Recent Advances on Variability, Morpho-Functional Adaptations, Dental Terminology, and Evolution of Sloths

Abstract

The occasion of the Xenarthra Symposium during the ICVM 9 meeting allowed us to reflect on the considerable advances in the knowledge of sloths made by the "Xcommunity" over the past two decades, particularly in such aspects as locomotion, mastication, diet, dental terminology, intraspecific variation, sexual dimorphism, and phylogenetic relationships. These advancements have largely been made possible by the application of cladistic methodology (including DNA analyses) and the discovery of peculiar forms such as Diabolotherium, Thalassocnus, and Pseudoglyptodon in traditionally neglected areas such as the Chilean Andes and the Peruvian Pacific desert coast. Modern tree sloths exhibit an upside-down posture and suspensory locomotion, but the habits of fossil sloths are considerably more diverse and include locomotory modes such as inferred bipedality, quadrupedality, arboreality or semiarboreality, climbing, and an aquatic or semi-aquatic lifestyle in saltwater. Modern tree sloths are generalist browsers, but fossil sloths had browsing, grazing, or mixed feeding dietary habits. Discovery of two important sloth faunas in Brazil (Jacobina) and southern North America (Daytona Beach and Rancho La Brea) have permitted evaluation of the ontogenetic variation in Eremotherium laurillardi and the existence of possible sexual dimorphism in this sloth and in Paramylodon harlani. A new dental terminology applicable to a majority of clades has been developed, facilitating comparisons among taxa. An analysis wherein functional traits were plotted onto a phylogeny of sloths was used to determine patterns of evolutionary change across the clade. These analyses suggest that megatherioid sloths were primitively semiarboreal or possessed climbing adaptations, a feature retained in some members of the family Megalonychidae. Pedolateral stance in the hindfoot is shown to be convergently acquired in Mylodontidae and Megatheria (Nothrotheriidae + Megatheriidae), this feature serving as a synapomorphy of the latter clade. Digging adaptations can only be securely ascribed to scelidotheriine and mylodontine sloths, and the latter are also the only group of grazing sloths, the remainder being general browsers.

Evolution of body size in anteaters and sloths (Xenarthra, Pilosa): phylogeny, metabolism, diet and substrate preferences

“…The main study subject is the phylogenetic tree proposed by Pujos et al (2012), complemented by those of Pujos et al (2007) and Gaudin (2004). While the current report was in review, Amson et al (2016) provided a new phylogenetic hypothesis, in which thalassocnines were removed from Nothrotheriinae and presented as closely related to Megatheriinae.…”

“…This optimisation, resulting in a reduction of muscular mass in modern sloths, would not apply to most fossil sloths, as they were almost certainly more muscular (Bargo et al 2000;Vizcaíno et al 2006;Toledo et al 2013Toledo et al , 2015. Pujos et al (2012) optimised substrate preference categories of sloths in their cladogram, which supported previous proposals that the suspensory habits of extant sloths are convergent (Patterson & Pascual 1968;Webb 1985;Gaudin 2004;McDonald & De Iuliis 2008;Nyakatura et al 2010). These authors considered a terrestrial habit basal for all sloths, and reconstructed a basal semiarboreal substrate preference for megalonychids, megatheriids and nothrotheriids, as proposed previously by Webb (1985).…”

“…Other genera evolving in continental environments (e.g., Megalonyx) experienced less geographic constraint on body sizes. Nevertheless, maintenance of a greater diversity of body sizes and substrate uses and locomotor habits in megalonychids, including arboreal and terrestrial forms (Pujos et al 2007(Pujos et al , 2012, requires further analysis.…”

“…The clade including Bradypus, according to the phylogenetic hypotheses of Gaudin (2004) and Pujos et al (2007) (see also McDonald & De Iuliis 2008), is currently only represented by the three-toed sloth, since no known or putative fossil bradypodids have been recorded. The diversity of fossil sloths encompassed arboreal, terrestrial, fossorial and aquatic or semiaquatic forms (McDonald & De Iuliis 2008, Pujos et al 2012Amson et al 2014Amson et al , 2015, with body masses ranging from dozens of kilograms to several tons (White 1993(White , 1997Fariña et al 1998;Bargo et al 2000;De Esteban-Trivigno et al 2008;Toledo et al 2014) (Fig. 1 Bargo et al 2012;Pujos et al 2016).…”

Skeletal Anatomy of the North American Pangolin Patriomanis americana (Mammalia, Pholidota) from the Latest Eocene of Wyoming (USA)

“…In two of the three modern genera, Phataginus (Kingdon, 1997) and Manis (Heath, 1992a(Heath, , 1995, the species broadly overlap in body size, and in Smutsia the giant pangolin is nearly twice the size of the ground pangolin (Kingdon, 1997). In the end, there are not enough specimens of Patriomanis to reliably discriminate among the various possibilities; indeed, distinguishing among individual differences, species-level differences, and gender-based differences remains a difficult problem for extinct taxa in general (see discussion in Pujos et al, 2012).…”