Sloths, Folivora, are today represented by only two genera, Bradypus and Choloepus, and constitute an endemic South and Central American Mammalian clade restricted to Neotropical rain forests. By contrast, their fossil record is extremely rich and diverse, spanning chronologically from the late Eocene to the early Holocene, and geographically covering nearly all of the American supercontinent. The Mylodontidae is one of the most important Folivoran clades, representing a major subdivision of Sloths diversity. Its representatives were mainly quadrupedal and terrestrial, and in some cases developed digging capabilities. Mylodontid Sloths were particularly widespread and abundant in South America, but they also reached Central and North America in different migratory events.
Among Mylodontids, the Mylodontinae includes medium to large-sized herbivorous forms and, with a few exceptions (e.g. Mylodon and Pseudoprepotherium), they show mediolaterally wide muzzles that are indicative of grazing diets. A different morphology is observed in the sister group to Mylodontinae, Scelidotheriinae. The narrow muzzle of Scelidotheriines have been associated with more selective diets. Indeed, Scelidotheriine Sloths have long been recognised as distinctive, based on their unusual dentition and skull morphology. In 1987 Hugh McDonald noted that all Scelidotheriine species possess elongated and narrow skulls. The dentition is also buccolingually compressed, so that every tooth has a long axis that is much longer than its orthogonal width, although the orientation of the long axis varies along the toothrow. In 2004 Timothy Gaudin identified additional unique synapomorphies of Scelidotheriinae. For example, the maxilla is deeper in its midsection to accommodate the elongated tooth roots, and narrows anteriorly and posteriorly, in lateral view. This results in a ventrally convex profile of the palate in lateral view that is also unique to the group.
The Scelidotheriine fossil record ranges chronologically from the middle Miocene to the Pleistocene/Holocene transition (Friasian to Bonaerian/Lujanian South American Land Mammal Ages). In contrast to Mylodontines, Scelidotheriines were restricted to South America, and did not participate in the Great American Biotic Interchange, as a probable consequence o their ecological restrictions due to specialised feeding habits. During the Pleistocene, this clade was represented by three genera: (i) Valgipes, endemic to the intertropical region of Brazil, (ii) Scelidotherium, from the Pampas region of Argentina and Uruguay, and (iii) Catonyx (Scelidodon) a more widespread taxon known from Argentina, Bolivia, Brazil, Chile, Ecuador, Peru, and Uruguay.
According to Timothy Gaudin, features that serve to differentiate the two scelidotheriine Catonyx and Scelidotherium include: the posterior extension of the temporal fossa, which is more pronounced in the former taxon; in Catonyx, the orbital portion of the lacrimal is more expanded than the facial, whereas the two portions are equal in size in Scelidotherium; a maxillo-lacrimal contact within the orbit is present in Catonyx, whereas it is absent in Scelidotherium; the sphenorbital fissure is more posteriorly located in Scelidotherium than in Catonyx, and Scelidotherium exhibits a smaller hypoglossal foramen than Catonyx. Finally, Catonyx possesses distinct fossae for the rectus capitis muscles on the basioccipital that are lacking in Scelidotherium.
Following the latest revisions of the group Catonyx includes three species: (i) Catonyx chiliensis from the Lujanian South American Land Mammal Age of Argentina, Bolivia, Chile, Ecuador, and Peru, (ii) Catonyx cuvieri from the Lujanian South American Land Mammal Age of Brazil and Uruguay, and (iii) Catonyx tarijensis, from the Ensenadan–Lujanian South American Land Mammal Ages of Argentina, Bolivia, and Uruguay. Only Catonyx chiliensis and Catonyx tarijensis have previously been recovered from Bolivia. The two species are easily recognisable by their general body size, the shape of the nasals, the development of the sagittal crest, and the morphology of the dentition in occlusal view.
In a paper published in the journal Frontiers in Ecology and Evolution on 7 April 2020, Alberto Boscaini of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Dawid Iurino of the Dipartimento di Scienze della Terra and Paleo Factory at the Sapienza Università di Roma, Bernardino Mamani Quispe and Rubén Andrade Flores of the Departamento de Paleontología at the Museo Nacional de Historia Natural de Bolivia, Raffaele Sardella, also of the Dipartimento di Scienze della Terra and Paleo Factory at the Sapienza Università di Roma, François Pujos, also of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, and Timothy Gaudin of the Department of Biology, Geology, and Environmental Sciences at the University of Tennessee at Chattanooga, describe an almost complete skull of Catonyx tarijensis from late Pleistocene deposits of the Department of Oruro, and provide a detailed description of its skull and mandible, with special emphasis on its poorly known ear region. Boscaini et al. also provide a description of the almost complete hyoid apparatus of this species, together with the first digital reconstructions of the main endocranial structures of a Scelidotherine Sloth.
