Side-Necked Turtles (Testudines, Pleurodira) from the Ancient Gulf Coastal Plain of Florida During Middle Cenozoic Megathermals
Abstract
Fossils from paleocoastal deposits in northwestern Florida represent at least 3 pleurodire taxa. Biochronologic and paleoclimatic correlates suggest that the presence and absence of side-necked turtles in Florida was synchronous with global climatic shifts in the middle to late Paleogene and early Neogene. The oldest pleurodire record in Florida is from the end of the Greenhouse World in the middle to late Eocene, followed by a hiatus of ∼ 11 million yrs during the onset of the Icehouse World in the early Oligocene. Their sojourn in Florida during the latest Oligocene (25–24 Ma) was concurrent with a brief late Oligocene warming (LOW), and a subsequent hiatus in the early Miocene (∼ 24–18 Ma) was contemporaneous with global cooling. Their resettlement and most populous occurrences in Florida during the middle to late Hemingfordian through early Barstovian (∼ 18–15.5 Ma) coincided with the Middle Miocene Climatic Optimum (MMCO), a significant global warming event. Pleurodires are indicators that paleoclimate in the southeastern United States was frost free during the Eocene, LOW, and MMCO with air and water temperatures warm enough to support these generally cold-intolerant turtles. A small-bodied podocnemidid existed in the Cucaracha Formation (∼ 19.05–18.8 Ma) of Panama during the early Miocene pleurodire hiatus in Florida, and this or a closely related taxon later dispersed into Florida by the early Barstovian, when temperatures were suitable during the MMCO. Previously reported pleurodire fossils from the temporally mixed Lee Creek Mine of North Carolina are most likely middle Miocene in age when pleurodires were common in the Atlantic Coastal Plain of the southeastern United States. The presence of pleurodires in the Seaboard Local Fauna could help constrain the age of the fauna to younger than 18 Ma and probably closer to 17 Ma based on hypothesized paleoclimate at that time. A pleurodire from the middle Eocene Point “A” Dam site in southern Alabama may represent a late occurrence of the Bothremydidae.
Extant pleurodires are distributed pantropically in parts of South America, Africa, Madagascar, the Seychelles, Indonesia, Papua New Guinea, and Australia (Ernst and Barbour 1989; Turtle Taxonomy Working Group 2014). By contrast, their fossil record throughout the Cenozoic is more latitudinally extensive and includes records from North America as far north as the Atlantic Coastal Plain of the United States. Pleurodire fossils are known from the Paleocene of South Carolina (Hutchison and Weems 1998), middle Eocene of Alabama (from the Point “A” Dam site; see Discussions and Conclusions below), Oligocene of South Carolina (Weems and Knight 2013), middle Miocene of Maryland (Collins and Lynn 1936) and Florida (Bourque 2013), and middle Miocene or early Pliocene of North Carolina (Zug 2001). Pleurodires were also present during the late Eocene through middle Miocene of Panama (Cadena et al. 2012) and inhabited Panama until at least the late Miocene (J.R.B., in prep.). Panamanian records indicate the steady presence of podocnemidids on the southern-most end of the North American continent over a span of more than 20 million yrs and well before the formation of the Isthmus of Panama and its connection with South America. However, seawater was not a biogeographic barrier for some pleurodires, such as the taxon Bairdemys, which occupied marine to semimarine habitats throughout its circum-Caribbean distribution that included northern South America (Sánchez-Villagra et al. 2004; Gaffney et al. 2008). Accounts of fossils attributed to Bairdemys include Bairdemys healeyorum Weems and Knight 2013 from the late Oligocene of South Carolina; Bairdemys miocenica (Collins and Lynn 1936) from the early middle Miocene of Maryland; Bairdemys hartsteini Gaffney and Wood 2002 from the middle Miocene of Puerto Rico; Bairdemys thalassica Ferreira, Rincón, Solórzano, and Langer 2015 from the middle Miocene of Venezuela; and Bairdemys venezuelensis Wood and Díaz de Gamero 1971, Bairdemys winklerae Gaffney, Scheyer, Johnson, Bocqentin, and Aguilera 2008, and Bairdemys sanchezi Gaffney, Scheyer, Johnson, Bocqentin, and Aguilera 2008 from the late Miocene of Venezuela. Other fossil pleurodire reports from the Caribbean include the late early or early middle Eocene of Jamaica (Portell et al. 2001), late Oligocene of Puerto Rico (Wood 1972; MacPhee et al. 2003), and early Miocene of Cuba (MacPhee et al. 2003).
Here I describe pleurodire fossils recovered from middle to late Eocene through middle Miocene deposits in northern Florida, which include the oldest record of the group in Florida and some of the latest records from the southeastern United States. I also discuss the paleobiogeographic significance of these and fossils from the Panama Canal and discuss pleurodires as indicators of paleoclimate.
Anatomical Abbreviations
Anatomical terminology of carapacial and plastral bones and scutes follow Zangerl (1969). Bones: C, costal; N, neural; P, peripheral. Scutes: M, marginal; V, vertebral.
Institutional Abbreviations
UF, Division of Vertebrate Paleontology, Florida Museum of Natural History, University of Florida, Gainesville, Florida; UF/FGS, formerly part of the Florida Geological Survey Collection, now housed at the Florida Museum of Natural History, University of Florida, Gainesville, Florida; UF/H, Division of Herpetology, Florida Museum of Natural History, University of Florida, Gainesville, Florida; UF/TRO, formerly part of the Timberlane Research Organization Collection, Lake Wales, Florida, now housed at the Division of Vertebrate Paleontology, Florida Museum of Natural History, Gainesville, Florida.
