Cenozoic

Published on April 20th, 2022 | by David Marshall

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Episode 138: Hispaniolan Sloths

Sloths (or do you pronounce it “sloths”?), are a group of tree-dwelling xenarthrans from South and Central America. They are well known for their sedentary lifestyles where they just hang around and seemingly do fairly little. But has this always been the case? When we look back at the fossil record of sloths, what kinds of ecologies do we see? How far back does their fossil record actually go?

In this episode, we speak to Dr Robert McAfee (Philadelphia College of Osteopathic Medicine – Georgia) about his research looking into the fossil record of sloths in all of its “beautiful absurdity”. His work has been focussed on the island of Hispaniola (Haiti and the Dominican Republic) and the remarkably rich cave deposits found there.

Hispaniola is a Caribbean island divided into Haiti in the West and the Dominican Republic in the East.

The island hosts numerous caves in which many fossils, including those of sloths, have been discovered. Many of these caves are underwater and would have been inhabited when sea levels were much lower than at present.

The underwater caves have preserved the bones of these animals remarkably well and thanks to the efforts of divers such as the Dominican Republic Speleological Society, fossil material is made available for researchers to study.

With so much material available to study, Dr Robert McAfee set out to explore the intraspecific variation within Hispaniolan sloths. Within each genus, were there any observable differences between the sexes, geographical location or age of deposit? The lengths of the arm bones humerus, raius and ulna were used as a proxy for body mass.

MHD-217B Neocnus cf. dousman, Radius right by VirtualNHM on Sketchfab

Radius of Neocnus (Mammalia: Pilosa: Megalonyhidae) from the late Pleistocene-early Holocene of Hispaniola (easternmost occurrence).

Radius of Acratocnus ye exhibiting extensive pathologies (left) compared to a normal radius (right). Elements are not from opposite sides of the body.
Length vs width dimensions for Parocnus specimens. The groupings of these points into two clusters suggests long and short forms which is strong evidence for sexual dimorphism. This was the case for all the Hispaniolan sloths. From McAfee & Beery 2021.
When compared across localities, Parocnus showed some significant size differentiation. The fossils from Parque Nacional del Este were approximately 15% shorter than other Parocnus specimens and these were ultimately described as the new species Parocnus dominicanus. This was an entirely unexpected result and only came to light thanks to this comparative analysis.
Image: Max element length of specimens separated by localities. 1. Haiti, 2. Central Dominican Republic, 3. Parque Nacional del Este. From McAfee & Beery 2021.
With so much material left to study, there could still be other new species hiding within the data. Here, Drs. Cooke and McAfee try to identify previously unstudied fossil specimens in the Museo del Hombre Domincano.

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