The Mammalian group Litopterna was coined by Florentino Ameghino in 1889, as a Suborder of the Perissodactyla, with the aim to include the aberrant Macrauchenia and its kin. Ameghino recognized affinities with the Laurasian clade Perissodactyla, a hypothesis sustained by some old workers. This idea was posteriorly criticised and refuted, and it was proposed that the similarities between Litopterns and Perissodactyls were acquired by convergence. In the same line of thought, together with Xenarthrans and Marsupials, South American native Ungulates were considered by George Gaylord Simpson. as comprising the 'Ancient Immigrants' Faunistic Stratum, coming from North America through a intercontinental bridge. Since then, the Litopterna weas regarded as an endemic clade exclusive of South America, with uncertain affinities to other Mammalian lineages. In line with Simpson proposal, most authors indicate that Litopterns were the descendants of 'ancient Ungulates' arriving at South America from North America by a land connection at the Latest Cretaceous–Early Paleocene. However eecent phylogenetic analysis based on protein spectrometry and DNA analyses resulted in the referral of Litopterna to Perissodactyla, in agreement with nineteenth century authors.
In a paper published in the journal Scientific Reports on 6 August 2020, Nicolás Chimento of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia' and the Consejo Nacional de Investigaciones Científicas y Técnicas, and Federico Agnolin, also of the Laboratorio de Anatomía Comparada y Evolución de los Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia' and the Consejo Nacional de Investigaciones Científicas y Técnicas, and of the Fundación de Historia Natural 'Félix de Azara' at Universidad Maimónides, present the results of a study which aimed to include representatives of Litopterna within a comprehensive morphological data matrix of basal Ungulates and to test, on the basis of morphology, the phylogenetic results obtained by previous authors, and discuss the palaeobiogeographical implications of Litoptern affinities.
The phylogenetic analysis performed by Chimento and Agnolin is congruent with recent claims, based on molecular evidence, in which Litopterna is nested within Pan-Perissodactyla, as the sister group of remaining Perissodactyls. The inclusion of Litopterna among Perissodactyls partially returns to the old ideas of Florentino Ameghino However, in contrast with Ameghino, and in agreement with Richard Cifelli, Chimento and Agnolin also consider Didolodontidae as closely related to Litopterns.
The analysis resulted in that Kollpaniidae, Didolodontidae, and Litopterna form successive stem-groups to Perissodactyla. All these taxa are united by features commonly regarded as diagnostic of perissodactyls, including metacone on P3 present but smaller than paracone, p3 metaconid present and close to protoconid, p4 entoconid absent, and m2 hypoconulid separate from hypolophid. This combination of characters is present in most known Pan-Perissodactyls, and sustains the Perissodactyl affinities of Litopterns, and South American 'Condylarths'. It is worthy to mention that such combination of characters is totally absent in North American Palaeogene Mioclaenidae 'Condylarths', such as Mioclaenus and Promioclaenus. These have been considered the group that most likely gave rise to the South American 'Condylarths' and Litopterns. Further, Kollpaniids as Molinodus, Simoclaenus and Tiuclaenus differ from typical Mioclaenids such as Promioclaenus, and resemble Didolodontids, basal Litopterns and Perissodactyls in having more bulbous lower molars, with apices of the cusps more approximated, in the longer trigonid of lower molars with paraconid more separated from metaconid, in the enlarged m3 and in the unreduced M3.
Dental similarities between South American Condylarths and Litopterns were previously noted by several authors, whom indicate that they may form a monophyletic clade, for which the name Panameriungulata is available. Chimento and Agnolin's partially agree with such proposal, being congruent in that South American Condylarths and Litopterns constitute successive stem-taxa of Perissodactyla.
South American Condylarths have been variously allied to the North American families Arctocyonidae, Hyopsodontidae, Phenacodontidae, Periptychidae, and Mioclaenidae. Richard Cifelli suggested that North American Mioclaenines could serve as structural ancestors for the South American Didolodontidae, and numerous workers sustained a close relationship between North American Mioclaenidae and South American Ungulates. However, it has recently been remarked that there is no support of close phylogenetic relationships between North American Mioclaenidae and South American Condylarths and native Ungulates. Even detailed morphological analysis did not find any derived character shared between Mioclaenidae and South American or African taxa.