Other Abbreviations
LF, local fauna; LOW, late Oligocene warming; Ma, millenianum; MMCO, Middle Miocene Climatic Optimum; NALMA, North American Land Mammal Age.
Specimens Examined
Fossils: UF 21952, UF 111760, UF 114996, 115697, UF 115705, UF 124093, UF 167039, UF 171521, UF 205740, UF 205741, UF 225674–UF 225681, UF 225683, UF 259076, UF 259077, UF 280644, UF 314928, UF/FGS 3529, UF/FGS 1876, UF/FGS 1887, UF/FGS 2680, UF/FGS 2678, UF/FGS 3556, UF/FGS 3563, UF/FGS 3584, UF/FGS 5201, UF/FGS 5202, UF/FGS 10131–10136, UF/FGS 10865, UF/FGS 10866, UF/FGS 19867, UF/TRO 1708, UF/TRO 2620. Podocnemis expansa: UF/H 57965, UF/H 118589.
LOCALITY BACKGROUNDS AND GEOLOGIC SETTINGS
Pleurodire fossils were recovered from Duchesnean or Chadronian through early Barstovian North American Land Mammal Age (NALMA) exposures in Gadsen, Hamilton, Walton, Leon, Columbia, and Citrus counties (Fig. 1). Most of the fossils were collected in association with terrestrial, freshwater, and coastal marine taxa, such as sharks, rays, bony fishes, frogs, salamanders, snakes, lizards, terrestrial and freshwater turtles, birds, terrestrial mammals, and marine mammals, such as sirenians and cetaceans (Olsen 1964a, 1964b; Morgan 1989; Bryant 1991; Albright 1998; MacFadden and Morgan 2003). The following localities have yielded pleurodire fossils in Florida, and pleurodire specimen vouchers are listed at the end of each site description.
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
Dolime Quarry
Citrus County, Florida, Ocala Limestone Formation, Lower Member, late middle to late Eocene (∼ 38–36 Ma; Blake and Portell 2009; R. Portell, pers. comm., March 2015). Vertebrates from the Dolime Quarry occur in the lower portion of the Ocala Limestone, just above the contact with the middle Eocene Avon Park Formation (Ivany et al. 1990; Blake and Portell 2009). To my knowledge, vertebrates from this locality have not been reported in detail but, based on specimens housed at the FLMNH, include marine fishes (e.g., diodontids and sharks), crocodilians, cheloniid sea turtles (e.g., UF 167039), pleurodires (discussed below), and sirenians. Seagrass beds and associated faunal communities have been recovered from exposures of the Avon Park Formation in the Dolime Quarry (Ivany et al. 1990). Specimen: UF 114996.
White Springs 2A (White Springs Local Fauna)
Columbia County, Florida, Parachucla Formation, middle Arikareean (25–24 Ma; Morgan 1989; Albright 1998; Tedford et al. 2004). The White Springs LF derives from a series of individual localities from the same stratigraphic horizon over a several kilometer long stretch of the Suwannee River. The White Springs 2A locality is one of those sites. The White Springs LF is composed of terrestrial, freshwater, and nearshore marine taxa, such as sharks, rays, bony fish, a frog, crocodilians (cf. Thecachampsa), typhlopid and boid snakes, terrestrial mammals, and dugongs (Domning 1989; Morgan 1989; MacFadden and Morgan 2003; Vélez-Juarbe and Domning 2014). The erycine Pterygoboa (a sand boa) was recently reported from the fauna (Mead and Schubert 2013). Turtles are rare or undersampled from the White Springs LF, with 2 or possibly 3 taxa represented by only 3 specimens. From White Springs 2A, a pleurodire is present and described below, and a posterior peripheral, UF 205741, is that of an undescribed genus of kinosternid and represents one of the oldest records of the family in Florida. It was possibly contemporaneous with the kinosternid Xenochelys floridensis from the Brooksville 2 LF (Bourque 2013). A small worn fragment of carapace from White Springs 3C, UF 314928, is difficult to classify more specifically than the rank Testudines but may represent a testudinoid. Specimen: UF 205740.
Seaboard Air Line Railroad, Switchyard B (Seaboard Local Fauna)
Leon County, Florida; late early Miocene, early Hemingfordian (18.5–17.5 Ma), Torreya Formation (Olsen 1964a; Hulbert 2001; Tedford et al. 2004). Vertebrates from this locality were discovered both within and below an exposed Miocene oyster bar layer uncovered by a dragline (Olsen 1964a, 1964b). Additionally, the presence of fishes such as a drum, Pagonias; eagle ray, Myliobatis; requiem shark, Carcharhinus; and sawfish, Pristis; and of long-legged wading birds suggests that the paleoenvironment was coastal bordering a shallow sea (Olsen 1964a, 1964b). The chelonians Pseudemys sp. (aff. P. floridana group) and a testudinid were reported by Olsen (1964a), but the former could not be confirmed from the Seaboard Air Line Railroad locality during this study. However, deirochelyine fossils (e.g., UF/FGS 3529, UF/FGS 3556, UF/FGS 3584, and UF/FGS 10136) that represent a taxon near Pseudemys are present in the nearby and similarly aged Griscom Plantation fauna and also common in other Florida Hemingfordian localities, such as Thomas Farm and the Miller Site. Other previously reported reptiles from the Seaboard LF include a crocodilian, perhaps Alligator olseni, and a boid snake (Olsen 1964a). Specimen: UF 21952.
Camp Creek
Walton County, Florida; late early to early middle Miocene, late Hemingfordian. The presence of the horse Merychippus gunteri from this locality suggests an age of between 17 and 16 Ma (R. Hulbert, Jr., pers. comm., June 2014). Specimen: UF/TRO 1708.
