Natemys sp. 1

Fallon, Bailey R. & Boessenecker, Robert W., 2020, Multispecies leatherback turtle assemblage from the Oligocene Chandler Bridge and Ashley formations of South Carolina, USA, Acta Palaeontologica Polonica 65 (4), pp. 763-776 : 766-772

Natemys sp. 1

Fig. 2 .

Material.— CCNHM 4405.1 – 4405.5 , five associated non-ridged carapacial ossicles collected by Steve Hildenbrandt in July 2017 from an unnamed upper unit (potentially Bed 3 correlative) of the Chandler Bridge Formation , Coosaw Preserve Subdivision ; CCNHM 4288 , a non-ridged carapacial ossicle collected by RWB on June 14, 2018 from the Givhan’s Ferry Member of the Ashley Formation, McKewn Subdvision, Ladson, SC ; CCNHM 5540 , 5541 , and 5542, three non-ridged carapacial ossicles collected by Steven Miller from Bed 1 ( Fig. 1 ) of the Chandler Bridge Formation, locality uncertain. All Oligocene of South Carolina, USA .

Description.—CCNHM 4405.1 is elongate (40.9×23.0 mm) and tabular in cross section (see Table 1 for size dimensions of all ossicles). The dorsal surface is smooth with seven foramina near the center ( Fig. 2C 1 ). The visceral surface is scattered with minute pores and five foramina ( Fig. 2C 2 ). The sutural margins are straight with six shallow notches ( Fig. 2C ). The sutural surface reveals three distinct internal layers: a dense dorsal layer, a thick and highly vascularized middle layer, and a thin, compact visceral layer ( Fig. 2C 3 ). CCNHM 5540 is also elongate and has seven deep sutural notches. It has similar dimensions and surface textures compared to CCNHM 4405.1 and a roughly similar internal structure ( Table 1, Fig. 2 ).

CCNHM 4405.2, 4405.3, 4405.4, 4405.5, 5541, and 5542 are all polygonal and approximately tabular in cross section, although CCNHM 4405.3 has a single rounded peak on the visceral surface ( Fig. 2A 2 –I 2 ). They have smooth dorsal surfaces with one to three scattered foramina ( Fig. 2A 1 –I 1 ). CCNHM 4405.2 and 4405.3 have smooth and round sutural margins, CCNHM 4405.4 has one crescent-shaped margin, and CCNHM 4405.5 has straight sutural margins ( Fig. 2A 1 – 1 ). The six ossicles have visceral surfaces that are comparable to CCNHM 4405.1 and have one to sixteen scattered foramina ( Fig. 2A 2 –I 2 ). All have internal structures that are comparable to CCNHM 4405.1 as revealed by their sutural surfaces ( Fig. 2A 3 –I 3 ). CCNHM 5541 has a fractured edge that reveals its internal structure, which is indeed comprised of the three described layers. CCNHM 4288 is comparable to the CCNHM 4405.1–4405.5 ossicles, is the thinnest (5.8 mm) of all the ossicles reported here, and is one of the thinnest non-ridged fossil leatherback ossicles reported to date. The thicknesses of these ossicles do not exceed 11.1 mm.

Remarks.— Specimens are assigned to Natemys sp. 1 owing to (i) the scalloped edges of some associated ossicles, (ii) to their triple-layered internal structure, and (iii) to their thinness.

The nine carapacial ossicles from the Chandler Bridge and Ashley formations (CCNHM 4405.1–4405.5, 4288, 5540, 5541, and 5542) are referred to the genus Natemys , which is historically represented by only one species, Natemys peruvianus . Wood et al. (1996) first identified this species based on a partial carapace that exhibits a distinct sunflower pattern. This pattern is defined by a linear series of enlarged ossicles with deeply scalloped edges that are surrounded by smaller, elongate “petal” ossicles arranged in a radial pattern ( Wood et al. 1996). Natemys peruvianus is one of three leatherback species that exhibit such a pattern, the others being Natemys rupeliensis and Psephophorus polygonus Meyer, 1847 ( Wood et al. 1996; Karl et al. 2012; Karl 2014; Peters et al. 2019; see taxonomic note in Introduction).