The specimen of Catonyx tarijensis MNHN-Bol V 13364 analysed by Boscaini et al. was recovered in 2013 from late Pleistocene deposits at Chokxo Pata, Ayllu Yuruma locality. This locality is situated on the western side of Lake Poopó (Municipality of Santiago de Andamarca, Sud Carangas Province, Department of Oruro, southwestern Bolivia). The specimen was recovered in a friable non-laminated yellowish sandstone of lacustrine origin, without any other associated faunistic elements. However, the late Pleistocene age of the fossils is suggested by the unconsolidated nature of the sediments and low degree of petrification of the remains, together with absolute dates obtained in neighboring areas. At the moment, a late Pleistocene age for these remains is the most plausible, given the wide distribution of Catonyx in other South American localities in this period. However, further palaeontological surveys and detailed analyses at Chokxo Pata are necessary for a reliable chronological assessment of the recovered remains. The specimen labeled as MNHN-Bol V 13364 includes an almost complete skull, both mandibular rami, most of the hyoid apparatus, an incomplete left humerus and radius, and other postcranial fragments.
MNHN-Bol V 13364 is particularly well-preserved and exhibits only slight deformation, allowing detailed descriptions of the cranial and mandibular remains, with special emphasis on the ear region and the hyoid apparatus. Moreover, Boscaini et al. report the first digital models of the endocranial cavities of a Scelidotheriine Sloth, based on scans of MNHN-Bol V 13364, taken with a Siemens Somatom Scope CT scanner at the 'Clínica Alemana' Institute of La Paz (Bolivia).
MNHN-Bol V 13364i s a complete skull, associated with both mandibular rami and most of the hyoid apparatus, incomplete left humerus and radius, and isolated postcranial fragments from late Pleistocene deposits at Chokxo Pata, Ayllu Yuruma locality (Sud Carangas Province, Department of Oruro, southwestern Bolivia). The specimen is from the Tolomosa Formation, which has been estimated to be 0.76 ± 0.03 million years old (Middle Pleistocene) based on uranium/lead dating, or 44 000-21 000 years old based upon carbon¹⁴ dating.
The skull of Catonyx tarijensis MNHN-Bol V 13364 is tubular in section, anteroposteriorly elongated, and somewhat low dorsoventrally, all typical of scelidotheriine cranial morphology.
The medial and lateral anterior processes of the nasal are separated by a deep notch, as is typical in Scelidotheriines. The anterior portion of the nasal is mediolaterally expanded and tapers posteriorly in a gradual and uniform fashion. The nasofrontal suture is almost transverse and straight.
Only the posteriormost portion of the right premaxilla is present, preserving a minor portion of the lateral ramus. The palatal portion of the maxilla exhibits a ventrally convex profile in lateral view, a characteristic feature of Scelidotheriines. The palate itself is elongated and narrow, and extends well posterior to the last uppermolariformas a broad postpalatal shelf that extends posteriorly along themedial surface of the pterygoid flange for almost half its anteroposterior length. This shelf is pierced by at least one large postpalatal foramen (the minor palatine foramen of other Mammals). The anterior palatal foramen is only preserved on the right side, where it forms a small opening emptying into a short, shallow anterior groove, roughly halfway between the lateral ramus of the premaxilla and the first molariform. The incisive foramina are largely hidden by the large medial palatal processes of the maxilla (as in Scelidotherium) and are located within an elongate median groove at the anterior end of the palate. The anterior end of the maxillopalatine suture lies medial to the third molariform, as it does in Scelidotherium.
The lacrimal is roughly quadrilateral in shape and has a relatively reduced facial exposure and an extensive orbital exposure. In this respect, Catonyx differs from Scelidotherium, in which the orbital exposure is no larger than the facial. There is a well-developed lacrimal tubercle just anterior to the single large and circular lacrimal foramen. The latter is located on the facial portion of the lacrimal and just anterior to the orbital rim. The facial exposure of the lacrimal contacts the maxilla anteriorly and the frontal posteriorly. This contrasts with the information presented by Timothy Gaudin in 2004, in which a contact between the nasal and lacrimal is considered a synapomorphy of Scelidotheres, suggesting that this feature may represent a variable feature at different taxonomic levels. The absence of contact between the nasal and the lacrimal in MNHN-Bol V 13364, differs from the arrangement depicted by Hugh McDonald for the same species. The variability of this feature was first mentioned by Lucas Kraglievich in 1923 for Scelidotherium and is now also confirmed for Catonyx. In MNHN-Bol V 13364, the orbital exposure of the lacrimal contacts the jugal and the maxilla ventrally and the frontal posteriorly. The contact between maxilla and lacrimal within the orbit is absent in Scelidotherium.
Another distinctive feature of MNHN-Bol V 13364 is the exceptional preservation of the nasopharyngeal roof. At the anterior end, both the alae and the median keel of the vomer are visible, the former well-outlined by sutures. A typical triangular presphenoid is observable posterior to the vomer. The basisphenoid is irregular in shape but reminiscent of the butterfly shape commonly observed in some Mylodontids, such as Mylodon, Glossotherium, and Paramylodon and other Sloths, such as Bradypus and Analcimorphus. The basisphenoid and basioccipital are completely fused. However, there are two strongly developed tubera which typically lie on the junction between the basisphenoid and the basioccipital, suggesting that the suture occurs in this region. The basioccipital is wide transversely and short anteroposteriorly, nearly flat but marked bilaterally by shallow depressions for the rectus capitis muscles. As noted above, these fossae are absent in Scelidotherium.