Fuller's Earth Company Mine (Midway Local Fauna)
Gadsen County, Florida; late early to middle Miocene, late Hemingfordian (17–16 Ma; Olsen 1964b; Bryant 1991; Hulbert 2001; Tedford et al. 2004). The Midway LF is composed of both terrestrial and estuarine to coastal taxa and probably represents a shallow paleocoastal setting (Olsen 1964b). Chelonians from the Fuller's Earth Company Mine are previously unreported and include a chelydrid (cf. Macrochelys; UF/FGS 10131), trionychids (UF/FGS 1876, UF/FGS 1887, UF/FGS 2680, UF/FGS 2678, and UF/FGS 10135), a deirochelyine emydid (UF/FGS 10134), a large-bodied testudinid (UF/FGS 5201 and UF/FGS 5202), and a small testudinid of the Hesperotestudo turgida group (UF/FGS 10132 and UF/FGS 10133). The chelydrid may be the oldest record of the family in Florida, but unfortunately the voucher UF/FGS 10131 is a very fragmentary partial neural. It is temporally intermediate between the taxa Macrochelys schmidti from the Marsland Formation of Nebraska and Macrochelys stricta and Macrochelys sp. from Lower Snake Creek, Nebraska, and the Willacoochee Creek LF, respectively (Thomas et al. 2014). Trionychids make up the largest chelonian sample from the Fuller's Earth Company Mine and may have been the most common turtle taxon there. Specimens: UF/FGS 10865–10867, UF/FGS 3563.
Suwannee Springs
Hamilton County, Florida; early Miocene, late Hemingfordian (17–16 Ma); Marks Head Formation (Scott 1988). This faunal assemblage consists of the horse M. gunteri, a small alligatorid (aff. A. olseni), the longirostrine tomistomine crocodilian Thecachampsa, testudinids (possibly a small and midsized species), mud turtle Kinosternon sp., and a pleurodire (Bourque 2013). Specimen: UF/TRO 2620.
Milwhite Gunn Farm Mine (Willacoochee Creek Local Fauna)
Gadsen County, Florida; middle Miocene, early Barstovian (16–15 Ma); Torreya Formation (Bryant 1991; Hulbert 2001; Tedford et al. 2004). The Willacoochee Creek LF preserves a diverse array of terrestrial, freshwater, and marine taxa indicative of a coastal deltaic setting. Marine invertebrates were common in the sediments that preserved vertebrate fossils, and marine tetrapods include mysticete and odontocete cetaceans and dugongids (Bryant 1991). The herpetofauna is diverse and includes notable records, such as the enigmatic giant salamander Batrachosauroides and a helodermatid lizard. Previously reported chelonians include a kinosternid, pleurodire, small and large testudinids, trionychid, and an emydid (Bryant 1991; Bourque 2013). The kinosternid likely represents a new genus endemic to the Atlantic Coastal Plain (Bourque 2016). Specimen: UF 111760.
Englehard La Camelia Mine (Willacoochee Creek Local Fauna)
Gadsen County, Florida; middle Miocene, early Barstovian (16–15 Ma); Torreya Formation; Dogtown Member (Bryant 1991; Hulbert 2001; Tedford et al. 2004). Chelonians from this site include Macrochelys sp., represented by a protuberantly keeled C8 (UF 259076) and associated partial left hyo-hypoplastra (UF 259077), a small trionychid, a small testudinid (aff. H. turgida group), and a pleurodire. The Macrochelys specimens are contemporaneous with the taxon M. stricta from Sioux County, Nebraska (Thomas et al. 2014). Specimens: UF 124093, UF 225674–225681.
SYSTEMATIC PALEONTOLOGY
Order Testudines Linnaeus, 1758
Suborder Pleurodira Cope, 1868
Pleurodira gen. et sp. indet.
(Fig. 2)
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
Referred Material
UF 114996, pygal.
Family Podocnemididae Cope, 1868
Podocnemididae gen. et sp. indet.
(Figs. 3–5)
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
Referred Material
UF 21952, right P9 or P10; UF 111760, right P8; UF 124093, left P7; UF 205740, right hypoplastron; UF 225674, left P2; UF 225675, left P2; UF 225676, right xiphiplastron (posterior-most cusp); UF 225677, N1; UF 225678, left C8 (proximal portion); UF 225679, right epiplastron; UF 225680, right epiplastron (antero-medial portion); UF 225681, right C1 (proximal portion); UF/FGS 3563, left P7; UF/FGS 10865, left xiphiplastron (posterior-most cusp); UF/FGS 10866 (V4178), partial neural; UF/FGS 19867 (V4178), right P1 (or P2); UF/TRO 1708, right P11; UF/TRO 2620, right mesoplastron.
Remarks
A late Miocene (Clarendonian) pleurodire fossil previously reported by Bourque (2013, p. 462) was reassessed during this study. The costal fragment, UF 225683, from the Suwannee River Mine was previously misidentified and is actually that of a cheloniid sea turtle.
DESCRIPTIONS AND COMPARISONS
Pleurodire specimens described here consist of isolated carapacial and plastral bones and no cranial material is known from Florida. Specimens preserved include a C8 and fragmentary xiphiplastra, which exhibit pelvic sutural scars, a synapomorphy of pleurodires (Gaffney and Meylan 1988). The presence of small round mesoplastra (e.g., UF/TRO 2620) lacking midline contact is diagnostic of pelomedusoides, including podocnemidids (MacPhee et al. 2003). Classification at the family level Podocnemididae for at least the Miocene pleurodires studied is supported by the overall likeness of the Floridian fossils to podocnemidids, such as Podocnemis (Fig. 3). This classification is further corroborated by the fact that the only pleurodires known from the southeastern United States and Caribbean during the Oligocene and Miocene are podocnemidids, such as Bairdemys. The absence of cranial material makes classification at generic or specific levels dubious at this time. For example, a pleurodiran taxon from the Willacoochee Creek LF possesses neural bones, whereas, by contrast, the shell of B. venezuelensis lacks neurals (Gaffney and Wood 2002; Gaffney et al. 2011), while some species of Bairdemys possessed neurals (Weems and Knight 2013; J.R.B., in prep.). The following specimen descriptions are organized in order of anatomical position (carapace: peripherals, costals, neurals; plastron: epiplastra, mesoplastron, hypoplastron, xiphiplastra).
