Turtles from the late Hemphillian (latest Miocene) of Knox County, Nebraska.
Records of Hemphillian Mammal Age (late Miocene) turtles from the Great Plains of North America are so few that the occurrence of several species from the late Hemphillian (about five million years ago) of northeastern Nebraska is noteworthy. Moreover, the turtles importantly add to the overall knowledge of the herpetofauna of this time period and region.
The fossil turtle bones reported here were collected by University of Nebraska State Museum (UNSM) and Michigan State University Museum (MSUVP) field parties from two sites in Knox County, northeastern Nebraska.
Santee Site. -- This site (UNSM locality Kn-111) is located in the NE 1/4, NW 1/4, sec. 25, T 33 N, R 5 W, northern Knox County (Voorhies, 1977). The site represents the late Hemphillian Mammal Age (Woodburne, 1987; Voorhies, 1988), and there is a fission-track date of 5.0 [+ or -] 0.2 on a volcanic ash overlying the site (Boellstorff, 1976).
Devils Nest Airstrip Site. -- This site (UNSM locality Kx-113) is a north-northwest trending dirt air strip with its north end near the center of SE 1/4, NW 1/4, sec. 24, T 33 N, R 4 W, Knox County. The site is temporally equivalent in time and space to the Santee site (Voorhies, 1988).
Following is an annotated list of the turtles of the Santee and Devils Nest Airstrip local faunas. Identifications are based on direct comparison of the fossils with Recent and fossil turtle material in the collections of the Michigan State University Museum and Georgia College.
Material. -- Santee: one left hypoplastron, UNSM 56967.
Remarks. -- The dorsal sculpturing of this well-preserved plastral plate easily places it in the genus Apalone. Diagnostic characters that identify the element to Apalone spinifera and separate it from living A. ferox and A. mutica, and the extinct Barstovian species Trionyx (?Apalone) guinni Holman are as follows (Holman, 1982): 1) posterior margin moderately concave; 2) posteromedial processes long and striated; 3) notch between posteromedial processes encroaches sculptured area; and 4) sculpturing moderately shallow. The genus Trionyx (?Apalone) is known from as early as the Lower Cretaceous (Carroll, 1987).
Apalone sp. indet.
Material. -- Santee: 15 coastal fragments, eight plate fragments, UNSM 560667. Devils Nest: four plate fragments, MSUVP 1304; two neurals, 19 costal fragments, one left xiphiplastron, 12 plate fragments, UNSM 56912.
Remarks. -- The sculpturing and thickness of these elements are typical of the genus Apalone, but I am unable to identify them to the species level.
Material. -- Santee: two nuchals, one epiplastron, one right hyoplastron, one left xiphiplastron, UNSM 56968. Devils Nest: seven peripherals, MSUVP 837, 839, 840, 841, 843, 844, 846; one right hyoplastron, MSUVP 852; one right xiphiplastron, nine nuchals, one hyoplastron, two right and two left hypoplastra, UNSM 56910.
Remarks. -- The nuchals are identified as Chrysemys picta and separated from those of Pseudemys sp. and Trachemys sp. based on the following combination of characters: 1) longer underlap of nuchal scute than in P. concinna and P. floridana; 2) longer nuchal scute than in species of Pseudemys and Trachemys; 3) smoother bone surface than in Trachemys scripta complex; and 4) anterior marginal areas serrated rather than smooth (or nearly so) as in most Pseudemys and Trachemys species.
The epiplastra plates resemble those of Chrysemys picta in having moderately developed epiplastra lips, and the xiphiplastra are similar to those of C. picta and differ from those of Pseudemys and Trachemys in being wide rather than constricted at the femoranal sulcus (Preston, 1979).
Chrysemys picta is know as early as the Barstovian (Holman and Sullivan, 1981), but this is the first record of the species from the Hemphillian.
[FIGURE 1 OMITTED]
Material. -- Devils Nest: One right hypoplastron, one left xiphiplastron, UNSM 56911.
Remarks. -- The hypoplastron of Emydoidea blandingii is easily distinguished from those of other North American emydid turtles of similar size on the basis of the following characteristics: 1) abdominal-pectoral sulcus joins hyo-hypoplastral suture at midline; 2) humeral-pectoral sulcus placed far anteriorly; 3) angular epi-hypoplastral suture; 4) bridge buttress limited in its anterior extent; and 5) posterior edge of bone lateral to bridge buttress bevelled for hypoplastral hinge attachment. The fossil hypoplastron is similar in these details to living E. blandingii and is confidently assigned to that species.
