of a derived position of the orbitotemporal canal running below the rhinal fissure along the lateroventral aspect of the piriform lobe. CT scanning also established that the maxillary nerve (CN V₂) leaves the skull through the sphenoorbital fissure, as in all other litopterns, a point previously contested for macraucheniids. The angle between the lateral semicircular canal and the plane of the base of the skull is about 26°, indicating that in life the head was oriented much as in modern horses. Depending on the variables used, estimates of the body mass of IANIGLA-PV 29 produced somewhat conflicting results. Our preferred body mass estimate is 250 kg, based on the centroid size of 36 3D cranial landmarks and accompanying low prediction error. The advanced degree of tooth wear in IANIGLA-PV 29 implies that the individual died well into old age. However, a count of cementum lines on the sectioned left M2 is consistent with an age at death of 10 or 11 years, younger than expected given its body mass. This suggests that the animal had a very abrasive diet. Phylogenetic analysis failed to resolve the position of IANIGLA-PV 29 satisfactorily, a result possibly influenced by intraspecific variation. There is no decisive evidence for the proposition that Huayqueriana, or any other litoptern, were foregut fermenters.
Since the late eighteenth century, fossils of bizarre extinct creatures have been described from the Americas, revealing a previously unimagined chapter in the history of mammals. The most bizarre of these are the 'native' South American ungulates thought to represent a group of mammals that evolved in relative isolation on South America, but with an uncertain affinity to any particular placental lineage. Many authors have considered them descended from Laurasian 'condylarths', which also includes the probable ancestors of perissodactyls and artiodactyls, whereas others have placed them either closer to the uniquely South American xenarthrans (anteaters, armadillos and sloths) or the basal afrotherians (e.g. elephants and hyraxes). These hypotheses have been debated owing to conflicting morphological characteristics and the hitherto inability to retrieve molecular information. Of the 'native' South American mammals, only the toxodonts and litopterns persisted until the Late Pleistocene-Early Holocene. Owing to known difficulties in retrieving ancient DNA (aDNA) from specimens from warm climates, this research presents a molecular phylogeny for both Macrauchenia patachonica (Litopterna) and Toxodon platensis (Notoungulata) recovered using proteomics-based (liquid chromatography-tandem mass spectrometry) sequencing analyses of bone collagen. The results place both taxa in a clade that is monophyletic with the perissodactyls, which today are represented by horses, rhinoceroses and tapirs.
No large group of recently extinct placental mammals remains as evolutionarily cryptic as the approximately 280 genera grouped as 'South American native ungulates'. To Charles Darwin 1,2 , who first collected their remains, they included perhaps the 'strangest animal[s] ever discovered'. Today, much like 180 years ago, it is no clearer whether they had one origin or several, arose before or after the Cretaceous/Palaeogene transition 66.2 million years ago 3 , or are more likely to belong with the elephants and sirenians of superorder Afrotheria than with the euungulates (cattle, horses, and allies) of superorder Laurasiatheria 4–6. Morphology-based analyses have proved unconvincing because convergences are pervasive among unrelated ungulate-like placentals. Approaches using ancient DNA have also been unsuccessful, probably because of rapid DNA degradation in semitropical and temperate deposits. Here we apply proteomic analysis to screen bone samples of the Late Quaternary South American native ungulate taxa Toxodon (Notoungulata) and Macrauchenia (Litopterna) for phylogenetically informative protein sequences. For each ungulate, we obtain approximately 90% direct sequence coverage of type I collagen a1-and a2-chains, representing approximately 900 of 1,140 amino-acid residues for each subunit. A phylogeny is estimated from an alignment of these fossil sequences with collagen (I) gene transcripts from available mammalian genomes or mass spectrometrically derived sequence data obtained for this study. The resulting consensus tree agrees well with recent higher-level mammalian phylogenies.