As is common in mylodontids, there is a large ovate depression near the bottom of the medial orbital wall that houses most of the major orbital foramina. The sphenopalatine/anterior palatal foramen, which is normally part of this complex, is not preserved on either side. However, the large opening, typical of Sloths, that serves as a common aperture for the optic foramen and sphenorbital fissure, is well-preserved bilaterally. Scelidotherium differs from Catonyx in the absence of a common depression for the orbital foramina, with the aperture of the optic foramen/sphenorbital fissure situated well posterior to the sphenopalatine/anterior palatal foramen. The sutural relationships of the bones surrounding the aperture for the optic foramen/sphenorbital fissure are complex in MNHN-Bol V. Indeed, four bones participate in the formation of the opening: frontal, orbitosphenoid, alisphenoid, and palatine. The frontal forms the anterior and dorsalmost portion of the aperture. The orbitosphenoid forms the roof and part of the lateral wall. The alisphenoid is the most complex, comprising a portion of the lateral wall, most of the medial wall, as well as a portion of the floor. The remaining part of the floor of the aperture is formed by the palatine. The orbitosphenoid has a small lateral exposure lateral to the aperture of the sphenorbital fissure/optic foramen, extending posteriorly to contact the squamosal and largely separating the alisphenoid and frontal. A lateral exposure of the orbitosphenoid like this is quite unusual, not only in Sloths, but for Xenarthrans in general, but has been reported in a single juvenile specimen of the extant Two-toed Sloth, Choloepus hoffmanni.
The foramen ovale of MNHN-Bol V 13364 is located between the alisphenoid, pterygoid, and squamosal. The last two elements form the bulk of the rim, with the alisphenoid contributing only a small portion of the anterior edge. In MNHN-Bol V 13364 there is also a rounded projection extending ventrally from the dorsolateral margin of the foramen ovale and partially shielding it in lateral view. This small projection is comprised of alisphenoid anteriorly and squamosal posteriorly, and bears a slightly concave articular surface laterally. This surface contacts a distinct medial facet on the mandibular condyle, and therefore we identify this as an accessory mandibular articulation, unknown in other Sloths.
Behind the foramen ovale, an ovoid, reduced sutural bone is present, just at the junction between the pterygoid and the squamosal. The pterygoid/squamosal suture extends posteriorly into the ear region. The pterygoid of Catonyx tarijensis appears less inflated basally than is the case in either Mylodon darwinii or Glossotherium robustum. In the latter two taxa, a pair of inflated areas, lateral and medial, are observed at the base of the pterygoid, whereas in Catonyx tarijensis, a single low pterygoid inflation is present. The ventral portion of the pterygoid extends ventrally into a large pterygoid flange, as is typical for Sloths. In Catonyx, the ventral margin of this flange is semicircular in shape, whereas in Scelidotherium it ismore nearly triangular.
In MNHN-Bol V 13364 many of the internal structures of the nasal cavity are visible in anterior view, including the nasal septum and the anteriormost portion of the nasoturbinates and maxilloturbinates. The nasoturbinates terminate anteriorly on the underside of the long lateral process of the nasal and delimit the dorsal nasal meatus ventrally. The surface of the maxilloturbinates form a relatively smooth and conical mass at its anterior end.
In dorsal view, the temporal lines are poorly developed and the sagittal crest is lacking, two features that are probably related to the immaturity of the individual. In MNHN-Bol V 13364, the temporal lines are only observable at the level of the poorly developed postorbital processes, and posteriorly, where they merge with the nuchal crest. Among Mylodontidae, the posterior convergence of the temporal lines and the nuchal crest is also observed in Nematherium, Octodontotherium, and Lestodontini. In Scelidotherium and other Mylodontines, the temporal lines turn ventrally, anterior to the nuchal crest. The frontoparietal suture is only partially detectable in MNHN-Bol V 13364, and shows an irregular and bilaterally asymmetric outline. The position of this suture, well posterior to the glenoid, represents a Scelidotheriine + Mylodontine synapomorphy as previously reported by Timothy Gaudin.
In posterior view, the occipital is subrectangular in shape, as is typical for Scelidotheriines, whereas the subtriangular occipital condyles are directed ventrally, as is typical for Catonyx.
The ectotympanic and themiddle ear ossicles aremissing on both sides of MNHN-Bol V 13364, but the remainder of the auditory region is well-preserved.
Both entotympanics are present and complete. As is typical for Sloths, they are located lateral to the basioccipital/basisphenoid tuber and ventral to the promontorium of the petrosal. Again as in most sloths, the entotympanic is divided into medial and lateral plates by a deep sulcus for the internal carotid artery. At the anterior end of this sulcus, the entotympanic encloses a canal through which the artery passed; the presence of this canal is a variable feature in Catonyx and Scelidotherium. A carotid foramen/canal in the entotympanic is also present in Nematherium and Pseudoprepotherium, but is missing in other Mylodontids.