The fossil xiphiplastron is identical to that of living Emydoidea blandingii in 1) having a relatively short, broad femoral lip; 2) long, narrow anal lip; and 3) deeply excavated external abdominal oblique muscle scar on its dorsal surface. Only Clemmys has a similarly shaped and structured xiphiplastron, but it is wider relative to its length than in Emydoidea (Preston and McCoy, 1971).
The genus Emydoidea is know as early as the Barstovian (Hutchinson, 1981), and E. blandingii previously as early as the early Blancan (Preston and McCoy, 1971).
Terrapene sp. indet.
Material. -- Devils Nest: two nuchals, UNSM 56913.
Remarks. -- These nuchal plates are easily identifiable as those of Terrapene based on having short, wide nuchal and marginal scutes, but I cannot determine for certain which species they represent. The genus is known from as early as the Barstovian (Holman, 1987).
Material. -- Devils Nest: four posterior peripherals, UNSM 56914.
Remarks. -- The posterior peripherals of Macroclemys temminckii are deeply notched at and between the interperipheral sutures rather than weakly notched as in Chelydra serpentina (Preston, 1979). The fossil plates are from a turtle(s) much smaller than the maximum size of 80 centimeters obtained by Recent M. temminckii (Conant and Collins, 1991).
Material. -- Santee: one nuchal, one xiphiplastron, UNSM 56969.
Remarks. -- The nuchal of Kinosternon flavescens may be distinguished from those of K. subrubrum, K. bauri, Sternotherus odoratus and S. minor in having the vertebral scute wider than long rather than longer than wide. It differs from K. hirtipes in being only slightly arched rather than strongly arched.
The fossil xiphiplastron clearly resembles that in Kinosternon in being thin and flat with nearly equal anal-femoral scute areas and narrow marginal lips. The plate is like that found in Recent Kinosternon flavescens in overall size, shape, and thickness, and differs from those of K. leucostoum, K. hirtipes, and K. subrubrum in having a wider posterior anal scute. It differs from K. cruentatum in having a wider inner anal scute. This is the first record of Kinosternon flavescesn from the Hemphillian and, to my knowledge, the oldest record of the species as a fossil.
[FIGURE 2 OMITTED]
Geochelone sp. indet.
Material. -- Santee: one nuchal, one neural, nine coastal fragments, six peripherals, three epiplastra, one left xiphiplastron, four plastral fragments, UNSM 56964, 56965. Devils Nest: four costal fragments, eight peripherals, one pygal, one right and one left epiplastron, three entoplastra, one right hyoplastron, 14 plate fragments, one osteoderm, UNSM 56908, 56909.
Remarks. -- Within the genus Geochelone, two Nearctic subgenera are currently recognized--Hesperotestudo and Caudochelys (Auffenberg, 1963). Members of the subgenus Hesperotestudo tend to be small with rugose shells, whereas those of Caudochelys tend to be larger with smoother shells. On this basis, both subgenera and three unidentified species appear to be represented in the collection of late Hemphillian Geochelone plates as follows.
Geochelone (Caudochelys) sp. indet.
Devils Nest: one costal fragment, one entoplastron, two plastral fragments, one osteoderm, UNSM 56908. These elements indicate a large kind of smooth-shelled tortoise similar to Geochelone (Caudochelys) rexroadensis (Oelrich).
Geochelone (Hesperotestudo) sp. indet.
Type A. -- Santee: one nuchal, seven costal fragments, six peripherals, three epiplastra, two plastral fragments, UNSM 56965; Devils Nest: one nuchal, three coastal fragments, eight peripherals, one right and one left epiplastron, two entoplastra, one right hyoplastron, 12 plate fragments, UNSM 56909. These fossil elements represent a moderately large species of Geochelone with a carapace sculptured with grooves and ridges (growth rings) making the bone quite rugose as in Hesperotestudo tortoises, possibly of the larger G. alleni line.
Type B: St.: one neural, two costal fragments, one left xiphiplastron, two plastral fragments, UNSM 56964. These elements represent a small species of tortoise, and the xiphiplastron is the best preserved and the most interesting of the fossils. Based on xiphiplastra of Recent Geochelone, the fossil represents a tortoise with a plastron length of about 160 mm, but at least 14 growth rings are evident on its ventral surface suggesting it represents an adult individual rather than a juvenile of a large species.
