No evidence for parallel evolution of cursorial limb adaptations among Neogene South American native ungulates (SANUs)

Abstract

During the Neogene, many North American ungulates evolved longer limbs. Presumably, this allowed them to move more efficiently or quickly in open habitats, which became more common during this interval. Evidence suggests that open habitats appeared even earlier in South America, but no study to date has investigated whether the ungulate-like mammals of South America (South American native ungulates or SANUs) evolved similar limb adaptations. We analyzed limb elongation in the two predominant SANU groups, notoungulates and litopterns, by compiling genus-level occurrences from the late Oligocene to the Pleistocene and calculating metatarsal/femur ratio (Mt:F). None of the groups or subgroups we analyzed show a pronounced increase in Mt:F across this interval, with the possible exception of proterotheriid litopterns. Proterotheriids are thought to have inhabited forested environments rather than open ones, which raises questions about the selective forces responsible for limb elongation in ungulates. Conversely, notoungulates, which are traditionally thought to have lived in open habitats, show no strong trend of increasing Mt:F across this interval. Our study suggests that the macroevolutionary trend of limb elongation in ungulate-like mammals is not universal and is highly influenced by the evolutionary affinities of the groups being analyzed.

Paleohistology of Caraguatypotherium munozi (Mammalia, Notoungulata, Mesotheriidae) from the early late Miocene of northern Chile: A preliminary ontogenetic approach

“…During the Cenozoic, the South American native ungulates (SANUs) represented a particular and diverse group of phylogenetically related clades, presently grouped in the orders Xenungulata, Pyrotheria, Astrapotheria, Notoungulata and Litopterna, plus few endemic, early Paleogene "condylarthra" [1][2][3]. These extinct mammals exhibited one of the most divergent variety of body sizes and morphological adaptations, including fully cursorial locomotion in the medium sized Proterotheriidae [4], mass graviportal standing in gigantic toxodontids, astrapotheres, and pyrotheres [5][6][7][8], and scansorial, cursorial and semi-fossorial habits in diverse lineages within the small to medium-sized typotheres [9][10][11]. Studies based on collagen protein chains (proteomics) extracted from Pleistocene fossils (i.e., Toxodontia and Litopterna), have placed SANUs within Laurasiatheria, specifically as a sister group to extant Perissodactyla (e.g.…”

Intraspecific craniomandibular and dental analysis of Pseudotypotherium exiguum (Mesotheriidae, Notoungulata) from Monte Hermoso, late Neogene, Buenos Aires Province, Argentina

“…Typotheria encompasses the smaller-sized notoungulates (Cifelli, 1985;Croft et al 2020;Reguero & Prevosti, 2010), and among them, Mesotheriidae reached the greatest body mass, which increased from the Miocene to the Pleistocene (Croft et al, 2004;Solórzano & Nuñéz-Flores, 2021). Typotheria includes species with some appendicular locomotor features reminiscent of modern cursorial taxa, such as some interatheriids and hegetotheriids (Cassini et al, 2012b;Croft & Anderson, 2008;Sinclair, 1909;Stirton, 1953), although a recent study did not support the presence of clear cursorial adaptations in this group (Croft & Lorente, 2021). The appendicular skeleton of Mesotheriidae has semifossorial traits (Shockey et al 2007;Fernández-Monescillo, et al 2018).…”