Isotope data from amino acids indicate Darwin\u2019s ground sloth was not an herbivore

Julia V Tejada,Lizette Bermudez,Tamsin C O'Connell,Natalie Wallsgrove,Brian N Popp,Carmen Capuñay,Ross Macphee,John J Flynn,Thure E Cerling

doi:10.1038/s41598-021-97996-9

Abstract

Fossil sloths are regarded as obligate herbivores for reasons including peculiarities of their craniodental morphology and that all living sloths feed exclusively on plants. We challenge this view based on isotopic analyses of nitrogen of specific amino acids, which show that Darwin’s ground sloth Mylodon darwinii was an opportunistic omnivore. This direct evidence of omnivory in an ancient sloth requires reevaluation of the ecological structure of South American Cenozoic mammalian communities, as sloths represented a major component of these ecosystems across the past 34 Myr. Furthermore, by analyzing modern mammals with known diets, we provide a basis for reliable interpretation of nitrogen isotopes of amino acids of fossils. We argue that a widely used equation to determine trophic position is unnecessary, and that the relative isotopic values of the amino acids glutamate and phenylalanine alone permit reliable reconstructions of trophic positions of extant and extinct mammals.

Highlights

Fossil sloths are regarded as obligate herbivores for reasons including peculiarities of their craniodental morphology and that all living sloths feed exclusively on plants
Determining the trophic relationships of xenarthrans is crucial for understanding the evolution of Neotropical mammalian biodiversity, as well as the interplay between resource availability, ecological functions among xenarthrans, and competition within and across mammalian clades in South America over the Cenozoic
Our results document that we can accurately reconstruct the trophic position of mammal species from δ15NGlx and δ15NPhe values alone, which are preferable to relying on an equation with poorly constrained constants introducing uncertainties

Summary

Introduction

Fossil sloths are regarded as obligate herbivores for reasons including peculiarities of their craniodental morphology and that all living sloths feed exclusively on plants. Characterizing the paleoecology of South American (SA) fossil mammals is challenging, because those communities bear little resemblance to modern or fossil guilds from other continents due to marked taxonomic and phylogenetic disparities and unusually large numbers of entirely extinct clades[3] Another aspect of this perceived uniqueness is the depauperate abundance and diversity of mammalian carnivores throughout the Cenozoic. Among source amino acids, phenylalanine (Phe) has been observed to experience minimal increase in its δ15N values per each trophic level, as its main initial metabolic step, the formation of tyrosine, does not involve breaking C–N bonds[20,21,22] Constancy of these distinct patterns between Glx and Phe across organisms makes them the two canonical “trophic” and “source” amino acids[15,16,17,23]. The most popular equation 20 to calculate trophic positions (hereafter referred to as “TP Eq”) uses the δ15N values of those two amino acids, a few studies have instead proposed a combination of multiple trophic and source amino acids to calculate trophic levels[14,24,25]

Methods

Findings

Discussion

Conclusion