September 3, 2018 report
A portrait of ancient elephant-like mammals drawn from multiproxy analysis
Although world-famous consulting detective Sherlock Holmes prided himself on his deductive prowess, in truth, a great many of his astounding observations resulted from inductive reasoning, by which he arrived at conclusions about events that he did not observe based on the evidence at hand. Similarly, biologists, ecologists and paleontologists strive to describe the world that existed before humans could observe or record it, based only on fossil information. Via induction, they attempt to reconstruct the prevailing climate during biological epochs, the dietary habits and behaviors of extinct animals, and the lineages of creatures for which sparse fossil evidence is available.
When Holmes examines the scene of a crime, he observes everything around him to collect multiple data points from which to draw conclusions. Modern paleontology might describe this as a multiproxy methodology, in which the analysis is complemented by multiple sources of information. A recent multiproxy analysis by an international collaborative of researchers has produced a vivid picture of the dietary habits of extinct proboscideans in Central Chile, thereby also informing a picture of South American microclimates that Holmes might approve of.
Gompotheres were elephant-like mammals that lived 12 to 1.6 million years ago during the Miocene and Pliocene epochs. Chilean gompotheres were the only group of proboscideans to reach South America, and survived to the end of the Pleistocene. Biologists refer to gompotheres as "ecosystem engineers," animals that significantly modify their habitats. They strongly affect species richness and geographic heterogeneity within their domains.
Paleontologists have recognized an array of dietary categories based on the dental morphology evidenced in fossils, including browsing, grazing and mixed feeding. However, because dietary patterns are strongly influenced by the environment, dental morphology alone may not provide enough evidence to draw conclusions about dietary habits. For the current study, the researchers analyzed multiple points of evidence to determine the diets of Chilean gompotheres, including stable isotopes, dental microwear, and dental calculus microfossils derived from molar fossils found at 30 Late Pleistocene sites.
"The advantage of this multiproxy approach over others lies in that it allows the interpretation of dietary patterns at different times in the individual's life history," the authors write. "Moreover, the fact that the studied gompotheres have been found at different time periods enables us to evaluate environmental and climatic shifts that may have happened in Chile between ~30,000 and 12,000 cal y. B.P."
Gompotheres have previously been classified as browsers based on their dental morphology. Browsers favor soft shoots, fruit and leaves, by contrast with grazers, which eat grass and ground vegetation. But consider modern elephants, which have grazing dental morphology. The authors observe that elephants are mixed-feeders, with a tendency toward browsing.
The multiple sources of evidence, with heavy weight on the decay of the isotopes present in the samples, led the researchers to the conclusion that most of the feeding took place in closed environments. They also conclude that the diet of gompotheres was more influenced by resource availability than by potential dietary range. However, samples from North Central Chile had evidence of an exclusive leaf-browsing environment. This reflects the environmental variability of the transition from the Pleistocene to the Early Holocene.
One Querero specimen had evidence of an open, dry environment, and was particularly valuable to the multiproxy analysis—although dental microwear and dental calculus microfossil analysis suggested it was a leaf browser, the bulk of the evidence favored the conclusion that it lived in a more arid environment in which woodland and shrub striatum were predominant.
The researchers believe that their approach prevents misleading conclusions that can be drawn from single sources, and that it is highly applicable to the study of such biologically rich regions as South America. The study, titled "Multiproxy evidence for leaf-browsing and closed habitats in extinct proboscideans (Mammalia, Proboscidea) from Central Chile," is published in the Proceedings of the National Academy of Sciences.
Proboscideans are so-called ecosystem engineers and are considered key players in hypotheses about Late Pleistocene megafaunal extinctions. However, knowledge about the autoecology and chronology of the proboscideans in South America is still open to debate and raises controversial views. Here, we used a range of multiproxy approaches and new radiocarbon datings to study the autoecology of Chilean gomphotheres, the only group of proboscideans to reach South America during the Great American Biotic Interchange (∼3.1 to 2.7 million years before present). As part of this study, we analyzed stable isotopes, dental microwear, and dental calculus microfossils on gomphothere molars from 30 Late Pleistocene sites (31° to 42°S). These proxies provided different scales of temporal resolution, which were then combined to assess the dietary and habitat patterns of these proboscideans. The multiproxy study suggests that most foraging took place in relatively closed environments. In Central Chile, there is a positive correlation between lower δ13C values and an increasing consumption of arboreal/scrub elements. Analyses of dental microwear and calculus microfossils have verified these leaf-browsing feeding habits. From a comparative perspective, the dietary pattern of South American gomphotheres appears to be constrained more by resource availability than by the potential dietary range of the individual taxa. This multiproxy study is aimed at increasing knowledge of the life history of gomphotheres and thus follows an issue considered one of the greatest challenges for paleontology in South America, recently pointed out by the need to thoroughly understand the role of ecological engineers before making predictions about the consequences of ecosystem defaunation.
© 2018 Phys.org