Carapace
UF/FGS 10867 is a partial right P1 (Fig. 4i, j). It measures 42.10 mm wide between the interperipheral sutures. Overall, the element is thin with very little marginal underlap on the ventral side, indicating that the skin attached along the carapacial rim in life, similar to that seen in the anterior carapace of Podocnemis.
UF 225674 is a partial left P2 (Fig. 4c, d). It is 45.74 mm wide between the interperipheral sutures. The proximal edge is broken, but it measures 33.58 mm in height. The bone is 12.41 mm thick, which is thicker than observed in the P2 of Podocnemis. It is smooth dorsally with very faintly incised scute sulci and has sutural teeth of low relief and narrow marginal underlap ventrally. UF 225675 is a complete left P2 (Fig. 4a, b) that measures 19.73 mm wide across the interperipheral sutures and 19.38 mm in height. As is the case for UF 124093, this specimen appears to be from a young adult, being small and with a well-developed peripheral-pleural suture. It is dorsoventrally thin with narrow marginal underlap on the ventral face, similar to Podocnemis.
UF/FGS 3563 is a partial left P7 (Fig. 4o, p) for which the ventral portion is not preserved. It measures 52.87 mm wide across the interperipheral sutures and 43.91 mm tall from the carapacial margin to the peripheral-costal suture. The peripheral is smooth and strongly angled at the outer margin.
UF 124093 is a relatively complete left P8 (Fig. 4g, h). It measures 23.63 mm wide across the interperipheral sutures and 25.3 mm in height. Its small size as well as the slightly underdeveloped nature of the peripheral-pleural suture suggest that the specimen is from a young adult. It is classified as a pleurodire by its overall thinness; smooth dorsal surface; thin, lightly incised sulcal impressions; and highly angled peripheral-pleural sutural teeth. UF 111760 is a partial right P8 (Fig. 3k, l). It is broken along the proximal sutural edge. It measures 42.65 mm wide between the interperipheral sutures and 45 mm tall. Ventrally, there is a smooth transition where the marginal underlap meets the visceral bone face, and there is no step at this juncture as seen in UF 21952 (described below). The dorsal face is smooth with faintly incised scute sulci. The position of the pleural sulcus indicates that the pleural moderately overlapped onto the peripheral, similar to that for UF/FGS 3563 and Podocnemis.
UF 21952 is a nearly complete right P9 or P10 belonging an adult pleurodire (Fig. 4u, v). It is from the largest-bodied specimen represented in the Florida sample and measures 83.52 mm long (from the peripheral-costal suture to the distal carapacial margin) and 66.05 mm wide (between the interperipheral sutures). The specimen probably belonged to an individual similar in size to a P. expansa (UF/H 57965) with a carapace length of approximately 54 cm. The smooth dermal surface, faintly incised nature of the scute sulci, overall thinness of the element, and high incline of the peripheral-costal sutural teeth help distinguish it as a pleurodire. It is overall similar in shape to Podocnemis examined; however, in UF 21952, a distinct step is present along the ventral marginal sulcus. In Podocnemis, the transition between the marginals and the visceral bone surface is essentially smooth or with only a faint step. Other Floridian pleurodire peripherals (e.g., UF/TRO 1708) lack the deep visceral step present in UF 21952.
UF/TRO 1708 is a right P11 (Fig. 4k, l). It is 48.83 mm tall and 48.06 mm wide but is chipped along all of its edges. The smooth dermal surface, thin faintly incised scute sulci, shallow sutural teeth, and overall thinness of the element help distinguish the specimen as a pleurodire. Ventrally, the transition from the marginal scute to the visceral bone surface is smooth and essentially flat as seen in Podocnemis and not deeply stepped like UF 21952.
UF 114996 is a pygal (Fig. 2) that measures 68.44 mm long, 65.68 mm wide at the posterior margin, and 43.3 mm wide anteriorly. Despite some asymmetry of the dorsal marginal sulcus, it is recognized as a pygal in having symmetrical corners proximally that would have articulated with the suprapygal in life. Also, the ventral M12 to visceral sulci form a symmetrical “V” shape located centrally on the bone, whereas the same sulci would be asymmetrical if the element were a posterior peripheral. The pygal is flat and lacks any carination or curvature along the midline. It is recognized as a pleurodire primarily by the sloped nature of the pygal-suprapygal sutural teeth, faintly incised scute sulci, and relatively smooth surface. It shows a strong resemblance to pygals of Podocnemis examined (UF/H 57965 and UF/H 118589), but classification of the fossil past the suborder level would be speculative.
UF 225681 is a partial right C1 that is broken distally (Fig. 3b, c). It is 24.97 mm wide between the intercostal sutures. It is pleurodiran in being smooth dorsally with thin, lightly incised sulci and in being highly buttressed ventrally with a distinct ovate sutural pocket that would have received the axillary process of the hyoplastron. The sutural pocket is relatively smooth and concave internally. By contrast, the axillary suture of testudinoids possesses more numerous and prominent sutural teeth. The proximal rib end is bifurcated. Overall, the specimen is thick for its size with well-formed sutures, indicating that it was from a small-bodied adult.