On the right side of MNHN-Bol V 13364, the lateral plate of the entotympanic has a small opening near its anterior end that likely accommodated the exit of the greater petrosal nerve (vidian nerve) from the tympanic cavity. A short groove for the greater petrosal nerve extends from the opening toward the nasopharynx. On the left side, there is a much larger aperture in roughly the same position, suggesting that this difference in size could be related to the immature development of this individual.
In fact, there are several additional features in the ear region which indicate the subadult status of MNHN-Bol V 13364 including: (i) a small gap between the medial entotympanic and the basioccipital/basisphenoid tuber (visible on the left side of the skull), (ii) an unusually deep stylohyal fossa, and (iii) the presence of an open groove for the occipital artery on themastoid exposure of the petrosal. In all other Mylodontids except Nematherium, the occipital artery is either partially or fully enclosed in a canal.
The petrosalmorphology ofMNHN-Bol V 13364 is typical for Scelidotheres, and indeed for Mylodontidae as a whole. One unusual aspect of this specimen is the small size of the anteroventral process of the tegmen tympani. The aptt is variable in size and shape among |Mylodontids, but was recorded as a 'large, rugose bony mass' in prior observations of Catonyx. The small and triangular shape of the aptt in MNHN-Bol V 13364 could also be related to its subadult status.
The large jugular foramen which marks the posteromedial boundary of the ear region is typical for Mylodontids. However, the hypoglossal foramen is distinctive in a number of aspects. First, in MNHN-Bol V 13364 it is nearly identical in size to the jugular foramen, whereas in Mylodon and Glossotherium the hypoglossal foramen is larger than the jugular. Moreover, it nearly abuts the jugular foramen, the two being separated by a sharp ventral crest. In other Mylodontids, the hypoglossal foramen is located farther posteriorly.
The mandible of MNHN-Bol V 13364 shows an almost horizontal ventral border, as is typical for Mylodontids. As in other Scelidotheres, it has a very elongated and narrow mandibular spout with a rounded anterior edge.
The very deep horizontal ramus of MNHN-Bol V 13364 is characteristic of Catonyx tarijensis and contrasts with the shallower ramus in Catonyx cuvieri and Catonyx chiliensis. The condyle is transversely extended as in other Scelidotheres, but with a peculiar concave articular surface, which differs from the flat articular surface that is typical of the group. The latter is bordered medially by a second articular surface for the accessory mandibular articulation, as discussed above.
Despite the apparent subadult status of the specimen, the grooves and ridges that mark the attachments of the masticatory muscles are very well-developed. This is particularly evident along the ventrolateral margin of the angular process, where two strong grooves are present.
A large part of the hyoid apparatus of MNHN-Bol V 13364 is preserved, the available elements including the right stylohyal and epihyal, the 'V-bone' (fused basihyal and thyrohyals), and the ossified thyroid cartilage. Both ceratohyals, however, are missing. On the whole, the hyoid morphology of Catonyx reflects the general Xenarthran pattern, which consists of three paired and unfused bones (representing the anterior cornua) and an unpaired V-bone (representing the posterior cornua).
The right stylohyal ofMNHN-BolV13364 is almost complete, lacking only the articular facet for the basicranium. The body of the stylohyal is cylindrical, with a slight enlargement toward the distal facet for the epihyal as is typical in Folivora. The shaft of the stylohyal is straight in Catonyx, a condition that is observed in other Scelidotheriines such as Scelidotherium and Valgipes, whereas the stylohyal shaft is curved in mylodontines such as Mylodon, Paramylodon, Glossotherium, and Lestodon. However, the curved condition can be occasionally observed in some specimens of Scelidotherium, and the straight condition in some Paramylodon specimens. In Paramylodon harlani, Chester Stock also identified an uncommon case of fusion between the stylohyal and the epihyal. This feature, commonly recovered among the extant Sloths is rarely observed in extinct taxa such as Paramylodon harlani, Megatherium americanum, and Eremotherium laurillardi. The stylohyal-epihyal contact is flat in lateral and medial views in MNHN-Bol V 13364. This feature is also typical of Sclelidotheriines, in contrast to the more concavo-convex articulation of Mylodontines.
The epihyal of Catonyx MNHN-Bol V 13364 is shorter in length than the stylohyal, a typical condition for Pilosans. The shaft of the epihyal is wide anteroposteriorly and compressed mediolaterally, resembling the shape of the epihyal of Scelidotherium and in contrast to that observed in Glossotherium and Paramylodon, in which the epihyal decreases uniformly in width from its proximal to its distal end. Both the facets for the stylohyal and the ceratohyal are flat, again resembling the condition in Scelidotherium and contrasting with the morphology of Paramylodon and Glossotherium, in which they are concave and convex, respectively. Both ceratohyals are missing in MNHN-Bol V 13364. As in all extinct sloths, and in contrast to the living forms, they were unfused to the epihyals.