In sum, Chimento and Agnolin's analysis indicates that South American Condylarths are probably not closely allied to Northern Hemisphere taxa. South American forms share a number of derived features with Perissodactyls that are absent in basal North American Ungulate taxa.
The monophyly of Kollpaniidae resulted unresolved, with Pucanodus, Molinodus, Simoclaenus and Tiuclaenus, conforming a basal polytomy to remaining Pan-Perissodactyla. Because it was not the aim of Chimento and Agnolin's analysis to resolve the internal relationships among Kollpaniids, they do not discuss the monophyly of this grouping in length.
Miguelsoria and Protolipterna were first included as belonging to Protolipternidae. In Chimento and Agnolin's analysis they are included in the Didolodontidae, following recent proposals. The clade including Didolodontidae + (Litopterna + Perissodactyla) is sustained by six unambiguous synapomorphies, namely: P4 with metacone subequal in size to paracone, M3 size subequal or larger than M2, M3 metacone lingually shifted, lingual metaconid buttress on lower molars, buccally tilted paracone on upper molars, and lower molars hypoconid large, extending on the lingual half of the talonid, invading talonid basin anterior to hypoconulid. Many of these features are typically considered as diagnostic of Perissodactyla, and were regarded as widespread among Didolodontids, such as Didolodus and Asmithwoodwardia, as well as Litopterns (e.g. Proterotherium, Victorlemoinea), and are also observed in Escribania. These traits are totally absent in other basal Ungulates including South American 'Condylarths; of the clade Kollpaniidae.
In addition to the above mentioned synapomorphies, some other key-traits shared by Didolodontids, Litopterns and Perissodactyls include a fused mandibular symphysis, twinned lower molar metaconids, and a well-defined third lobe on the last lower molar, a combination of traits previously considered as unique to Perissodactyls. Didolodontidae shares with basal Perissodactyls such as Cambaytheriids and Anthracobunids many plesiomorphic features including bunodont cheek-teeth with well-developed conules on upper molars, and the lack of any hint of lophodonty. In fact, very prominent conules are usually considered to be diagnostic of Didolodontids, but are present also in Cambaytheriids and Anthracobunids, sustaining close relationships between these clades.
Litopterns and Perissodactyls share a number of apomorphies absent in basal Ungulates and all South American 'Condylarths', including Didolodontids. These traits include a saddle-shaped navicular facet of astragalus, P3 parastyle protruding, with mesial edge concave, paracone and metacone of M1-2 about the same size, p4 paralophid well developed without paraconid, and mesially directed, m1 paralophid extending lingually and connected to mesial crest from metaconid, well-developed lower molar cristid obliqua obliquely oriented and contacting lingual cusps, resulting in a reduced valley between trigonid and talonid, m3 hypolophid complete, lingual and labial cristids subequal in length, lower molar posthypocristid absent, and m2 hypoconulid closely appressed to hypolophid. Most of the listed dental traits are related with the rearrangement of cusps due to the development of cristids and lophids, resulting in the progressive acquisition of selenodont dentition characterising Perissodactyls and Litopterns. Presence of saddle-shaped navicular facet of astragalus was recently regarded as one of the key-characters diagnosing Perissodactyla. Regarding the latter feature, it appears that the Didolodontids had a primitive-like astragali, showing an homogeneously convex navicular facet, very different from the saddle-shaped morphology reported for Litopterns and Perissodactyls.
In addition, Litopterns share a large number of postcranial traits previously regarded as typical of Perissodactyla, including mesaxonic foot symmetry with reduced metapodials I and V, and hoof-like terminal phalanges, femur with large third trochanter and prominent greater trochanter, and very expanded greater trochanter on humerus (much more expanded than in basal Condylarths as Phenacodus, Arctocyon, or Tetraclaenodon), the distal humeral articulation is strikingly narrow and high, proximally delimited by a large foramen, and the radius is anteriorly located to the ulna. These features are correlated with an increased stride length and joints with reduced rotation, a combination of characters typical of Perissodactyls.