The Cenozoic South American endemic pyrotheres, mostly characterized by a bilophodont dentition, are one of the most mysterious
ungulates ever known. Even though many specialists have investigated this group, the fascinating question of its origin is
still unresolved after more than a century of study. This paper provides a new description of the only known pyrothere skull,
that of Pyrotherium from the Deseadan of Patagonia. Detailed comparison of the cranial anatomy indicates strong similarities with the Notoungulata,
especially in the auditory region, as already mentioned by some authors. Intriguing similarities are also detected in the
anterior dentition of Pyrotherium and the Casamayoran notoungulate Notostylops. These resemblances suggest a unique relationship between Pyrotheria and Notoungulata, specifically between Pyrotheria and
Notostylops. These hypotheses are tested through a series of phylogenetic analyses of South American ungulate craniodental anatomy, primarily
focusing on Notoungulata and Pyrotheria. These phylogenetic analyses are the first to encompass such a diversity of South
American ungulate taxa. When including the bunodont taxon Proticia usually attributed to the Pyrotheria, the strict consensus of the analysis does not present much resolution. When excluding
it, the analysis supports the nesting of the pyrotheres within the Notoungulata via an exclusive relationship with Notostylops. This relationship is supported by both cranial and dental anatomy. The analysis also supports the position of Astrapotheria
as the sister group of the Notoungulata. It does not support, however, the monophyly of the Litopterna. The clustering of
Pyrotheria, Notoungulata, and Astrapotheria supports an isolated evolutionary history of these ungulates in South America
similar to that of afrotherian mammals in Africa. These results give rise to new perspectives in research on South American
endemic ungulate evolution.
Astraponotus Ameghino, 1901, the only valid Mustersan (late Eocene) astrapothere, typifies the Ameghino's "Capas Astraponotenses". This taxon is traditionally interpreted as structurally ancestral to all the Oligocene-Miocene astrapotheriids. However, it was imperfectly known: only isolated teeth and very partial mandibles have hitherto been described. In this contribution we provide the first description of the skull, mandible, and complete dentition of Astraponotus based on new materials from the Gran Hondonada and other Mustersan localities in central Patagonia, Argentina. The features observed in the dentition of Astraponotus are intermediate between the Casamayoran (middle Eocene) and the Oligocene-Miocene astrapotheres in the degree of hypsodonty, reduction of the dental formula, and development of accessory occlusal elements. Concordantly, the skull retains some plesiomorphies, also observed in Trigonostylops, whereas the auditory region and the basicranium are much closer to those of Parastrapotherium, Astrapotherium, and Granastrapotherium. On the other hand, the skull of Astraponotus differs from all known astrapotheres by the disproportioned height and narrowness of the braincase, the extreme reduction of the nasals and the premaxillaries, the absence of anteorbital rim, and the reduction of the frontal region. Some of these features represent cranial specializations exactly opposite to that of Astrapotherium. These characters look astonishingly derived for an Eocene astrapothere, suggesting that extreme cranial specializations occurred independently during the evolution of the order, and that Astraponotus represents a distinctive lineage from that of Astrapotherium and other Miocene forms.
Skull morphology in tapirs is particularly interesting due to the presence of a proboscis with important trophic, sensory and behavioral functions. Several studies have dealt with tapir skull osteology but chiefly in a comparative framework between fossil and recent species of tapirs. Only one study examined an aspect of cranial ontogeny, development of the sagittal crest (Holbrook. J Zool Soc Lond 2002; 256; 215). Our goal is to describe in detail the morphological changes that occur during the postnatal ontogeny of the skull in two representative tapir species, Tapirus terrestris and Tapirus indicus, and to explore possible functional consequences of their developmental trajectories. We compared qualitative features of the skull on a growth series of 46 specimens of T. terrestris ordered on the basis of the sequence of eruption and tooth wear, dividing the sample into three age classes: class Y (very young juvenile), class J (from young juvenile to young adult) and class A (full and old adult). The qualitative morphological analysis consisted of describing changes in the series in each skull bone and major skull structure, including the type and degree of transformation (e.g. appearance, fusion) of cranial features (e.g. processes, foramina) and articulations (sutures, synchondroses, and synovial joints). We then measured 23 cranial variables in 46 specimens of T. terrestris that included the entire ontogenetic series from newborn to old adults. We applied statistical multivariate techniques to describe allometric growth, and compared the results with the allometric trends calculated for a sample of 25 specimens of T. indicus. Results show that the skull structure was largely conserved throughout the postnatal ontogeny in T. terrestris, so class Y was remarkably similar to class A in overall shape, with the most significant changes localized in the masticatory apparatus, specifically the maxillary tuber as a support of the large-sized permanent postcanine dentition, and correlated changes in diastemata, mandibular body, and sagittal and nuchal crests. In the nasal region, ontogenetic remodeling affected the space for the meatal diverticulum and the surfaces for the origin of the proboscis musculature. Overall, ontogenetic trajectories exhibited more negative allometric components in T. indicus than in T. terrestris, and they shared 47.83% of allometric trends. Tapirus indicus differed most significantly from T. terrestris in the allometry of postcanine toothrows, diastemata and mandibular body. Thus, some allometric trends seem to be highly conserved among the species studied, and the changes observed showed a strong functional and likely adaptive basis in this lineage of ungulates.