The fusion between the basihyal and the thyrohyals, forming the so-called V-bone, is a typical Xenarthran feature. The two anterior eminences that articulate with the ceratohyals are well-developed in Catonyx and similar in shape to those of Glossotherium and Paramylodon. Posteriorly, the V-bone contacts the ossified thyroid cartilage through two roughly circular and flat facets. In Catonyx, the V-bone and ossified thyroid cartilage are markedly V-shaped in anterior view, similar to the morphology observed in Scelidotherium and distinct from the U-shaped homologous elements in Glossotherium and Paramylodon.
The facial part of the respiratory tract is preserved, both externally and internally. The main turbinates of this region, the nasoturbinate (an olfactory structure) and the maxilloturbinate (a respiratory structure), are observable in the anterior view of the nasal cavity and their morphology can be traced back posteriorly thanks to the digital endocranial reconstruction.
The nasoturbinate of Catonyx tarijensis MNHN-Bol V 13364 is anteroposteriorly elongated and mediolaterally narrow. It is also inclined ventromedially in anterior view, facing the nasal septum. It constitutes the floor of the relatively large and undivided dorsal nasal meatus. The nasoturbinate reaches the anterior edge of the nasal bone anteriorly (as noted above), and extends posteriorly to the level of the postorbital process of the frontal. Unfortunately, the ethmoidal crest and the ethmoturbinates are not preserved, preventing observation of the posterior attachment point for the nasoturbinates. The nasoturbinates appear to be somewhat shorter in Chester Stock’s of Paramylodon.
Themaxilloturbinates are also partially observable. They show the same anteroposterior elongation as the nasoturbinates, but their anatomical structure is more complex. In fact, even if they are only partially preserved (broken in places and likely lacking the most delicate bony scrolls), they are clearly enrolled in a corkscrew fashion throughout most of their length. The outermost scroll begins with a dorsolateral attachment to the inner wall of the nasal cavity, and then extends medially, curving ventrally as it approaches the nasal septum, then curving again medially, dorsally, and so on. Similarly scrolled maxilloturbinates are present in Paramylodon harlani. In ventral view, the maxilloturbinates of Catonyx appear thicker, forming a continuous surface, likely corresponding to the ventral concha.
Cranial sinuses are well-developed in Catonyx tarijensis MNHNBol V 13364, extending for about two thirds of the entire skull length. Pneumatisation reaches its largest extent in the middle cranial area, where the sinuses mirror the external morphology of the cranium. More posteriorly, at the level of the squamosal, parietal, and occipital bones, pneumatisation is less dense or missing. This set of features is more similar to that of Choloepus than that of other extinct Giant Sloths like Glossotherium robustum and Paramylodon harlani, where the sinuses invade every bone all the way back to the occiput.
The frontal shows the highest level of pneumatisation, with extensive sinuses extending from its anterior to posterior edges, in both dorsal and lateral views. In lateral view, the extent of the sinuses reflects the anterior contact with both the lacrimal and the maxilla. Frontal sinuses merge posteriorly with the orbitosphenoid and the alisphenoid sinuses. In fact, the latter bones are also largely pneumatised at the level of the common aperture for the optic foramen and sphenorbital fissure, a common feature in extant and extinct Sloths.
The squamosal of MNHN-Bol V 13364 is partially pneumatised, with large sinuses in its anteriormost portion, invading the tip of the zygomatic process. However, the posterior half of the squamosal is not pneumatised, in contrast to Glossotherium, Choloepus and Bradypus. It should be noted that the squamosal pneumatisation of the extant Sloths is an epitympanic sinus continuous with the tympanic cavity ventrally, whereas that of Catonyx and Glossotherium is confined to the squamosal itself, as is typical for Mylodontids. Parietal sinuses are observable in Catonyx, but only in proximity to the frontoparietal suture, as in Choloepus. In comparison to the frontal sinuses, the parietal sinuses are smaller in size, allowing observation of the underlying brain cavity in dorsal and lateral views. In MNHN-Bol V 13364, sinuses at the level of the parietal represent the posteriormost extension of pneumatisation, which is also lacking in most of the basicranium.
Ventrally, some sinuses were detected on the cranial base. As already noted from the external aspect of MNHN-Bol V 13364, isolated sinuses are present in the pterygoid. They are placed in the middle of the descending lamina, but also posteriorly, at the posterior edge of the descending lamina, as in Glossotherium and Choloepus. Moreover, some globose and individualized sinuses, similar to those of Choloepus, are visible in the basisphenoid, forming a 'v-shaped' pneumatisatized area on the cranial base. As in extant Sloths, sinuses are lacking in the occipital.
The CT images of the mandible reveal the entire height of the lower teeth and the morphology of the mandibular canal. The left first, second, and fourth molariforms are pushed dorsally far out of their alveoli due to infilling sediment. The strongly hypselodont teeth once penetrated deeply in the mandible, reaching the mandibular canal.