CCNHM 4405.1 and 5540 resemble the central ossicles of the sunflower pattern because they have deeply scalloped edges. As such, we refer these and their associated ossicles to Natemys sp. 1 ( Peters et al. 2019; see taxonomic note). However, we differentiate these nine ossicles ( Natemys sp. 1 ) from those we refer to Natemys sp. 2 based on ossicle thickness and internal structure. Natemys sp. 1 ossicles are thinner (5.8‒11.1 mm) than Natemys sp. 2 ossicles 14.8‒18.0 mm). Additionally, Natemys sp. 1 ossicles have a relatively thick middle internal layer whereas Natemys sp. 2 ossicles have a relatively thick visceral-most internal layer see below for description).

Natemys sp. 2

Fig. 3 .

Material.— CCNHM 4287.1 and 4287.2, a pair of associated non-ridged carapacial ossicles collected on different dates but within about a meter of each other by Shaun Coates on August 11, 2018 from the? Givhan’s Ferry Member of the Ashley Formation, Chandler Bridge Creek ; CCNHM 4910 , a non-ridged ossicle collected by Shaun Coates on April 1, 2018 from the? Runnymede Marl member of the Ashley Formation, Sawmill Branch Canal, Summerville. All Oligocene of South Carolina, USA .

Description.—CCNHM 4287.1 is the largest ossicle reported here, measuring 74.9 mm in length and 18.0 mm in maximum thickness. The fragment measures 44.1 mm in width and appears to be fractured in half, revealing its internal structure ( Fig. 3A ). The original dimensions may therefore be projected to be approximately 88× 74.9 mm. CCNHM 4287.1 is slightly curved in cross section ( Fig. 3A 3 ). The dorsal surface is smooth and imperforate ( Fig. 3A 1 ). The visceral surface is also smooth, but has many small, dispersed pores ( Fig. 3A 2 ). The non-fractured sutural surfaces are highly vascularized and are heavily scalloped with four distinct notches ( Fig. 3A 1 ). The ossicle fragment is divided into three distinct internal layers that differ from the internal layers of Natemys sp. 1 ossicles ( Figs. 2A 3 –I 3, 3A 3 ). The dorsal layer of CCNHM 4287.1 is thin (~ 3 mm) and compact with no discernable vascularization. The middle layer is thicker (~ 4 mm) and is moderately vascularized and the visceral-most layer is highly vascularized and is the thickest (~ 11 mm) internal layer ( Fig. 3A 3 ).

CCNHM 4287.2 is another large (54.9× 37.8 mm) ossicle that is approximately tabular. Its dorsal, visceral and sutural surfaces are comparable to those of CCNHM 4287.1 ( Fig. 3 ). These shared surface textures suggest CCNHM 4287.2 may also exhibit a stratified internal structure. CCNHM 4287.2 most notably differs from all other ossicles in having a fissure that runs along the visceral surface, splitting the ossicle approximately in half ( Fig. 3B 2 ). This fissure expands about halfway into the ossicle laterally (i.e., toward the dorsal surface) as revealed by its sutural surface ( Fig. 3B 3 ). CCNHM 4910 is a third, very thick (14.8 mm) carapacial ossicle. It is tabular in cross section, and has dorsal, visceral and sutural surfaces comparable to those of CCNHM 4287.1 and 4287.2 ( Fig. 3 ).

Remarks.— Specimens are assigned to Natemys sp. 2 owing to (i) the scalloped edges of some of the ossicles, (ii) to their triple-layered internal structure, and (iii) to their thickness.

CCNHM 4287.1 and 4287.2 have scalloped edges that are reminiscent of the central ossicles of the carapacial sunflower pattern unique to Natemys peruvianus and Natemys rupeliensis , leading us to refer these ossicles to the genus Natemys . We also refer CCNHM 4910 to Natemys sp. 2 because its thickness is most comparable to CCNHM 4287.1 and 4287.2. As mentioned, we distinguish these three Natemys sp. 2 ossicles from the Natemys sp. 1 ossicles because they are considerably thicker (14.8‒18.0 mm) than the Natemys sp. 1 ossicles and because they have a relatively thick visceral-most, not middle, internal layer.

