Paleobiology

A mara or Patagonian hare (Dolichotis patagonicus), an ungulate-like rodent from southern South America.

A mara or Patagonian hare (Dolichotis patagonum), an ungulate-like rodent from southern South America. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

Understanding how extinct mammals lived is what attracted me to the field of paleontology in the first place. Living mammals encompass an amazing diversity of lifestyles and adaptations that are even greater when extinct species are included. Paleobiological studies aim to reconstruct the characteristics and habits of these species. This can be particularly challenging for groups like notoungulates, which have no living representatives, and sloths, which have only a few living members that are quite unlike most of their extinct relatives. In such cases, it is difficult to know which assumptions can be safely made and what living mammals be might the most appropriate models for a particular feature. One way to deal with this issue is to compare an extinct species to a wide variety of living mammals rather than a single group. For example, one might compare the limb bones of notoungulates to those of modern rodents, carnivorans, and ungulates in order to understand how they moved. Another approach is to use a “taxon-free” method, which is one based on physical laws or similar principles that should apply equally to all mammals regardless of their evolutionary relationships. A good example of this approach is finite element analysis, which uses the microscopic structure of a skull or a limb bone (as determined by CT scanning) to understand the types of forces it experienced and how it functioned when the animal was alive.

  • Diet: What an animal eats is one of the most fundamental aspects of its biology. Knowing an animal’s diet can also help determine the type of environment in which it lived. Most of my investigations of diet have focused on native ungulates, but I am also starting to look at diet in rodents and meat-eating marsupials (sparassodonts). Determining whether these animals mainly ate plants (herbivorous) or meat (carnivorous) it is fairly straightforward based on the structure of their teeth. What is more difficult is determining the types and proportions of different foods they ate. For example, in the case of an herbivore, it is useful to know whether it mainly ate fruit (frugivorous) or leaves (folivorous) and, if the latter, whether those leaves were mainly of trees and shrubs, grasses and other low-growing plants, or a mixture of the two.
Microwear from a modern camel (Camelus dromedarius) at 35x magnification.

Microwear from a modern camel (Camelus dromedarius) at 35x magnification. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

  • Microwear: The very small marks that are left on a mammal’s teeth by the food it eats are collectively known as microwear. It is called microwear because such marks can only be seen well using a microscope. In living mammals, microwear varies depending on the food an animal eats. Therefore, the microwear on a fossil tooth can provide information about what an animal ate during the last week or two of its life. In 2008, Beth Townsend and I were the first to apply this technique to notoungulates. We came to the surprising conclusion that the particular species we studied were not primarily eating grass, as had traditionally been supposed, but rather less abrasive vegetation. Since then, we have been collecting data on other species from the same site in order to gain a better understand of the entire native ungulate fauna. Also in 2008, we published a study of microwear in modern South American rodents. We plan to apply our findings to extinct rodents as well as smaller, rodent-like notoungulates.
High, rounded upper molar cusps of a modern elk (Cervus elaphus), an ungulate that feeds on a mixture of vegetation.

High, rounded upper molar cusps of a modern elk (Cervus elaphus), an ungulate that feeds on a mixture of vegetation. Photo by D. Croft. Reuse permitted under CC BY-NC-SA 2.0.

  • Mesowear: Like microwear, tooth cusp shape changes depending on an animal’s diet. This is known as mesowear because such changes occur at an intermediate rate: more slowly than microwear (over several months) but more rapidly than the overall form of the tooth (which only changes over many generations). In 2007, an undergraduate student and I were the first to apply this technique to notoungulates. Other students and I are now extending these studies to other native ungulates from other time periods.
  • Incisors: The large, ever-growing incisors of rodents are their most conspicuous trait. In 2011, two students and I published a study linking the size and shape of a rodent’s incisors to its diet. This pilot study examined a relatively small number of South American (caviomorph) rodents, with the idea that it might be a useful technique for determining the diet of extinct species.
Posterior views of the upper arm bone (humerus) of the notoungulate Protypotherium (center) compared with several modern carnivorans (left) and rodents (right) of similar size. Scale bar equals 5 cm.

Posterior views of the upper arm bone (humerus) of the notoungulate Protypotherium (center) compared with several modern carnivorans (left) and rodents (right) of similar size. Scale bar equals 5 cm. From Croft and Anderson (2008:fig. 3).

  • Locomotion and Body Mass:  Two other important aspects of an animal’s biology are its size (body mass) and how it moved. Inferring these characteristics can be challenging for notoungulates, not only because they lack living representatives, but also because they display an unusual mixture of traits that do not resemble those seen in living hoofed mammals. As an example, my colleagues and I published a paper in 2007  showing that mesothere notoungulates were diggers (fossorial) and probably more similar to marsupial wombats than any type of ungulate (or any other mammal for that matter).  A student of mine and I published a similar study in 2008  focused on the interathere Protypotherium. We concluded that this animal had a catlike skeleton that was rather generalized but more adapted for speed and efficiency than power and flexibility. Studies of other native ungulates are in progress.

In the fall of 2015, two local colleagues and I (Scott Simpson and Denise Su) organized a symposium at the Cleveland Museum of Natural History on these and related studies and how they can be combined with other types of data to reconstruct ancient ecosystems. We are now co-editing a volume based on the symposium that will be published as part of Springer’s Vertebrate Paleobiology and Paleoanthropology series. We anticipate a publication date of 2017. A PDF with a brief description of the volume and a preliminary table of contents is available here.

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