UF 225678 is the proximal end of a left C8 (Fig. 3g, h). It measures 18.56 mm wide between the intercostal sutures. It is diagnostic as a pleurodire in possessing a well-formed sutural concavity ventrally just distal to the proximal rib end, which would have articulated with the ilium. The proximal rib end is slightly bifurcated and protrudes anteriorly along the C7–C8 suture, where it would have overlapped C7. The position of the V4–V5 scute sulci indicates that the vertebrals barely overlapped onto the antero-medial-most portion of the costal, and as such these scutes were very narrow toward the posterior of the carapace.
UF 225677 (Fig. 4w, x) is an N1 that measures 38.87 mm long (despite being chipped) and 23.99 wide. It is ovate in shape. The V1 sulcus crosses the dorsal surface midway on the specimen. UF/FGS 10866 (Fig. 4s, t) is the anterior portion of a neural. It is anteriorly symmetrically short-sided and measures 34.61 mm wide across the sutures.
Plastron
UF 225679 is a complete right epiplastron (Fig. 5a–c). It is 41.12 mm wide between the medial suture and epi-hyoplastral suture and 32 mm long from the anterior intergular margin to the epi-ento-hyoplastral sutural junction. It is 16.79 mm long at the midline along the interepiplastral suture. The gular scute is restricted to the epiplastron and did not overlap onto the entoplastron in life. Its proximal terminus is at or just anterior to the epi-entoplastral suture. The humero-pectoral sulcus is positioned along the posterior of the element, just anterior to the epi-hyoplastral suture, and would have extended onto the middle of the entoplastron in life. This sulcus is angled anteriorly from the midline and oriented almost parallel to the epi-hyoplastral suture. The intergular and gular scutes are slightly protuberant along the distal epiplastral margin and not smooth along this edge as in Podocnemis. There is a slight but distinct notch between the intergular and gular scutes. The dorsal overlap of the intergular, gular, and humeral scutes is very narrow and almost absent, indicating that skin would have attached along the outer margin of the forelobe in life. UF 225680 is the medial portion of a right epiplastron (Fig. 4e, f) of a slightly larger individual than UF 225679. It measures 25.86 mm long at the midline suture. Like UF 225679, it has an anteriorly protuberant intergular scute indicated by a slight notch between the intergular and gular scutes.
The epiplastron UF 225679 shows a striking resemblance to a partial plastral forelobe UF 280644 from the Centenario Fauna of central Panama (Fig. 5d–f). The specimens are nearly identical in size and belong to relatively small-bodied adults. The right epiplastron of UF 280644 is 41 mm wide and 33.86 mm long. The midline suture is 18.33 long dorsally and 15.58 mm long ventrally. In both UF 225679 and UF 280644, the forelobe is short and broad. Both specimens possess a distinct swelling along the medial suture. The gulars do not overlap the entoplastron. The humeral-pectoral sulcus crosses the posterior-most corner of the epiplastron; however, in UF 225679, this sulcus is oriented antero-posteriorly almost parallel with the epi-hyoplastral suture, while in UF 280644, the sulcus is oriented much straighter transversely and perpendicular to the midline. UF 280644 lacks a distinct notch between the intergular and gular scutes.
UF/TRO 2620 is a right mesoplastron (Fig. 3d). It measures 28.43 mm long by 33.33 mm wide. The specimen is from a relatively small-sized adult pelomedusoid. That it is from an adult is indicated by its overall small size and in possessing fully formed sutural teeth.
UF 205740 is a right hypoplastron of a small-bodied pelomedusoid (Fig. 4m, n). All edges and sutures are water-worn smooth on the specimen, so it is difficult to say if it is from an adult or a subadult. It is 37.18 mm long at the midline, 46.76 mm maximum length, and 53.28 mm wide. The bridge length at the distal suture between the inguinal buttress and mesoplastral suture is 26.99 mm, and the bridge length at the inguinal notch is 29.24 mm. The presence of a mesoplastral suture well anterior to the inguinal bridge buttress helps identify this as a pelomedusoid (MacPhee et al. 2003). The inguinal bridge buttress is highly inclined. The abdominal-femoral scute sulcus is barely discernible on the ventral surface and is curved and angled posteriorly away from the midline. The abdominal scute overlaps 13.02 mm at the midline. The outer margin of the femoral is straight, and the margin lacks substantial dorsal overlap.
UF/FGS 10865 is the posterior-most tip of a left xiphiplastron (Fig. 3n, o). The fragment measures 29.53 mm long by 26.69 mm wide. It is overall very thin, much more so than in Podocnemis, and identified as a left xiphiplastron in being thickest along the outer left margin. The ischial suture scar is preserved and runs parallel with the distal margin of the bone and almost perpendicular with the caudal notch margin. The fragment is broken along the pubic suture scar. UF 225676 is the partial posterior-most end of a right xiphiplastron (Fig. 4q, r). It measures 43.78 mm long by 34.61 mm wide and is from a relatively large adult. It is identified as a right xiphiplastron in that it is thickest on the outer right margin. The ischial scar is well preserved, highly protuberant dorsally, and oriented parallel with the outer margin and nearly perpendicular with the caudal notch margin, similar to that observed in UF/FGS 10865. However, UF 225676 is larger, overall much thicker, and more pointed and elongate than UF/FGS 10865.