Ossicle thickness, size and internal structure have historically been helpful in distinguishing ossicles of the genus Natemys from those of other leatherback genera. The triple-layered internal stratification of our Natemys sp. 1 and 2 ossicles is consistent with that of ossicles described by Köhler (1996: figs. 82, 83) and by Karl et al. (2012), which we consider to be Natemys ( Fallon and Boessenecker 2019; see taxonomic note). Furthermore, Peters et al. (2019) noted that Natemys peruvianus and Natemys rupeliensis have considerably thickened ossicles when compared to other leatherback species like Cosmochelys dolloi Andrews, 1919 , Egyptemys eocaenus Andrews, 1901 , E. oregonensis Packard, 1940 , Psephophorus polygonus , and “ Psephophorus calvertensis ” Palmer, 1909 . CCNHM 4287.1, 4287.2, and 4910 are some of the thickest non-ridged ossicles reported to date ( Fallon and Boessenecker 2019), which supports their identification as Natemys sp. 2 . Peters et al. (2019) also used ossicle size to distinguish between leatherback genera even though this trait is ontogenetically variable and depends on the ossicle’s original position in the shell. They noted that N. peruvianus and N. rupeliensis ossicles tend to be larger than the ossicles of the genera Cosmochelys , Egyptemys and Psephophorus ( Peters et al. 2019) . This finding further supports our identification of CCNHM 4287.1 and 4287.2 as Natemys sp. 2 since they are some of the largest (88 and 54.9 mm, respectively) leatherback ossicles recorded ( Wood et al. 1996; Peters et al. 2019). Moreover, CCNHM 4287.1 ( Natemys sp. 2 ) is larger (88× 74.9 mm) than the enlarged ossicles of Natemys peruvianus , which are 40‒65 mm in maximum length, and is more comparable to the enlarged ossicles of Natemys rupeliensis , which measure up to 102 mm in maximum length ( Wood et al. 1996). Therefore, we note that Natemys sp. 2 compares well with N. rupeliensis in terms of having very large ossicles. Furthermore, Natemys sp. 2 and N. rupeliensis both date to the early Oligocene and both are found in the North Atlantic Ocean basin ( Köhler 1996; see taxonomic note in the Introduction). In the future, it is possible that Natemys sp. 2 may prove to be synonymous with N. rupeliensis . However, we advise caution against such a determination until more complete specimens of Natemys sp. 2 are discovered and described. Still, the differences in thickness, size and internal structure between Natemys sp. 1 and 2 highlight the necessity in reporting such features in future work.

Interestingly, CCNHM 4287.2 preserves a fissure that runs along its mid-visceral surface. This is the first formal report of a leatherback ossicle exhibiting such a trait. The only other mentions of a visceral fissure-like feature on leatherback ossicles are from a “ Psephophorus calvertensis ” (synonymy P. polygonus sensu Peters et al. 2019 ) shell fragment found in the Calvert Formation of Maryland, USA, and from unpublished Belgian fossils ( Köhler 1996; Roger Wood personal communication to Peters et al. 2019). While Köhler (1996) attributed this feature to an infection, we agree with Roger Wood that this is likely a fusion of two ossicles due to an individual’s ontogenetic aging process. The expansion of this fissure about halfway to the dorsal surface suggests the ossicle may have resulted from the incomplete fusion of two separate ossicles late in ontogeny. Such a fusion raises the possibility that the enlarged ossicles of Natemys (and perhaps Psephophorus ) may have been formed by the coalescence of smaller ossicles.

cf. Egyptemys sp. Wood, Johnson-Gove, Gaffney, and Maley, 1996

Fig. 4 .

Material.— CCNHM 4289 , a ridged carapacial ossicle collected by RWB on June 26, 2018 from Bed 1 of the Chandler Bridge Formation , McKewn Subdivision , Ladson; Oligocene of South Carolina, USA .

