Research history , in which
Leptomeryx is the most common fossil mammal, in
Badlands National Park in
South Dakota The first fossils from the badlands of
South Dakota were collected by
trappers in the 1840s and soon sparkled the interest of scientists. From 1849, John Evans, an assistant of
David Dale Owen, conducted expeditions into the badlands and forwarded collected fossils to
Joseph Leidy from the
Academy of Natural Sciences of Philadelphia. Among these fossils was a fragmentary skull of a ruminant (specimen number
USNM 157), which is now thought to come from the
Brule Formation and is therefore early
Oligocene in age. In 1853, Leidy
scientifically described this skull as a new genus and species,
Leptomeryx evansi, which at that time was the oldest known ruminant from North America. Leidy noted similarities with
chevrotains and compared it with the
Java mouse-deer. The genus name
Leptomeryx is derived from the
Ancient Greek words () meaning or and () meaning . The
specific name evansi honors Evans. In 1869, Leidy provided a more comprehensive treatment of the skull in his second
monograph on the fossil fauna of the badlands, in which he reaffirmed its close relationships to chevrotains.
Ludwig Rütimeyer instead suggested a closer relationship with
camels in 1881, while
Max Schlosser supported Leidy's interpretation in 1887. Beginning in the 1870s, mayor American museums excavated in the badlands, leading to a surge in fossil discoveries, including of
Leptomeryx. In 1884,
Edward Drinker Cope described the foot anatomy of the genus, and
William Berryman Scott presented a detailed account of the entire skeleton in 1891, supporting the close relationship with chevrotains. In 1886,
John Bell Hatcher, one of the fossil collectors of
Othniel Charles Marsh from the
Yale Peabody Museum, started collecting in the badlands. In 1940, Scott published a second detailed account about
Leptomeryx. In 1873, Cope described a skull from the
Great Plains of
Colorado under the name
Trimerodus cedrensis. One year later, he declared this species as a synonym of
Leptomeryx evansi, which was followed by other authors. In 1922,
Richard Swann Lull described a second species,
L. obliquidens, based on a poorly preserved skull from
Hermosa, South Dakota.
Harold J. Cook named several additional species in 1934, of which only one,
L. exilis, is still accepted. Both
L. obliquidens and
L. exilis date to the early Oligocene. In 1935,
Erich Maren Schlaikjer described
L. yoderi based on a mandible from
Goshen County,
Wyoming, that dates to the late Eocene. In 1885, Cope expanded the known range of the genus north to
Saskatchewan, Canada, when he described a mandible from the
Cypress Hills Formation in
Assiniboia as
L. mammifer. From the same region,
Lawrence Morris Lambe named
L. speciosus in 1908 based on teeth. In the later 1960s, a mandible of the genus was documented from the
Rancho Gaitan local fauna in
Chihuahua in northern Mexico, marking the southernmost occurrence of the genus known at the time. Almost 50 years later,
Leptomeryx was discovered even further south, in the north of
Oaxaca, Mexico, in the
Yolomécatl Formation.
Systematics and species In 1893,
Karl Alfred von Zittel classified
Leptomeryx within the new subfamily Leptomerycinae, within the family
Tragulidae, the chevrotains. Although William Berryman Scott elevated the group to family level (as Leptomerycidae), subsequent authors continued to use the group at subfamily level.
Henry Fairfield Osborn classified the subfamily within the extinct family
Hypertragulidae in 1910, which was followed by Scott in a 1940
monograph. In a 1945 publication by
George Gaylord Simpson, the group was only recognised at the level of a
tribe. It was only in 1955 that
C. Lewis Gazin differentiated the group from the Hypertragulidae and established Leptomerycidae as a family of its own. Leptomerycids are classified within
ruminants but outside of
Pecora (horn bearers), and only occurred in North America from the middle Eocene to the early
Miocene. They did not possess horns, and their skulls were relatively flat, long-snouted, and lacked upper incisors. The species
L. elissae was named by
William W. Korth and
Margaret E. Diamond in 2002 based on numerous mandibles from the Brule Formation of Nebraska, but moved into its own genus,
Santuccimeryx, in 2023. Another species,
L. minimus, is possibly identical with
L. exilis, while
L. lenis is probably a synonym of
L. evansi. Two evolutionary lines can be distinguished within the genus: The first line leads begins with
L. yoderi, leading to
L. mammifer and
L. exilis, and finally to
L. obliquidens. The second line leads from
L. speciosus to
L. evansi. Both lines existed from the late Eocene until the Oligocene. They differ in the shape of the
entoconid, a
cusp of the lower
molars, which is conical in the first line but cylindrical in the second line. The
L. yoderi-
L. mammifer-
L. exilis-
L. obliquidens line showed a fluctuation in body size; size increased from
L. yoderi to
L. mammifer, decreased from
L. mammifer to
L. exilis, and increased from
L. exilis to
L. obliquidens. The second line showed a continuous reduction in body size, which probably culminated in the genus
Santuccimeryx. The second line might also include
L. agatensis, although this species is often assigned to
Pronodens, a successor of
Leptomeryx. The position of another closely related genus,
Hendryomeryx, is uncertain. Described in 1978,
Hendryomeryx combines several species, some of which were initially assigned to
Leptomeryx. Some authors consider
Hendryomeryx as a synonym of
Leptomeryx, while others see it as a predecessor of
Leptomeryx. A 2023 study by Mattison Shreero and colleagues recognizes the following species: •
L. evansi, the
type species, named by
Leidy in 1853. This is the by far most common species; in some collections it is 50 times more common than the other species. •
L. exilis, named by
Cook in 1934 based on a mandible fragment from the lower Oligocene of the Brule Formation near
Chadron, Nebraska. This was a small species. •
L. mammifer, named by
Cope in 1885 based on teeth from the
Cypress Hills Formation, Assiniboia, Saskatchewan. •
L. obliquidens, named by
Lull in 1922 from the lower Oligocene of South Dakota. This was a very large form. •
L. speciosus, named by
Lambe in 1908 based on isolated teeth from the Cypress Hills Formation of southern Saskatchewan. •
L. yoderi, named by
Schlaikjer in 1935 based on a mandible and teeth from the upper Eocene of Wyoming. This
cladogram shows the relationships within
Leptomerycidae according to Shreero and colleagues, 2023 }} == Description ==