Mammals are the dominant large animals of today, occurring in virtually every environment. This book is an account of the remarkable fossil records that document their origin since the extinction of the dinosaurs. Tracing their evolution over the last 35 million years. For the first time presented in one single volume Kemp unveils the exciting DNA sequence evidence which coupled with fossil evidence challenges current thinking on the relationships amongst mammal and their inferred history.
A detailed comparison of the skulls of Cadurcodon (Rhinocerotoidea, Amynodontidae) and a recent tapir (Tapirus pinchaque) presents strong evidence for the existence of a proboscis in the Oligocene amynodont. A review of other fossil mammals (Astrapotherium, Palaeomastodon, Brachy-crus, and Macrauchenia) shows the range of variation among probable proboscis bearers and suggests that the key characters indicating a proboscis are the reduction of the nasals, the expansion of the nasal incision, and the presence of large snout muscle scars. Six genera of amynodonts displaying a range from primitive, intermediate, to advanced grades with regard to snout structure, are described. These grades show a good correlation with the strati-graphic history of the family. The primitive square-lipped pattern is exhibited by AMNH 107635 from the late Eocene of Mongolia. Sharamynodon was an early intermediate form which may have had a prehensile upper lip. Two Oligocene genera, Amynodontopsis and Cadurcodon, represent the line of derived amynodonts which probably had a tapir-like proboscis. The second derived group, the metamynodonts, diverged along a separate adaptive line but retained the prehensile upper lip developed in the intermediate common ancestor of both groups.
The author, referring to his previous paper on the Equine fossil remains from the cavern of Bruniquel, finds, in the preliminary illustrations of the dental characters of existing species of the Horse-kind, the requisite and much-needed basis of comparison for the determination of other fossils of the Solidungulate group, and he devotes the present paper to the elucidation of those which have reached him from Central and South America. He commences by referring to the type-specimens of teeth, from two localities in South America, on which he founded the species E. curvidens , describing it (in 1840) “as one coexisting with the Megatherium, Toxodon, &c. in that continent, and which had become extinct at a prehistoric period.”
The São Raimundo Nonato (Piauí, Northeastern Brazil) area yielded about 50 Macraucheniidae remains. The study of this material and of other fossils from Northeastern Brazil (Paraíba, Pernambuco, Rio Grande do Norte) shows a great similarity to Macrauchenia patachonica as it was defined in Argentina, which is the most abundant of the three species pertaining to the genus. Thus the taxon Xenorhinotherium bahiense created in 1988 by Cartelle et Lessa for a material discovered in the Bahia State and assigned to use for all the Quaternary Litopterns from tropical Brazil appears to be no more than a junior synonym of it. M. patachonica seems to have occupied very varied biotopes from sea level up to a high of 4000 m, and from the cold Southern Patagonia to the Equatorial Venezuela. The species lived from the Middle Pleistocene up to the beginning of the Holocene.
Tapirs (Perissodactyla: Tapiridae) are the only living vertebrates, beyond the order Proboscidea, found to possess a true proboscis, defined as a flexible tubular extension of the joint narial and upper labial musculature that serves, at least in part, to grasp food. Tapirs show only partial homology and analogy with elephants in the narial and upper labial structures, as well as in the skull bones and teeth. However, superficially similar extensions in other extant vertebrates differ greatly in anatomy and function. Therefore, they deserve new names: prorhiscis (e.g. Mammalia: Saiga tatarica), prorhinosis (e.g. Chondrichthyes: Callorhinchus spp.), prorhynchis (e.g. Osteichthyes: Campylomormyrus spp.) and progeneiontis (e.g. Osteichthyes: Gnathonemus spp.). Among non-mammalian vertebrates, no bird or reptile is known to possess a proboscis. Among fishes, there are various extensions of the rostrum, jaws, 'nose' and 'chin' that lack the required narial involvement. The skulls of extinct mammals within (e.g. deinotheres) and beyond (e.g. astrapotheres) the Proboscidea confirm that a proboscis evolved independently in several mammalian lineages before the Pliocene. This convergence with tapirs presumably reflects, in part, the advantages of concentrating the olfactory sensor on what is, effectively, the tip of a long mobile upper lip. However, the proboscis does not appear to have arisen de novo in any vertebrate post-Pliocene, and its continued evolution has apparently depended on the further development of its length, flexibility and innervations, as epitomized by elephants.
