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(Arundel Formation) (Aptian Age), Maryland, USA
PETER M. KRANZ Dinosaur Fund
300 50th St. SE #103
Washington, D.C. 20019
In late 1887 and early 1888, J.B. Hatcher was sent by O.C. Marsh at the request of the U.S. Geological Survey to collect dinosaur fossils from the Potomac Group (Arundel Formation)
(Aptian Age) in Maryland.
Hatcher was very successful and also recovered other fossils among which were parts of a turtle's shell. This shell material (USNM #5800) was given the name Glyptops caelatus by O.P. Hay in his monograph Fossil Turtles of North America (Hay, 1908). Other specimens found in the late 1800s by Hatcher and A.B. Bibbins were referred to this genus and species. No other new specimens of any type have been reported in the scientific literature since Hay's monograph.
(See figure A for an example of part of the type specimen of Glyptops caelatus)
Since 1988 I and many others have been recovering new fossil material from the Potomac Group beds, especially from the brick clay pit formerly owned by Maryland Clay Products, now Cherokee, Sanford Co., at Muirkirk, Maryland, USA. All the new specimens discussed in this paper were collected by me from that site between 1989 and 1991. All either presently have or are in the process of receiving USNM numbers, and are or will soon be part of the collections of the U.S. National Museum, Smithsonian Institution.
Figure B, a shell fragment, can be seen to closely resemble in texture figure A and can therefore be reasonably assumed to have come from a Glyptops caelatus individual. All the remaining figures represent distinctly different textures and are clearly not from Glyptops caelatus.
Figure C shows two shell fragments with papillate texture. They bear a close similarity to Naomichelys speciosa described by Hay (1908). The fragmentary nature of the remains makes any more specific statements or definitive assignment risky. Naomichelys speciosa is described from the Upper Jurassic of Montana, USA.
Figure D is the interior and exterior view of a smooth thick shell fragment. Little can be said about it except that it seems to be the smoothest and least textured of all the specimens. It may represent a separate genus or may be just a very smooth fragment from the turtle represented by figure E.
Figure E is a maxilla, articulated phalanges, and a marginal shell fragment from USNM #466063. This extraordinary, remarkably complete, partially articulated individual is the best specimen to come from the Muirkirk clay pit to date. The specimen awaits description by a competent turtle specialist. Several experts, including Gaffney, Hutchinson, and Weems have viewed the specimen and have stated that it is "something new." The shell is almost a meter long and nearly as wide and relatively high. The shape of shell and shortness of toes suggest a possible terrestrial rather than aquatic habit, though this is uncertain. The relatively smooth, largely untextured shell shows that it is clearly different from the specimens in figures A, B, and C, although D may be just a very smooth example of this genus.
From the shell texture it is clear that while specimens in figures A and B are obviously Glyptops caelatus, it is equally clear that those figured in C and E are neither like A and B or each other, while D may be just an odd example of E. Regardless of the outcome with D, one must recognize at least two new genera of turtle from the Potomac Group, specimen USNM #466063 being as good a type as any specialist could hope for.
Life has come full circle. My Ph.D. thesis adviser, the late Ralph Gordon Johnson, began his career working on turtles and moved to marine invertebrate paleoecology. I thank him for giving me my start in marine invertebrate paleoecology from whence I have now reached fossil turtles.
I also thank M.K. Brett-Surman for his help in making arrangements for me to photograph the Smithsonian's fossil turtle specimens.
Figure A (1.5X) A fragment of shell from holotype of Glyptops caelatus (USNM #5800).
Figure B (2X) A recently discovered shell fragment presumably from a Glyptops caelatus individual.
Figure C (2X) Two shell fragments from previously unrecognized turtle genus in the Potomac Group resembling Naomichelys speciosa.
Figure D (1X) Interior and exterior view of a shell fragment from an unknown genus of turtle.
Figure E (1X) Maxilla (Note uneven cutting edge), articulated phalanges and marginal shell fragment undescribed specimen (USNM #466063).
Hay, O.P., 1908, Fossil Turtles of North America, Washington, D.C. Carnegie Institution of Washington, p. 568.