The mandibular canal housed the mandibular division (V₃) of the trigeminal nerve, which exited the skull at the foramen ovale, entering the mandible through the internal aperture of the mandibular canal, located on the medial side of the coronoid process. As in all Folivorans, the mandibular canal also presents a posteroexternal opening on the lateral surface of the mandible.
The main branch of the mandibular canal is located ventrally in the dentary, extending medially to the lower dentition. The most anterior portion of the mandibular canal diverges into several rami at the level of the mandibular spout, emerging from the numerous mental foramina. The development of the mandibular canal and its rami reveal the many neurovascular terminations that occurred in this area.
The brain endocast of Catonyx tarijensis MNHNBol V 13364 is generally similar to that of other Mylodontid Sloths and Choloepus, characterised by prominent olfactory bulbs, a globose telencephalon and mediolaterally expanded cerebellar hemispheres.
In dorsal view, the olfactory bulbs of Catonyx are mediolaterally wide, as in Glossotherium and Choloepus. In MNHN-Bol V 13364, the olfactory bulbs are particularly protruded anteroposteriorly, and are connected to the telencephalon by two divergent olfactory peduncles. The olfactory bulbs are also dorsally directed in lateral view and appear elliptical in shape in anterior view.
Compared to Megatherium, Glossotherium, and Lestodon, the cerebrum of Catonyx is less domed in lateral view. It is similar to Choloepus in this respect. In the same view, the telencephalon of Catonyx appears anteroposteriorly elongated, with prominent convolutions that are well-preserved on the left hemisphere. This convolution pattern consists of orbital, olfactory, suprasylvian, ectosylvian, and occipital gyri bordered by shallow furrows represented by the lateral, presylvian, suprasylvian, and pseudosylvian sulci, as well as by the rhinal and the transverse fissures. The pyriform lobe is less developed than its homologue in Glossotherium, where it appears as a prominent bulge. In dorsal view, the longitudinal fissure is barely visible on the anterior and posterior edges of the cerebrum, appreciable as slight surface inflections, whereas it is completely missing along its middle part. A weakly marked longitudinal fissure is also present in Scelidotherium and Choloepus and differs from that in Bradypus, Megatherium, Mylodon, Lestodon, and Glossotherium, in which it is more marked and wider, particularly between the frontal lobes. Dorsally, both the frontal and temporal lobes are laterally expanded, crossed by wide gyri separated by slightly marked sulci. In dorsal view the lateral, suprasylvian, and occipital gyri, as well as the lateral and the entolateral sulci, are observable. The general pattern of convolutions in Choloepus tarijensis is comparable to that reported for Glossotherium and Choloepus.
More posteriorly, the cerebellum is mediolaterally expanded and separated from the cerebrum by a marked transverse fissure. As in Lestodon, Mylodon, Scelidotherium, Megatherium, Choloepus and Glossotherium, the cerebellar hemispheres are mediolaterally expanded and separated by a well-developed vermis. In Catonyx, the strong development of the vermis and the marked transverse fissure are also particularly clear in lateral view. As in Glossotherium, and in contrast to Megatherium, Scelidotherium and the extant Sloths, the cerebellum appears wider mediolaterally in dorsal view than the posterior portion of the cerebrum. However, when compared with Glossotherium, Catonyx shows a more anteroposteriorly compressed cerebellum, similar to the condition observed in Choloepus.
Neither the ramifications of the olfactory nerves (I) nor the canals for the optic nerves (II) are preserved in MNHN-Bol V 13364. However, the impression of the optic chiasma itself is preserved, and it is likely that the optic nerve traveled some distance forward to merge with the sphenorbital fissure as in other Mylodontids. In ventral view, the common groove for the optic foramen and the sphenorbital fissure (accommodating cranial nerves II, III, IV, V₁, V₂, and VI), as well as that for the foramen ovale (housing V₃, themandibular branch of the trigeminal nerve), are the largest andmost extended neurovascular canals in Catonyx. These openings are located at the level of the anteriormost portion of the brain and are more widely separated at their base than is the case in Glossotherium, in which the foramina converge at their origin. In lateral view, the sphenorbital fissure and the foramen ovale project forward strongly, forming an angle of about 30◦ to one another. The internal acoustic meatus appears as a small bump located under the cerebellar hemispheres, close to the well-developed jugular and hypoglossal foramina. The internal acoustic meatus is wider at its base and relatively larger than that of Glossotherium. In Catonyx tarijensis MNHN-Bol V 13364, the jugular and hypoglossal canals are similar in shape and close to each other, whereas in Glossotherium robustum the hypoglossal is relatively larger and originates farther posteriorly. This is also reflected in the external cranial anatomy of Catonyx, and a similar condition was also observed in Scelidotherium. In contrast, the hypoglossal foramen is larger than the jugular in Mylodon and Glossotherium.