DISCUSSION AND CONCLUSIONS
Megathermals and other significant climatic events are characterized by rapid faunal and floral turnover in the fossil record, and previous accounts have documented the effects of climatic shifts on chelonian faunas (Hutchison 1996; Holroyd et al. 2001; Prothero et al. 2003; Wing et al. 2005; Wing and Currano 2013; Bourque et al. 2014; Holroyd et al. 2014). The disappearance of pleurodires in the United States is directly correlated with the end of the Middle Miocene Climatic Optimum (MMCO) and the onset of northern hemisphere glaciation. However, during the MMCO, the northeastern United States was frost free, with an estimated mean annual temperature of 17°C as far north as Vermont (Zachos et al. 2001, 2008; Woodburne 2004). Extant pleurodires are generally cold intolerant and exist today only near equatorial regions of the world (Ernst and Barbour 1989; Turtle Taxonomy Working Group 2014). For this reason, pleurodires are good climatic indicators of warm and frost-free mean annual temperatures (MATs). Their presence in the ancient coastal plain of northwestern Florida supports previous hypotheses of seasonally stable climate with warm ocean temperatures during the middle to late Eocene, the latest Oligocene, and the middle part of the Miocene (Zachos et al. 2001, 2008).
Turtles are rare in the Eocene and Oligocene fossil record of Florida. A single fossil pygal, UF 114996, represents the oldest and only confirmed Eocene record of a pleurodire in Florida, although some very fragmentary specimens (e.g., UF 115705, and parts of UF 115697) might also represent pleurodire shell pieces. Furthermore, UF 114996 is significant in occurring during the time of the Greenhouse World, when global temperatures were significantly higher than those of the Icehouse World, an interval that spans the beginning of the Oligocene to Recent (Zachos et al. 2001, 2008; Prothero et al. 2003). Only a single pleurodire hypoplastron, UF 205740, is known from the Oligocene of Florida, from the White Springs 2A locality on the upper Suwannee River. Significantly, the age of this record (25–24 Ma) is coincident with the height of a brief but immense climatic event that entailed a rapid increase in global temperature as inferred by decreased deep-sea ∂18 oxygen isotope values (Zachos et al. 2001, 2008; Fig. 6). During this late Oligocene warming event (LOW), MATs were approximately as high as or higher than those reached during the MMCO but occurred 7–6 million yrs prior. The LOW is also significant in that the onset of this interval is coincident with the last occurrence of the kinosternid genus Xenochelys, represented by the species X. floridensis from the Brooksville 2 LF, ∼ 26–25 Ma (Bourque 2013). Florida may have been a post–Greenhouse World refugium for Xenochelys during LOW because the latest records prior are from the late Eocene (Chadronian) of the US Western Interior (Hutchison 1991, 1996). Notably, a kinosternid (UF 205741) is present in the White Springs LF but probably represents a different taxon than X. floridensis.
Citation: Chelonian Conservation and Biology 15, 1; 10.2744/CCB-1159.1
The next subsequent oldest records of pleurodires in Florida are from around the middle Hemingfordian and are similarly contemporaneous with the onset of increased global warming during the early part of the MMCO. In the interval between the temperature spike at LOW and the MMCO onset, there exists a pleurodire hiatus in Florida, or at least an ∼ 6-million-year-old gap in the Floridian pleurodire fossil record. This gap could arguably be the result of poor sampling or a lack of early Miocene shallow coastal deposits that represent suitable paleohabitats for these pleurodires or an indication that paleoenvironmental factors such as MATs were not suitable for these turtles during the early Miocene. Early Hemingfordian Florida localities such as Thomas Farm (Gilchrist County) and the Miller Site (lower Suwannee River) are fossil rich with terrestrial and freshwater vertebrate taxa but lack marine-associated faunal components and pleurodire fossils. During the hiatus, MATs are predicted to have been lower than those experienced at the LOW and MMCO (Zachos et al. 2001, 2008). However, at ∼ 22 Ma, a brief post-LOW spike in MAT may have approached temperatures experienced during the MMCO (Zachos et al. 2001, 2008), and as such it might be predicted that pleurodire fossils will be discovered from deposits that represent the proper paleoenvironment during the interval between the LOW and MMCO. Pleurodires were most abundant and perhaps most diverse in Florida in the latest Hemingfordian and early Barstovian during the height of the MMCO.
At least two pleurodire taxa are represented in Florida during the span of the MMCO based primarily on differences in adult body size (a large and a small form) and on the presence or absence of a deep visceral marginal step of the posterior peripherals. It is, however, plausible that the size discrepancy is attributable to sexual dimorphism within a single taxon. The most plentiful fossil sample from this study comes from the early Barstovian (∼ 16–15.25 Ma) Englehard La Camelia Mine and is part of the Willacoochee Creek LF (Bryant 1991; Bryant et al. 1992). Within this fauna, the small form is most common and perhaps the only taxon represented there, while the larger taxon is present in the late Hemingfordian Midway (∼ 17–16 Ma) and Seaboard (∼ 18–17 Ma) LFs. Notably, both large- and small-bodied pleurodire shell fossils have been recovered from the early Miocene Las Cascadas, Culebra, and Centenario faunas through recent collecting efforts along the Panama Canal (see also Cadena et al. 2012). In portions of the Las Cascadas and Culebra formations (∼ 21–19 Ma), large pleurodire taxa occur, and in the latter, a nearshore marine taxon is represented, perhaps the genus Bairdemys, or a closely related taxon. However, in the lower portion of the Cucaracha Formation (19.048–18.781 Ma; MacFadden et al. 2014), a small-bodied and seemingly rare pleurodire taxon has been collected. This taxon shows striking similarities in size and plastral forelobe morphology to the small-bodied taxon represented in the Willacoochee Creek LF from the Englehard La Camelia Mine (Fig. 5). While there are attributes that may differentiate these specimens at the species level (e.g., presence/absence of an intergular-gular notch and different orientation of the humeral-pectoral sulcus from the midline), the fossils from these disjunct proveniences more confidently represent the same genus. It is therefore likely that the small-bodied pleurodire lineage present in Panama during the early Miocene emigrated northward via the Gulf Coastal Plain during the Hemingfordian and populated northern Florida by the early middle Miocene (early Barstovian) using coastal and/or a network of closely connected fluvial habitats. The larger-bodied form may represent the genus Bairdemys, which had an extensive range across North America, some of the Caribbean Islands, and South America during the Miocene and likely dispersed freely across seawater. Bairdemys miocenica (Collins and Lynn 1936) is known from the Calvert Cliffs of Maryland (∼ 16–15.7 Ma; Weems and Knight 2013), and records of this taxon are contemporaneous with the more abundant pleurodire records discussed here from the Willacoochee Creek and Midway LFs and only slightly younger or equivalent in age to pleurodires from Suwannee Springs, Camp Creek, and the Seaboard LF. It is unclear at this time, pending more complete fossils, if any of the Floridian pleurodires represent B. miocenica or a closely related taxon, but this scenario is not unlikely.