It is likely that the fossils represent a new small species of Geochelone, but a formal specific description must wait until more complete material is available. Nonetheless, the rugosity and size of the bones, and the deeply notched xiphiplastron suggest a small species of the western Geochelone (Hesperotestudo) turgida line. According to Holman (1972), the G. turgida line of the Great Plains were small, rugose, thick-shelled tortoises with huge epiplastral beaks; members increased in size and carapace rugosity through time. The lineage began with G. turgida (Cope) of the latest Hemphillian, continued with G. riggsi (Hibbard), and terminated with the late Blancan species G. oelrichi Holman. The Santee turtle fits neatly into this evolutionary continuum on the basis of its small size, shell thickness (Table 1) and rugosity, and early appearance in the geologic record. Thus, it likely represents the ancestral morphotype of the G. turgida line.
COMMENTS AND PALEOENVIRONMENTAL CONSIDERATIONS
At least seven turtles are identified from the late Hemphillian faunas. From a temporal standpoint, the collective turtle fauna is quite modern, with Geochelone sp. representing the only known extinct species. Among the Recent taxa, Macroclemys temminckii and Kinosternon flavescens do not occur in northeastern Nebraska today (Lynch, 1985). Macroclemys temminckii presently has a distribution mainly in the southeastern United States (Conant and Collins, 1991), whereas K. flavescens occurs in Nebraska but considerably to the west and south of Knox County (Lynch, 1985).
Many of the fossils are badly water worn, suggesting considerable postmortem transport. On the basis of similar biological and sedimentological evidence, both sites appear to reflect ecologically equivalent stream or river deposits (Voorhies, 1977, 1988; Parmley, 1989). The turtles are characteristic of a river or stream with some deep, slow-moving water; shallow sandy areas; quiet vegetated coves and backwaters; and at least some marshy edge areas. The surrounding country probably consisted of upland grasslands interspersed with riparian woodlands and possibly small playa-type ponds or creeks, or both (Fig. 3).
As would be expected, turtles of the permanent river-stream community make up the majority (71 percent) of the late Hemphillian turtle fauna, and include Apalone spinifera, Chrysemys picta, Emydoidea blandingii, Macroclemys temminckii, and Kinosternon falvescens. Based on known ecological preferences of these turtles (Conant and Collins, 1991), they probably inhabited the stream as follows. The deeper, quiet waters, particularly deep coves, may have been inhabited by Macroclemys temminckii, a turtle that seldom leaves its aquatic environment except to lay eggs. Chrysemys picta and Emydoidea blandingii may have been most common in the shallow backwaters and coves where ample vegetation for cover existed. Apalone is a highly riverine species that may have preferred shallow sandy areas for hiding and sandy flats or sand bars for basking. Kinosternon flavescens may have been common in two aquatic settings: 1) in the quiet muddy areas of the stream, or 2) in nearby mud-bottomed playa-type ponds or creeks.
Terrestrial turtles of the upland grasslands would have included the Geochelone sp, and Terrapene sp. It is likely that theses species moved into lower riparian habitats from time to time in search of shelter or food. In fact, Terrapene may have been most common in the woodlands.
There is some evidence in the late Hemphillian turtle fauna that the climate of northeastern Nebraska was quite different than today. The genus Geochelone is widespread in North American Tertiary deposits. It is generally excepted that the presence of these nonburrowing land tortoises in fossil faunas indicates a climate with mild winters with temperatures seldom (if ever) reaching freezing (Hibbard, 1960). The presence of Macroclemys temminckii is probably also of climatic importance. This species is mainly confined to the Southeastern United States today, (Conant and Collins, 1991), an area with comparatively mild, short winters. Thus it is likely that mild late Hemphillian winters in northeastern Nebraska allowed this species to survive north and west of its present range.
TABLE 1. Measurements of the Santee Type B Geochelone sp. and members of the western G. turgida line. Geochelone turgida group measurements are from Holman (1972:148), and the plastron length of the Santee Geochelone is estimated. Santee Type B G. turgida G. riggsi G. oelrichi Plastron length 160 216 190 235 Xiphiplastron length 30 59 48 75 Xiphiplastron width 31 51 48 64 Xiphiplastron midline thickness 7.5 -- 9 13 Xiphiplastron notch length 15 19 18 24 Xiphiplastron notch width 23 55 41 57
For the loan of Hemphillian turtle fossils in their care, I am grateful to Dr. Michael Voorhies and George Corners (UNSM), and Dr. J. Alan Holman (MSU). Dr. Holman is further acknowledged for his informative discussions with the author on Miocene turtles. I kindly thank Lisa Hallock and Irene Rinchetti (MSU staff artists) for preparing the figures. Funding was provided in part by the University of Nebraska State Museum and the Michigan State Museum, for which I am most grateful. I wish to kindly thank Julie, Amanda, and Zac Parmley for all their help during our Nebraska field season.
[FIGURE 3 OMITTED]
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Department of Environmental and Biological Sciences, Georgia College, Milledgeville, Georgia 31061
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|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 1992|
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