The Pleistocene Scelidotheriine Sloth Catonyx tarijensis was first described on the basis of fossil remains recovered in the Tarija Valley (southern Bolivia). This locality is therefore the type locality of Catonyx tarijensis, but it also represents the site that has provided the greatest number of skeletal remains for this species. After the original description, many other remains referable to Catonyx tarijensis were recovered in Argentina and Uruguay, extending its palaeobiogeographic distribution to more southern latitudes in South America. The remains reported by Boscaini et al. represent the first record of this species from latitudes north of its type locality. They extend the geographic distribution of this taxon northward and also expand the spectrum of palaeoenvironments in which Catonyx tarijensis could survive. Palaeoenvironmental reconstructions of the Pleistocene of Tarija suggest the presence of open-county grasslands, with rivers and lakes surrounded by shrubs and scattered trees. The Bolivian Altiplano is distinguished by its higher altitude, but also its colder temperatures and dryer conditions relative to those recorded for the Tarija Valley. During the late Pleistocene, the Altiplano was characterized by periods of extreme dryness and shrinking bodies of water. The recovery of Catonyx tarijensis in the late Pleistocene of the Bolivian Altiplano suggests that this species could have been able to survive under a broad spectrum of palaeoecological conditions.
The exceptional preservation of the skull of MNHN-Bol V 13364, together with its subadult ontogenetic stage, offer the opportunity for detailed observation of its external and internal anatomy, and to characterize some anatomical areas previously unknown for Catonyx tarijensis. The subadult age of MNHN-Bol V 13364 is indicated by the presence of many sutural contacts and the poorly developed temporal ridges and sagittal crest, as well as other features detected in the ear region, such as the narrow gap between the entotympanic and the basioccipital/basisphenoid tuber, the unusually deep stylohyal fossa, the presence of an open groove for the occipital artery on the paraoccipital process of the petrosal, and the reduced anteroventral process of the tegmen tympani. The presence of clear sutures in this skull has allowed us to observe the distinct outlines of many of the cranial bones for the first time, and have revealed at least one noteworthy, unusual feature, the small lateral exposure of the orbitosphenoid posterior to the aperture for the optic foramen/sphenorbital fissure. Another probable ontogeny-related feature is the occurrence of the lacrimal-nasal contact, which is absent in MNHN-Bol V 13364, but figured as present in an adult Catonyx tarijensis specimen by Hugh McDonald. This feature was observed as variably present in Scelidotherium leptocephalum but its presence was later considered a synapomorphy of Scelidotheriinae by Timothy Gaudin.
Other anatomical regions, such as the nasopharyngeal roof, the orbital foramina, and the hyoid apparatus are described in Catonyx for first time. Among these, the hyoid apparatus has been considered indicative of lingual anatomy and function, providing valuable information on the procurement and processing of dietary items in the oral cavity. Although Pleistocene Mylodontids were generally supposed to have had strong tongues, a 2010 study suggested that Scelidotherium had significantly reduced lingual protrusion, when compared with the Mylodontine Glossotherium. This was inferred mainly from the robust and barely mobile hyoid elements of Scelidotherium, which differ from the less rigidly articulated apparatus of Glossotherium. As a result, browsing species such as Scelidotherium leptocephalum possessed long skulls and mandibular symphyses, and likely had strong prehensile lips but reduced lingual protrusion. In contrast, grazing species like Glossotherium robustum were characterised by shorter and broader skulls and mandibles, associated with reduced lips and increased lingual protrusion. On the one hand, the hyoid apparatus of Catonyx tarijensis bears a strong resemblance to that of Scelidotherium leptocephalum, further demonstrating that the morphology of the hyoid elements is phylogenetically informative. On the other hand, the strong similarity in the morphology of the hyoid apparatus among Scelidotheriine Sloths suggests similar feeding behaviors, characterised by limited lingual protrusion, with prehensile upper lips compensating for the less mobile tongue. It has been proposed that in narrow-muzzled Sloths such as Scelidotherium and Megatherium, the upper lip was prehensile and used to select the plants. However, extinct Sloths generally show a large mandibular canal, several foramina connected to the mandibular canal through multiple rami, including multiple mental foramina, as well as a wide posterior external opening of the mandibular canal, suggesting that the whole mandible was probably well-innervated and vascularised, especially in its anteriormost part. Accordingly, it is plausible that the lower lip also had greater mobility, contributing to better sensitivity and greater control of their grip while browsing.
Strong bite movements concentrated in the mandible are also in accordance with the presence in MNHN-Bol V 13364 of two facets for the articulation of the mandible with cranial glenoid fossae. Moreover, the well-developed muscle scars in the ventral margin of the dentary are indicative of marked masticatory strength.