The facial region of moose Alces alces is highly divergent relative to other cervids and other ruminants. In particular, the narial region forms an expanded muzzle or proboscis that overhangs the mouth. The nose of moose provides a case study in the evolution of narial novelty within a phylogenetically well-resolved group (Cervidae). The function of the nasal apparatus of moose remains enigmatic, and new hypotheses are proposed based on our anatomical findings. Head specimens of moose and outgroup taxa were subjected to medical imaging (CT scanning), vascular injection, gross anatomical dissection, gross sectioning, and skeletonization. Moose noses are characterized by highly enlarged nostrils accompanied by specialized musculature, expanded nasal cartilages, and an increase in the connective-tissue pad serving as the termination of the alar fold. The nostrils are widely separated, and the rhinarium that encircles both nostrils in outgroups is reduced to a tiny central patch in moose. The dorsal lateral nasal cartilage is modified to form a pulley mechanism associated with the levator muscle of the upper lip. The lateral accessory nasal cartilage is enlarged and serves as an attachment site for musculature controlling the aperture of the nostril, particularly the lateralis nasi, the apical dilatators, and the rectus nasi. Bony support for narial structures is reduced. Moose show greatly enlarged nasal cartilages, and the entire osseocartilaginous apparatus is relatively much larger than in outgroups. The nasal vestibule of moose is very large and houses a system of three recesses: one rostral and one caudal to the nostrils, and one associated with the enlarged fibrofatty alar fold. As a result of the expanded nasal vestibule, osseous support for the nasal conchae (i.e. turbinates) has retracted caudally along with the bony nasal aperture. The nasoturbinate and its mucosal counterparts (dorsal nasal concha and rectal fold) are reduced. The upturned maxilloturbinate, however, is associated with an enlarged ventral nasal concha and alar fold. Moose are the only species of cervid with these particular characteristics, indicating that this anatomical configuration is indeed novel. Although functional hypotheses await testing, our anatomical findings and published behavioural observations suggest that the novel narial apparatus of moose probably has less to do with respiratory physiology than with functions pertaining specifically to the nostrils. The widely separated and laterally facing nostrils may enhance stereolfaction (i.e. extracting directional cues from gradients of odorant molecules in the environment), but other attributes of narial architecture (enlarged cartilages, specialized musculature, recesses, fibrofatty pads) suggest that this function may not have been the evolutionary driving force. Rather, these attributes suggest a mechanical function, namely, an elaborated nostril-closing system.
The trunk-like proboscis of tapirs provides a prime case study in the evolution of anatomical novelty. Morphological study of this unique structure was undertaken employing several specimens and a combination of analytical techniques: gross anatomical dissection, radiographic imaging and histological sectioning. Evolution of the proboscis of tapirs entailed wholesale transformation of the narial apparatus and facial architecture relative to perissodactyl outgroups. This transformation involved retraction and reduction of the bony and cartilaginous facial skeleton, such that several structures present in outgroups are completely absent in tapirs, including cartilages surrounding the nasal vestibule (e.g. alar and medial accessory cartilages, rostral portion of the nasal septum) and associated musculature (dilatator naris apicalis, lateralis nasi pars ventralis). At the same time, soft tissues surrounding the upper lip and nose became elaborated to form a mobile, ¯eshy proboscis. Several key facial muscles (e.g. levator labii superioris, levator nasolabialis, caninus, lateralis nasi) have been co-opted to function in movement of the proboscis. The nasal vestibule is expanded and occupies approximately 75% of the nasal cavity. Vestibular expansion has compressed and simpli®ed caudal components of the nasal cavity (e.g. reduction of dorsal and middle nasal conchae, loss of plica recta and plica basalis). The airway has become dorsally arched causing the ventral conchal complex to become inclined relative to the long axis of the skull. A few anatomical enigmas remain, such as the complicated maxilloturbinate that rostrally contacts the nasal septum and vomeronasal organ. Similarly, the meatal diverticulum, despite being both ancient and anatomically complex, has no obvious functional signi®cance; it is clear that it is not homologous to the nasal diverticulum of horses and other equids. The reduction of the osseocartilaginous portion of the proboscis, coupled with expansion of the muscular and connective tissue components, has resulted in an organ that is best interpreted as a muscular hydrostat.
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