There were likely multiple pleurodiran colonization and dispersal events throughout the southeastern United States during the Cenozoic. Paleogene records are known from the Paleocene Williamsburg Formation of South Carolina (Hutchison and Weems 1998); middle Eocene (Tallahatta or Lisbon Formation) Point “A” Dam site in Covington County, Alabama (previously unreported); middle to late Eocene of Citrus County, Florida (presented here); and Oligocene, early Arikareean (∼ 28 Ma), Chandler Bridge Formation of southeastern South Carolina (Weems and Knight 2013). The taxon represented at the Point “A” Dam site possesses a short and broad plastral forelobe similar to that of the small taxon/taxa present from the much younger Miocene Willacoochee Creek and Centenario faunas. However, it differs significantly in the positions of the gular and humeral scutes, in epiplastron morphology (e.g., very short at the midline), and in having a very broad intergular and more likely represents a different lineage. In fact, it most closely resembles a bothremydid and is strikingly similar to the forelobe of Taphrosphys congolensis figured by Gaffney et al. (2006:fig. 266). It is unclear at this time how morphologically similar or closely related the taxon from the Eocene Dolime Quarry in Florida is to the taxon from the Point “A” Dam site because fossils from both localities are so rare and fragmentary. It is plausible that pleurodires were able to colonize northern Florida only as sea level fell during cooling in the late Eocene and that the taxon from the Dolime Quarry was descended from older, more northern populations that dispersed south (Huddleston 1993; Holroyd et al. 2005). Early Paleogene records from the southeastern United States are not unexpected given the warm climate of the Greenhouse World (Zachos et al. 2001, 2008). However, given how strongly pleurodire records are correlated with the warmest temperatures during the Neogene, the occurrence of B. healeyorum Weems and Knight 2013 from the early Arikareean of South Carolina seems extraordinary when considering that MATs are estimated to have been the coolest over the 11-million-year span of the Arikareean and much lower than temperatures hypothesized during the LOW and MMCO. Perhaps not all pleurodire taxa at that time were so dramatically influenced by MAT as that observed in the Miocene. Alternatively, the Chandler Bridge Formation could be either older (e.g., ∼ 30 Ma using the ∂18 oxygen curve of Zachos et al. [2008]) or younger than currently thought, perhaps closer in age to the onset of LOW (∼ 26–25 Ma). Similarly, pleurodire fossils collected from the Seaboard LF of Florida indicate an age of ∼ 17 Ma or slightly younger, based on correlation of these records to the onset of the MMCO. The Seaboard LF is currently thought to be ∼ 17 Ma or older, with the lower boundary unknown (Tedford et al. 2004).
Pleurodires from the Willacoochee Creek LF represent some of the latest occurrences of the group in the United States. Zug (2001) reported the possibility of similarly aged pleurodires with his record of “Bothremys” (now B. miocenica sensu Weems and Knight [2013]) from the Lee Creek Mine of North Carolina, but the age of those fossils was uncertain and thought to be either middle Miocene or Pliocene. However, Zug (2001:216) questioned the Pliocene age of the Lee Creek pleurodires due to the fact that they were not collected in situ, and a middle Miocene component within the Lee Creek Mine, the Pungo River Formation, is also exposed and discarded out of place throughout the mining process. Based on results of the current study, I also question a Pliocene age for the Lee Creek pleurodires and concur that these specimens are derived from the Pungo River Formation, which spans ∼ 18–15 Ma (Weems and George 2013), approximately contemporaneous with the interval in which pleurodires inhabited Florida and Maryland during the MMCO. Based on the pleurodire records from Florida and Maryland, the Pungo River records are probably more accurately between 17 and 15 Ma, during the height of the MMCO, and suggest that pleurodires inhabited coastal habitats as far north as Maryland and as far south as Florida contemporaneously. Additionally, the Lee creek B. miocenica fossils are less likely to be Pliocene in age because the Pliocene Yorktown Formation is temporally and paleoenvironmentally similar to the Bone Valley Formation of Florida in that both represent turtle-rich nearshore marine coastal environments. The Pliocene component of the turtle fauna from Lee Creek resembles the upper Bone Valley fauna, sharing taxa such as Psephophorus, Caretta, Chelonia, Eretmochelys, Lepidochelys, trionychids, Pseudemys, Trachemys, and the giant tortoise Caudochelys (Dodd and Morgan 1994; Zug 2001; unpubl. data). However, despite an extensive survey over a sample that includes many hundreds of Bone Valley turtle specimens, no pleurodires have yet been found from the Pliocene of Florida or post-MMCO in the United States.