In Litopterns, as occurs in Perissodactyls, the deltopectoral crest of humerus is not protrudent, and is restriced to the proximal half of the bone, whereas in Phenacodontids and Cambaytheriids the crest is distinct and plesiomorphically extends towards the distal end of the bone. Further, the entepicondyles and the lateral supinator crest are reduced, contrasting with Condylarths and basal Perissodactyls such as Cambaytheriids. In addition, the posterior astragalar facet of the calcaneum is angular and interlocks with the atragalus, whereas in Cambaytheriids and Condylarths this facet is rounded.
One surprising result of present analysis was the nesting of the South American Condylarth Escribania among Palaeogene Indian Cambaytheriidae and Anthracobunidae. These taxa share some unambiguous synapomorphies, including absence of lower molar metaconid buttress, individualized protostyle on upper molars, and distinct entoconulid on lower molars. Chimento and Agnolin interpret the large and well-developed cusp in the lower molars of Escribania, and sometimes described as the 'accesory cusp 2' as the entoconulid, because it is located anteromedially to the entoconid cusp.
Escribania shares with Didolodontids, Litopterns and Perissodactyls several features (e.g., m3 with entoconid similar in size to hypoconulid, entoconid and hypoconulid separate, absence of entocristid, and presence of additional cusp mesial to entoconid). However, it differs from Didolodontids in several dental traits: m3 with inflated metaconid that invades the talonid basin, relatively narrower talonid, and large trigonid with well-developed paraconid. Further, Escribania shows a large parastyle as large as the mesostyle. These features are clearly present in Cambaytheriids, such as Cambaytherium.
Perissodactyls sensu stricto, excluding Litopterns, and South American Condylarths are joined by a large combination apomorphies: absence of first metacarpal, metaconule mesially displaced on P4, preparaconule crista on upper molars joined with paracone, and m3 hypoconulid connection joining mid-hypolophid, among others.
Cambaytheriidae and Anthracobunidae result included in the sister-group of remaining Perissodactyla, in agreement with recent contributions.
Recently, on the basis of protein analysis, it has been suggested that Notoungulates and Litopterns may belong to Perissodactyla. Regarding Notoungulates, many authors indicate that they are probably not phylogenetically close to Litopterns, and that Notoungulates share features with Afrotherians. This last proposal resulted in a hot debate about Notoungulate origins. In this way, present discussion will focus on the biogeographic implications of Perissodactyl affinities for Litopterns.
Seminal studies by Florentino Ameghino on fossil Mammals from Patagonia resulted in a number of biogeographical relationships for the entire Mammalian clade. This palaeontologist proposed that most Mammals originate in the Southern Cone and from there dispersed trough the entire world, a point of view known as 'Extreme Australism'. This was refuted by Albert Gaudry, who considered that most characters linking Argentinean fossils with those of other landmasses are the result of convergences through a long time of isolated and parallel evolution, a 'Splendid Isolation' as coined by George Gaylord Simpson.
In spite that most authors (with exception of Christian de Muizon and Richard Cifelli) were not able to find special similarities between North American and South American basal Ungulates, it was clear to them that South American Condylarths undoubtedly arrived from North America. Present work failed to find a clade encompassing South American and North American Condylarths, suggesting the possibility that South American Litopterns may not be necessarily related to Northern Hemisphere taxa, in agreement with some previous authors.
