Another Use for Shovel Tusks

Whenever I visit  New York’s American Museum of Natural History, I can’t leave without briefly passing through the fourth floor fossil halls. The rows of skeletal displays are those rare places where natural history and the history of science intertwine. Out of all the exhibits, though, one of my favorites features a shovel-mouthed relative of modern elephants named Platybelodon. Tucked away in a glass case in the shadow of the museum’s mammoth skeleton is a growth series documenting the development of the Platybelodon jaw from juvenile to adult, and, like many AMNH displays, the fossils within it are closely tied to the history of the museum.

During the 1920s and early 1930s, the AMNH sent teams of experts to the rich fossil outcrops of Mongolia and China in search of unknown chapters in prehistory. The naturalists found an abundance of wonderful fossil creatures, and among their discoveries were two quarries full of Platybelodon skeletons in China’s Tunggur Formation. Found in 1928 and excavated in 1930, the sites yielded a nearly complete growth series for this strange, elephantine beast, which paleontologists Henry Fairfield Osborn, Walter Granger, and Roy Chapman Andrews described in 1932.

At first, the strange anatomy of the animals from China seemed to most closely resemble that of an extinct proboscidean which had recently been found in the strata of Nebraska by Erwin Hinckly Barbour. Barbour called his animal Amebelodon, and it was unique among fossil elephants in have an elongated lower jaw tipped with two flattened tusks that created a kind of thin spade at the tip. Amebelodon was a shovel-tusker.

But, by 1931, Osborn changed his mind. The new fossils from China were dead ringers for a genus the paleontologist Aleksei Alekseevich Borissiak named Platybelodon in 1927 from bones found in southwestern Russia. Next to the Amebelodon of Barbour, Platybelodon had a shorter lower jaw with a broader and deeper scoop at the end. The comparison with a shovel was unavoidable, and Osborn speculated that Platybelodon was “[a]dapted to uprooting bulbous plants, [and] it frequented the shallow lake waters of ancient Gobi.”

Even better, the two Platybelodon bonebeds Granger and his colleagues worked yielded about twenty six individual animals of different ages, including a specimen Osborn regarded as a fossilized fetus. (Why so many individuals became preserved together is unknown, and may not be discernible if the AMNH expeditions destroyed the geologic context in which the fossils were found.) The bizarre elephants were not born with broad shovel-jaws. The jaw of the youngest individual had almost no scoop to speak of, and as these animals grew the middle of the jaw pinched in while the tip became broad, creating the shape of a shovel with a Y-shaped handle.

Paired with the idea that the Tunggur Formation indicated a landscape dotted with shallow, still bodies of water around 12 million years ago, the thought that these elephants dwelt around the edges of lakes seemed reasonable. Platybelodon families would wade into the lakes and swamps to shovel heaping masses of juicy water plants into their mouths, and Osborn supported his hypothesis by citing a peculiar pattern of wear on the tips of their squared-off incisor teeth:

It seems probable that this bevel was produced by abrasion against a smooth rock or against the bottom of a shallow pond or stream in the process of scooping up vegetation, a process in which the broad short trunk was used in connection with the mandibular scoop.

The elongated scoop-jaw of North America’s Amebelodon was taken as an indication that it must have been doing something similar, and for decades afterward both fossil elephants were restored wandering the edges of lakes or standing among ancient lily pads. But just because a lower jaw looks like a shovel doesn’t mean it was used like one. For one thing, subsequent research showed that Platybelodon lived in a mosaic environment of grasslands, tree stands, and meandering rivers. There were almost no large bodies of still water, and Amebelodon occupied similar, relatively dry haunts.

Paleontologist David Lambert also proposed a different explanation for the “bevel” Osborn described. As reported in a 1992 Paleobiology paper, Lambert examined the wear patterns on Platybelodon and Amebelodon teeth with the intention of reexamining the classic restorations of these animals.

Lambert did not find any solid evidence that either animal was scooping up soft food. The wear patterns on the lower Amebelodon tusks appeared to indicate that it scraped bark from trees and was a cosmopolitan feeder. Platybelodon, on the other hand, may have been doing something truly unique. The distinct bevel on Platybelodon incisors, Lambert proposed, was created by the living animals rubbing plants on them in order to cut them. Instead of the flat, floppy trunk of the classic restorations, Platybelodon had a typical, flexible elephant trunk that it used in conjunction with its unique teeth to saw through tough plants.

Could Platybelodon have scooped up water plants in its jaws? Probably – at least in the right circumstances – but there is no indication that they did so with any regularity. Lambert’s hypothesis has not fully resolved the mystery of this shovel-tusker, though. The idea that Platybelodon shred their food on their specialized tusks is reasonable, but, since the shovel-tuskers are relatively poorly studied, there may be other explanations for the tooth-wear patterns that have not been considered yet. Amebelodon and Platybelodon were not sifters of Miocene muck, but we still have only a very general understanding of their unique natural history and evolution.

Author’s Note: I’m a little too short on time to finish up a new post for this afternoon, so here’s a revised essay from the archives that is a fitting follow-up to Saturday’s post on American mastodon tusks.

References:

Lambert, D. (1992). The feeding habits of the shovel-tusked gomphotheres: evidence from tusk wear patterns Paleobiology, 18 (2), 132-147.

Osborn, H.; Granger, W. (1931). The shovel-tuskers, Amebelodontinae, of Central Asia American Museum novitates (470), 1-12.

Osborn, H.; Granger, W.; Andrews, R. (1932). Platybelodon grangeri, three growth stages, and a new serridentine from Mongolia American Museum novitates (537), 1-13.

Wang, X.; Qiu, Z.; Opdyke, N. (2003). Litho-, bio-, and magnetostratigraphy and paleoenvironment of Tunggur Formation (middle Miocene) in central Inner Mongolia, China. American Museum novitates (3411), 1-31.