Dental Variation in Pleistocene Marsh Rabbits from Ichetucknee River, Florida

Abstract

The Ichetucknee River flows through north-central Florida and intersects several sinkhole deposits that contain a diverse assemblage of Rancholabrean (late-Pleistocene) fossil vertebrates from both forest and open-terrain ecosys-tems (Lambert and Holling 1998; Webb 1974). This assemblage, housed at the Florida Museum of Natural History (UF), contains two species of lagomorphs: Sylvilagus palustris (marsh rabbit) and Sylvilagus palustrellus (Ruez 2003). Al-though the extant marsh rabbit, S. palustris, is abundant in many fossil and modern faunas of Florida, the Ichetucknee River sample of 22 lower third premolars (p3), the most diagnostic element of fossil rabbits, is the largest known fossil collection of the species. The crenulate enamel on the PER of the p3 (Figure 1) differentiates S. palustris from other species of Sylvilagus except for S. hibbardi, S. webbi, and S. aquaticus. In the two extinct species, S. hibbardi and S. webbi, the PER does not reach the lingual border of the p3. Sylvilagus aquaticus (swamp rabbit) is larger than S. palustris and exhibits more extreme complexity of the ARs. The length of the teeth ranges from 2.83 mm to 3.72 mm, with an average of 3.31 mm. Width ranges from 2.30 mm to 2.97 mm, with an average of 2.70 mm. The number of ARs varies from 1 to 6 (average = 2.9), but there is no correlation between the number of reentrants and the size of the tooth. Additionally, there is no correlation between ARs and the depth of the AER or the length/width ratio. On all teeth, the PER has thick enamel anteriorly, and thin enamel posteri-orly. The PER becomes constricted about a third to half across the tooth. Labially, the thick enamel exhibits either one or two anterior curves, which widen the PER. In one tooth (UF 48460) the PER extends completely through the lingual edge; no enamel closes the PER. Although this pattern occurs in ontogenetically young rabbits, this tooth is parallel sided and larger (length = 3.38 mm; width = 2.72 mm) than the averages for this sample of S. palustris. Additionally, this tooth only has one well-developed AR; S. palulstris individuals typically show an increased number of ARs as they mature. I am not aware of other published records of a neotenic fossil lagomorph tooth. This specimen coupled with many others from other localities is yielding insight into the evolutionary patterns with Sylvilagus. References Cited

Full text

Dental Variation in Pleistocene Marsh Rabbits
from Ichetucknee River, Florida
Dennis R. Ruez, Jr.
➤➤
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➤Keywords: Sylvilagus, Pleistocene, Florida
The Ichetucknee River flows through north-central Florida and intersects
several sinkhole deposits that contain a diverse assemblage of Rancholabrean
(late-Pleistocene) fossil vertebrates from both forest and open-terrain ecosys-
tems (Lambert and Holling 1998; Webb 1974). This assemblage, housed at the
Florida Museum of Natural History (UF), contains two species of lagomorphs:
Sylvilagus palustris (marsh rabbit) and Sylvilagus palustrellus (Ruez 2003). Al-
though the extant marsh rabbit, S. palustris, is abundant in many fossil and
modern faunas of Florida, the Ichetucknee River sample of 22 lower third
premolars (p3), the most diagnostic element of fossil rabbits, is the largest
known fossil collection of the species.
The crenulate enamel on the PER of the p3 (Figure 1) differentiates S.
palustris from other species of Sylvilagus except for S. hibbardi, S. webbi, and S.
aquaticus. In the two extinct species, S. hibbardi and S. webbi, the PER does not
reach the lingual border of the p3. Sylvilagus aquaticus (swamp rabbit) is larger
than S. palustris and exhibits more extreme complexity of the ARs.
The length of the teeth ranges from 2.83 mm to 3.72 mm, with an average of
3.31 mm. Width ranges from 2.30 mm to 2.97 mm, with an average of 2.70 mm.
The number of ARs varies from 1 to 6 (average = 2.9), but there is no
correlation between the number of reentrants and the size of the tooth.
Additionally, there is no correlation between ARs and the depth of the AER or
the length/width ratio.
Dennis R. Ruez, Jr., Department of Environmental Studies, University of Illinois at Springfield, One
University Plaza, MS PAC 308, Springfield, IL 62703-5407; email: druez2@uis.edu
CRP 28, 2011 RUEZ 183

On all teeth, the PER has thick enamel anteriorly, and thin enamel posteri-
orly. The PER becomes constricted about a third to half across the tooth.
Labially, the thick enamel exhibits either one or two anterior curves, which
widen the PER. In one tooth (UF 48460) the PER extends completely through
the lingual edge; no enamel closes the PER. Although this pattern occurs in
ontogenetically young rabbits, this tooth is parallel sided and larger
(length = 3.38 mm; width = 2.72 mm) than the averages for this sample of S.
palustris. Additionally, this tooth only has one well-developed AR; S. palulstris
individuals typically show an increased number of ARs as they mature. I am not
aware of other published records of a neotenic fossil lagomorph tooth. This
specimen coupled with many others from other localities is yielding insight
into the evolutionary patterns with Sylvilagus.
References Cited
Lambert, W. D., and C. S. Holling 1998 Causes of Ecosystem Transformation at the End of the
Pleistocene: Evidence from Mammal Body-mass Distribution. Ecosystems 1:157–75.
Ruez, D. R., Jr. 2003 A New Record of Sylvilagus palustrellus from the Rancholabrean (Late
Pleistocene) of Florida. Current Research in the Pleistocene 20:112–13.
Webb, S. D. 1974 Chronology of Florida Pleistocene Mammals. In Pleistocene Mammals of Florida,
edited by S. D. Webb, pp. 5–31. University of Florida Press, Gainesville.
White, J. A. 1987 The Archaeolaginae (Mammalia, Lagomorpha) of North America, Excluding
Archaeolagus and Panolax. Journal of Vertebrate Paleontology 7:425–50.
——— 1991 North American Leporinae (Mammalia: Lagomorpha) from Late Miocene
(Clarendonian) to Latest Pliocene (Blancan). Journal of Vertebrate Paleontology 11:67–89.
184 RUEZ Paleontology
Figure 1. Camera lucida
drawings of the occlusal view of
four p3s of Sylvilagus palustris
from the Ichetucknee River
fauna. Terminology and
measurements follow White
(1987, 1991); measurements
were made with a optical
micrometer attached to a Wilde
monocular microscope. On each
illustration, enamel is shown in
black, dentine is white, and
cementum is gray. The bottom
two illustrations are reversed to
facilitate comparisons.
AR = anterior reentrant;
AER = anteroexternal reentrant;
PER = posteroexternal reentrant.