The digital model of the brain cavity of Catonyx tarijensis MNHN-Bol V 13364 is extremely similar to that of Glossotherium robustum and Choloepus hoffmanni and other giant extinct Mylodontid Sloths. In lateral view, only minor differences are observable between Catonyx tarijensis and Glossotherium robustum in the shape of the olfactory bulbs, cerebral hemispheres, and cerebellum. Moreover, the pattern of sulci and gyri is extremely similar among extinct Mylodontids Sloths and the extant Two-toed Sloths. Although Catonyx and Scelidotherium show a weakly marked longitudinal fissure in comparison to other Mylodontidae, the general pattern of convolutions is similar to that in other members of the clade. In general, the similar shape of the brain cavity and the shared pattern of convolutions among extinct Mylodontid Sloths and the living Two-toed Sloths, as well as the similar development of cranial sinuses in Cholopeus and the Scelidotheriines, is noteworthy. Indeed, recentmolecular-based phylogenetic studies recovered the Two-toed Sloths as living representatives of Mylodontidae. Further CT-scanning data on other Mylodontids and Antillean taxa is required to test this hypothesis, but the present study reveals the importance of accounting for novel morphological information from previously unexplored sources.
It should also be noted that there are also endocranial features which lack phylogenetic consistency among Mylodontid Sloths. Differences are evident in the pattern by which cranial nerves leave the brain cavity. As in Glossotherium robustum, but unlike extant Sloths, Catonyx tarijensis possesses grooves for the optic foramen/sphenorbital fissure, the foramen ovale, and the hypoglossal foramen that are enlarged relative to other foramina transmitting cranial nerves. This suggests a greater development of the trigeminal and hypoglossal nerves, compared to other cranial nerves. However, in Catonyx tarijensis, this relative difference in the size of cranial nerve grooves is not as marked as in Glossotherium robustum. This is particularly true for the groove for the hypoglossal nerve (XII), which is the only nerve leaving the skull from the hypoglossal foramen. In Catonyx tarijensis the hypoglossal foramen is roughly equivalent in size to the jugular foramen, whereas in Glossotherium robustum the hypoglossal foramen is larger. The hypoglossal nerve, which innervates the glossal musculature, controls most of the intrinsic and extrinsic lingual musculature. Together with evidence from the hyoid apparatus discussed above, this morphology can be viewed as further indirect evidence of more limited lingual protrusion of Catonyx tarijensis in comparison to Glossotherium robustum.
Other differences between Catonyx and Glossotherium are noted in the morphology of their cranial sinuses. In general, cranial pneumatisation is markedly reduced in the posteriormost portion of the skull of Catonyx tarijensis, relative to that observed in Glossotherium robustum. As in Choloepus, the sinuses of Catonyx become smaller and more separate from one another at the frontoparietal suture, and are absent in the more posterior parts of the braincase. Preliminary observations suggest that this morphology is present in other Scelidotheriinae, emphasising the possible phylogenetic value of cranial pneumatization anatomy. In fact, for other large Mammalian herbivores, the extent of cranial sinuses varies at the interspecific level according to both phylogeny and general body size. New body size estimations for Mylodontid Sloths predicted a larger body mass for Catonyx tarijensis (about 1640 kg) than Glossotherium robustum (about 1480 kg), suggesting that among both Xenarthrans and Bovids, sinus organisation could be driven primarily by phylogeny, and only secondarily related to general body mass.
However, the presence of functional or developmental constraints for explaining this alternative morphology cannot currently be refuted. Scelidotheriines have narrower heads than Mylodontines and so may not have required as much pneumatisation to reduce the weight of the head. However, it is also true that sinuses in Sloths are more frequently present in the middle cranial region, extending posteriorly only in some cases suggesting that this feature could be ontogeny-related. Thus, the question of the relative contributions of phylogeny and function to the pneumatic patterns observed in extinct sloths is clearly one that will require further study.
Boscaini et al. report novel data on the external and internal cranial anatomy of the Scelidotheriine Sloth Catonyx tarijensis, further extending knowledge on the cranial morphology of South American extinct Sloths. This discovery, from late Pleistocene deposits of the Department of Oruro (southwestern Bolivia), allowed them to extend the palaeobiogeographic range of Catonyx tarijensis to more northern latitudes, as well as to the high altitudes of the Bolivian Altiplano.
The specimen described by Boscaini et al., a particularly well-preserved skull with associated mandible and hyoid apparatus, corresponds to a subadult individual of Catonyx tarijensis. Combined information from the external and the internal anatomy, obtained through CT-scanning followed by digital modeling techniques, allowed Boscaini et al. to analyse several anatomical regions that were unknown for this taxon.
Among these, the ear region, the nasopharyngeal area and the hyoid elements revealed several phylogenetically and functionally informative features. Digital models permitted observations of the brain cavity, neurovascular grooves and cranial sinuses, and comparisons of these features with other Pleistocene Mylodontids.
The information presented in Boscaini et al.'s report confirms previous hypotheses on inferred modes of food intake among extinct Scelidotheriine Sloths. According to the data now available, Catonyx tarijensis was likely a browsing species, which tore vegetation mainly using its strong lips, rather than the tongue. This habit was probably common among Scelidotheriinae and contrasts with that present in its sister clade, Mylodontinae, whose members were predominantly grazing species with smaller lips and more strongly protruding tongues.
Boscaini et al.'s study represents a further step in assembling broader morphological comparisons of digital endocranial models among extinct Sloths, and emphasises the importance of applying these new methodologies for understanding the evolution of this Mammalian group.
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