Description.—CCNHM 4289 is one of two (see below) ossicles that exhibits a keel, or ridge. The ossicle has a shallow, transverse arch and a weak middorsal keel that is ~ 2 mm in height ( Fig. 4A 3 ). The dorsal surface is smooth but is scattered with shallow linear depressions ( Fig. 4A 1 ). The visceral surface shows little concavity corresponding to the dorsal keel, is weakly dimpled, and has several small pores Fig. 4A 2 ). The sutural margins are smooth and scalloped with eight notches ( Fig. 4A 1 ). The sutural surface suggests an internal structure that is comparable to that of the five CCNHM 4405.1–4405.5 ossicles ( Natemys sp. 1 ).

Remarks.—Specimen is assigned to cf. Egyptemys sp. owing to (i) its weakly keeled, dorsal ridge that (ii) shows little expression on the visceral surface, and (iii) to its thickness.

CCNHM 4289 is referred to cf. Egyptemys sp. Leatherback fossils now referred to this genus were originally referred to the species Psephophorus eocaenus and Psephophorus oregonensis by Andrews (1901) and Packard (1940). However, Wood et al. (1996) revised this classification based on carapacial ridge distinctions between the two genera. The genus Egyptemys is distinct in having weakly keeled ridges that lack a corresponding trough on the visceral surface, are semi-circular in cross section, and are confined to a narrow middorsal band on ridge-bearing ossicles ( Wood et al. 1996). Köhler (1996) supported this anatomical distinction in his depiction of E. eocaenus keels. Parmley et al. (2006) noted that indeterminate dermochelyid ossicles from the late Eocene of Georgia ( USA) share these ridge characteristics, and tentatively compared these ossicles to those of the genus Egyptemys . Furthermore, CCNHM 4289 does not compare well with most Psephophorus - type ossicles. The ridges on ossicles referred to the genus Psephophorus are very prominent, may or may not show a visceral concavity, and appear on ossicles that are usually anteroposteriorly elongated ( Köhler 1996; Wood et al. 1996; Chesi et al. 2007; Delfino et al. 2013). CCNHM 4289 has a weak ridge that is semi-circular in cross section, is confined to a narrow middorsal band, is transversely wide, and mostly lacks visceral expression. This specimen is therefore best identified as cf. Egyptemys sp. based on its ridge characteristics. However, recent work suggests that weakly ridged ossicles with slight visceral concavity also exist on the accessory ridges of Psephophorus - type shells. Furthermore, the scalloped margins of CCNHM 4289 also mark it as a candidate for the sunflower pattern that has been reported in shells assigned to the genus Psephophorus ( Peters et al. 2019) . As such, our assignment of CCNHM 4289 to Egyptemys is tentative.

The geochronological age and thickness of CCNHM 4289 also support its identification as cf. Egyptemys sp. This genus is known from the late Eocene of northern Egypt and the early Miocene of Oregon and California, USA ( Andrews 1901; Packard 1940; Mitchell and Tedford 1973; Köhler 1996; Wood et al. 1996). With an age of 24.7–23.5 Ma, CCNHM 4289 compares well with the geochronological ages of other Egyptemys fossils ( Table 2). Although CCNHM 4289 falls within the thickness range recorded for Psephophorus ossicles, the substantial variation in Psephophorus ossicle thickness (4.8‒19.9 mm) offers little help in identification ( Chesi et al. 2007; Delfino et al. 2013; Fallon and Boessenecker 2019). Still, Wood et al. (1996) noted that the ridged ossicles of Egyptemys eocaenus are no thicker than 12 mm, which is consistent with the 9.0 mm thickness of CCNHM 4289.

cf. Psephophorus sp. Meyer, 1847 Fig. 5 .

Material.— CCNHM 5460 , an isolated non-ridged carapacial ossicle collected by Sarah J. Boessenecker on September 4, 2019 from the Givhan’s Ferry Member of the Ashley Formation, Wescott Plantation Subdivision, Summerville; CCNHM 5543 , an isolated ridged ossicle collected by Steven

Miller from Bed 1 of the Chandler Bridge Formation, locality uncertain. All Oligocene of South Carolina, USA.