Pleurodire fossil localities in the state of Florida, USA (from left to right): • Camp Creek, Walton County; ♦ Milwhite Gunn Farm Mine, Gadsen County; ✶ La Camelia (Englehard) Mine, Gadsen County; F+ Fuller's Earth Company Mine, Gadsen County; ▪ Seaboard Air Line Railroad, Switchyard B, Leon County; ▴ Suwannee Springs, Hamilton County; ★ White Springs 2A, Columbia County; x Dolime Quarry, Citrus County. Scale bar = 100 miles.
Isolated pygal, UF 114996, from the middle to late Eocene Dolime Quarry, Citrus County, Florida. This specimen represents the oldest and southern-most fossil record of a pleurodire in Florida. Scale bar = 1 cm.
Miocene podocnemidid fossils from northern Florida compared to shell bones of the extant taxon P. expansa, UF/H 118589. (a) Right C1 of P. expansa in ventral aspect. (b) and (c) Partial right C1, UF 225681, from the Willacoochee Creek LF in (b) ventral and (c) dorsal aspects. (d) Right mesoplastron, UF/TRO 2620, from the Suwannee Springs locality in ventral aspect. (e) Right mesoplastron of P. expansa in ventral aspect. (f) Left C8 of P. expansa. (g) and (h) Partial left C8, UF 225678, from the Willacoochee Creek LF. (i) and (j) right P8 of P. expansa in (i) dorsal and (j) ventral aspects. (k) and (l) Right P8, UF 111760, from the Willacoochee Creek LF in (k) dorsal and (l) ventral aspects. (m) Left xiphiplastron of P. expansa in dorsal aspect. (n) and (o) Posterior-most tip of left xiphiplastron, UF/FGS 10865, from the Midway LF in (n) ventral and (o) dorsal aspects. Arrows indicate positions of the following: (a) and (b) sutural pocket that accepts the axillary plastral buttress; (d) and (e) marginal scute sulcus; (f) and (g) ilium suture; (m) and (o) pubic suture (upper) and ischium suture (lower). Upper-left scale bar is for all of the P. expansa specimens (a, e, f, i, j, and m) and equals 3 cm. All other scale bars below the fossil specimens equal 1 cm.
Podocnemidid fossils from the latest Oligocene and middle Miocene of northern Florida. (a) and (b) Left P2, UF 225675, from the Willacoochee Creek LF in (a) dorsal and (b) ventral aspects. (c) and (d) Partial left P2, UF 225574, from the Willacoochee Creek LF in (c) dorsal and (d) ventral aspects. (e) and (f) Partial right epiplastron, UF 225680, from the Willacoochee Creek LF in (e) ventral and (f) dorsal aspects. (g) and (h) Left P8, UF 124093, from the Willacoochee Creek LF in (g) dorsal and (h) ventral aspects. (i) and (j) Partial right P1, UF/FGS 10867, from the Midway LF in (i) dorsal and (j) ventral aspects. (k) and (l) Right P11, UF/TRO 1708, from Camp Creek in (k) dorsal and (l) ventral aspects. (m) and (n) Right hypoplastron, UF 205740, from White Springs 2A in (m) ventral and (n) dorsal aspects. (o) and (p) Left P7, UF/FGS 3563, from the Midway LF in (o) dorsal and (p) ventral aspects. (q) and (r) Partial right xiphiplastron, UF 225676, from the Willacoochee Creek LF in (q) ventral and (r) dorsal aspects. (s) and (t) Partial neural, UF/FGS 10866, from the Midway LF in (s) dorsal and (t) ventral aspects. (u) and (v) Right P9 or P10, UF 21952, from the Seaboard LF in (u) dorsal and (v) ventral aspects. (w) and (x) Partial N1, UF 225677, from the Willacoochee Creek LF in (w) dorsal and (x) ventral aspects. Scale bar = 3 cm. Abbreviations: isc, ischium suture; mes, mesoplastral suture; ms, marginal-visceral step.
Plastral forelobe comparison of small-bodied podocnemidids from the middle Miocene of Florida and early Miocene of Panama. (a) and (b) Right epiplastron, UF 225679, from the early Barstovian Willacoochee Creek LF of northern Florida in (a) ventral and (b) dorsal aspects. (c) UF 225679 reconstructed as a partial plastral forelobe in ventral aspect. (d) and (f) Partial plastral forelobe, UF 280644, from the early Hemingfordian Centenario LF of the Panama Canal in (d) ventral and (f) dorsal aspects. (e) Illustration of UF 280644 in ventral aspect. Both specimens shown are nearly identical in size, possess a medial swelling on the dorsal epiplastra, lack gular overlap on the entoplastron, and have pectoral overlap on the posterior epiplastron. However, the humeral-pectoral sulcus of UF 225679 is antero-posteriorly angled, while in UF 280644 it is oriented straight transversely. Abbreviations: acp, acromial pit; axb, axillary plastral buttress; ent, entoplastron; epi, epiplastron; gul, gular scute; igl, intergular scute; hum, humeral scute; hyo, hyoplastron; pec, pectoral scute. Scale bar = 3 cm.
Correlation of middle Cenozoic pleurodire occurrences in North America and mean global deep-sea oxygen isotope records from Zachos et al. (2001). Gray bars indicate approximate temporal ranges of pleurodire records. Record of B. healeyorum after Weems and Knight (2013). Records of B. miocenica after Weems and George (2013). Locality ages after Tedford et al. (2004) and MacFadden et al. (2014). Identification of cf. Bothremydidae based on UF 171521. Ages for the Point “A” Dam site after Feldman and Portell (2007) for the Tallahatta Formation and Clayton et al. (2013) for the base of the Lisbon Formation. Age of the Alhajuela Formation after D. Jones and B. MacFadden (in prep.) and record of Bairdemys n. sp. after J.R.B. (in prep.).
Contributor Notes
Handling Editor: Jeffrey E. Lovich