In this sense, the model of South America isolation may be too biotically simplistic, as demonstrated by several studies which indicate that several Animal and Plant lineages reached South America from Africa by the Late Cretaceous and Tertiary (e.g., Legumes, Lauraceans, and several others). On this basis, authors indicate that Africa and South America may have been united by Walvis Ridge-Río Grande Rise, and Sierra Leone-Ceará Rises during the Early Tertiary. This is sustained by a large number of taxa shared between Africa and South America, but also with other landmasses and especially India, including Hystricognath Rodents, Anthropoid Monkeys, Afrotherian Mammals, Pipid Frogs, freshwater Fish (Cichlids and Aplocheiloids), Birds (Parrots, Hoatzins, Phororhacoids), and Lizards (Geckos), and Malpighiaceae, Asteraceae, and Bromeliaceae among plants. Further support for this interchange includes the finding of several lineages of Metatherians, Anthropoid Monkeys and Hystricognath Rodents in South America, indicating multiple dispersals between South America and Africa and vice-versa during the Palaeogene. As enumerated above, the evidence indicating a fluid interchange between South America and other Southern Hemisphere landmasses and India has been greatly increasing during the last years. This is in agreement with the seminal idea of René Lavocat, who suggested that the fossil record indicates closer biogeographical ties between South America and Africa than between North and South America.
A strong biotic connection between South America and former Gondwanan landmasses appears to come to light. This point is crucial for understanding early biogeographical relationships of Mammals, and more efforts are urgently need in order to analyse and criticise in detail different biogeographical scenarios.
There are striking similarities between the Latest Cretaceous and Palaeogene faunas and floras of former Gondwanan continents, including South America, Africa, and India. Jose Bonaparte noted that Mesozoic faunas from India were undoubtely Gondwanan in origin. In contrast, authors agree that the collision of India with Asia during the latest Cretaceous or Palaeogene resulted in an important faunistic exchange, and conclude that Palaeogene faunas from India were entirely composed by Laurasian taxa.
However, some recent workers sustained an important influence of Gondwanan biogeographical ties on India up to the early Tertiary. New findings suggest that by Eocene times Indian faunas were 'mixed', having both European and Gondwanan lineages. Typically Gondwanan taxa include Madtsoiid Snakes, Dyrosaurid Crocodiles and Pelomedusoid Turtles. More recently, Adapisoriculid Mammals with strong Gondwanan ties were reported for the first time in the early Eocene of India.
Chimento and Agnolin's analysis resulted in the shared presence of basal Perissodactyls in both India and South America. Further, the genus Escribania was included as the sister group of the Indian clade Cambaytheriidae + Anthracobunidae. In this way, Perissodactyls constitute another clade that adds to the list of taxa shared by India and South America. It is possible that as soon as the fossil record of Palaeogene faunas of India becomes improved, the list of taxa shared by both landmasses might increase.
Two main hypotheses explaining occurrence of Gondwanan faunas on India have become predominant. The first hypothesis proposed that these Gondwanan taxa may be the descendants of taxa already present by Cretaceous times that survived the Cretaceous/Palaeocene boundary. The second hypothesis sustain that a dispersal of Gondwanan taxa occurred from North Africa along the margins of the Neotethys to India. In this regard, an island arch (Oman–Kohistan–Dras) has been the route of migration proposed between Africa and India, during the Latest Cretaceous. Because of the meagre fossil record, both hypotheses still lack important empiric support. However, because Perissodactyls lack Cretaceous records, the shared presence of these taxa in both South America and India (and possibly Africa) may indicate Early Tertiary dispersal of Gondwanan taxa between India and North Africa.
The first works that deal with the origin of Hoofed Mammals indicate an Holarctic craddle for the Perissodactyla, particularly North American or Asiatic origins.
However, in the last decades many authors proposed that Perissodactyls may have originated on India prior to its collision with Asia. Under this hypothesis the Indian plate may have acted as a 'Noah´s Ark' during the Cretaceous and Palaeocene. Then, India carried Gondwanan forms to Asia after the break-up of the Gondwana super continent. This 'Out of India' model was followed with modifications by some authors whom sustained that Indo-Pakistan area was most likely the center of origin for the Perissodactyls. Further, it has been suggested that stem-Perissodactyla could have dispersed to India from Africa, by early Palaeocene, and then, given rise to Perissodactyla before contact of India with Asia. In partial agreement with these contributions, present phylogenetic analysis indicates that pan-perissodactyls were widespread on southern continents, particularly in India and South America (and possibly in Africa) by early Tertiary times. This suggests that the southern continents may have played an important role in the early evolution and radiation of Hoofed Mammals.
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