Description.—CCNHM 5460 is a polygonal ossicle that is approximately tabular ( Fig. 5A 3 ). It is distinguished from the other ossicles in having shallow radial grooves on its otherwise smooth, slightly convex dorsal surface ( Fig. 5A 1 ). Its slightly concave visceral surface is ornamented with a woven lacework pattern of shallow ridges and grooves, and its sutural surface reveals an internal structure consisting of a dense dorsal layer and a vascularized visceral layer ( Fig. 5A 2 , A 3 ). CCNHM 5543 is thicker (14.0 mm) than CCNHM 5460 (8.9 mm), but is otherwise similar in length and width ( Table 1). It has a smooth dorsal surface and a somewhat smooth visceral surface that is crossed by four deep grooves intersecting in the center of the ossicle

Fig. 5B 2 ). CCNHM 5543 has a broad, low ridge that spans the entire width of the ossicle and is pronounced on the visceral surface ( Fig. 5B 3 ). Its internal structure is comparable to CCNHM 5460 as revealed by its sutural surface.

Remarks.—Specimens are assigned to cf. Psephophorus sp. owing to (i) the dorsal, radial ornamentation on CCNHM 5460, (ii) the presence of a broad ridge with prominent visceral expression on CCNHM 5543, and (iii) to the ossicles’ diploic internal structure.

CCNHM 5460 is referred to the genus Psephophorus based on its dorsal surface texture and internal structure. Most notably, it has a dorsal, radial ornamentation that has also been described for the genera Arabemys Tong, Buffetaut, Thomas, Roger, Halawani, Memesh, and Lebret, 1999 , Cosmochelys and Dermochelys , though the ornamentation is much less pronounced in CCNHM 5460 when compared to that of the former extinct genera ( Andrews 1919; de la Fuente et al. 1995; Tong et al. 1999; Zvonok et al. 2013; Zvonok and Danilov 2019). Albright et al. (2003) also noted this distinction, and tentatively assigned Antarctic Eocene ossicles discussed in their study to cf. Psephophorus sp. Finally, the diploic internal structure of CCNHM 5460 also resembles ossicles of the genus Psephophorus as outlined by Delfino et al. (2013) and Fallon and Boessenecker (2019).

We assign CCNHM 5543 to cf. Psephophorus sp. based on the presence and structure of its ridge. Like other ridged ossicles referred to the genus Psephophorus, CCNHM 5543 has a ridge that is very prominent, demonstrates visceral concavity, and spans the entire width of the ossicle ( Köhler 1996; Wood et al. 1996; Chesi et al. 2007; Delfino et al. 2013).

Figures

Fig. 2. Ossicles of leatherback turtle Natemys sp. 1 from Oligocene of South Carolina, USA. CCNHM 5542 (A), CCNHM 5540 (B), CCNHM 4405.1– 4405.5 (C–G, respectively), CCNHM 4288 (H), and CCNHM 5541 (I), in dorsal (A1–I1), visceral (A2–I2), and sutural (A3–I3) views.

Fig. 1. The Ashley and Chandler Bridge formations and geologic context of CCNHM ossicles. A. Map of the southeastern United States.B. Map ofAshley and Chandler Bridge formation exposures on the coast of South Carolina. Stars denote Oligocene dermochelyid localities. C. Stratigraphic column of the Ashley and Chandler Bridge formations.

Fig. 3. Ossicles of leatherback turtle Natemys sp. 2 from Oligocene of South Carolina, USA. CCNHM 4287.1 (A), CCNHM 4287.2 (B), and CCNHM 4910 (C), in dorsal (A1–C1), visceral (A2–C2), and sutural (A3–C3) views. Arrows indicate the fissure on the visceral and sutural surfaces of CCNHM 4287.2, and note the stratified internal structure of CCNHM 4287.1.

Fig. 4. Ossicle of leatherback turtle cf. Egyptemys sp. (CCNHM 4289) from Oligocene of South Carolina, USA, in dorsal (A1), visceral (A2), and sutural A3) views.

Fig. 5. Ossicles of leatherback turtle cf. Psephophorus sp. from Oligocene of South Carolina, USA. CCNHM 5460 (A) and CCNHM 5543 (B), in dorsal A1, B1), visceral (A2, B2), and sutural (A3, B3) views.