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Journal of Paleontology 89(5):821–844
subrubrum and flavescens groups and K. angustipons); ventral M11 sulcus situated on distal half of pygal (shared with the K. flavescens and subrubrum groups, K. angustipons,and K. sonoriense); plastron reduced, ~83%of carapace length (shared with K. baurii, K. hippocrepis, Kinosternon steindachneri, K. angustipons, K. dunni, K. herrerai, K. pojoaque,and K. skullridgescens); presence of grooved and inclined plastral bridge (shared with extant members of the K. flavescens and subrubrum groups); axillary scute moderately anteriorly exten- sive, terminating midway on M4 at the P3–4 suture (shared with some K. herrerai and Kinosternon oaxacae; by comparison the axillary scute is much less anteriorly extensive in K. flavescens and is more anteriorly extensive in K. leucostomum); femoral scute with broad overlap (~37%) on the plastral hindlobe (shared with Kinosternon sonoriense longifemorale).
Description.—Carapace: The carapace of FAM 12778 is com- prised of a nuchal, five neurals, one suprapygal, a pygal, eight costal pairs, and ten peripheral pairs (Fig. 4.3). The total length of the carapace is 150.94mm. There are five vertebral scutes, a single cervical scute, four pairs of pleural scutes, and eleven pairs of marginal scutes. The carapace of Kinosternon panne- kollops is smooth, with no trace of dorsal carinae on the nuchal, neurals, suprapygal, pygal, and costals. The bridge peripherals are smooth and lack a pronounced lateral keel that is present on the type specimens of Kinosternon pojoaque and Kinosternon rincon. The overall shape of the carapace is elongate and the carapacial rim is not
flared.Amedial depression is present along the dorsum of the carapace, similar to that discussed for K. pojoaque and Kinosternon skullridgescens (Bourque, 2012a, 2012b), although this depression has likely been enhanced due to some distortion in the preservation of the specimen. The nuchal broadly contacts N1. The nuchal lacks
substantial costiform processes. Instead, these are short and nub- like and contact only P1 without P2 contact. The P1 set broadly contacts the C1 set dorsally, similar to Kinosternon herrerai, Kinosternon hippocrepis,and Kinosternon skullridgescens. Conversely, Kinosternon subrubrum, Kinosternon baurii,and Kinosternon steindachneri lack substantial contact between P1 and C1. The five neurals are relatively large with N5 being the smallest, and all are posteriorly symmetrically short-sided. Costals 1–6 contact the neural series and C6–8 contact their respective pairs at the midline of the carapace. The shape of the posterior carapace of Kinosternon
pannekollops is steep sided and abruptly rounded over, similar to the subrubrum group and Kinosternon angustipons. The posterior carapacial rim inK. pannekollops is thick (most clearly observed in ventral view) along M8–9 and viscerally stepped at an angle of ~ 90 degrees (Figs. 4–6), and the marginal rim tapers to thin along M10–11. The posterior carapacial rim is not flared, but is instead straight sided. Likewise, in K. pannekollops the distal edge of M8–10 is rounded over. This combination of features is shared by Kinosternon subrubrum, Kinosternon baurii, and K. angustipons (and probably Kinosternon notolophus) and these attributes give the shells of these taxa an egg-shaped appearance. The posterior carapacial rim is much thinner and more sharply edged along M8–10 in Staurotypus Wagler, 1830, Sternotherus, and most species of Kinosternon, and the marginal scute to visceral transition is smoother and
much less abruptly stepped and closer to 180 degrees or at least more obtuse than 90 degrees. The arrangement of the features associated with the axillary
musk duct is unique in Kinosternon pannekollops compared with congeners (Fig. 4.4). In FAM 12778 the posterior origin of the axillary musk duct groove is marked by a foramen that is situated on the anterior of P3, which is more anteriorly situated than observed in other species of Kinosternon. In most Kinosternon this foramen is situated on P4, or on the posterior-most P3 such as in Kinosternon leucostomum and Kinosternon integrum. The axillary musk duct groove is very deeply incised, more so than in any other kinosternid examined for this study. The terminus for the axillary musk duct groove is also unique amongst other kinosternids in that it becomes enclosed as a foramen approximately halfway on P2 at the visceral M2–3 sulcus. This foramen terminates externally on the anterior-most P2 as a pore on the ventral M2. The peculiar morphology of the axillary musk duct terminus could in fact be the result of a pathological pock on M2 so deeply bored that it intersects the axillary musk duct groove, prompting the illusion that it is a natural structure of the axillary musk duct. FAM 12778 does exhibit a number of pockmarks, particularly on the peripherals. The axillary musk duct terminus is preserved only on the left side of FAM 12778, and it is difficult to inspect (or photograph) as it is obscured in part by the epiplastra. For this reason, the axillary musk duct has been illustrated on the right side of the shell in Figure 4.4, despite the fact that the anterior of the right axillary musk duct is not preserved in the fossil. FAM 12778 possesses thick secondary axillary buttressing on the visceral face of P3–4 that diminishes approximately halfway on P2. The great thickness of the peripheral buttressing was not observed in any other fossil or extant Kinosternon examined and like the composition of the axillary musk duct may be autapomorphic for K. pannekollops. Similar but less robust buttressing was observed in specimens of Kinosternon angustipons, some Kinosternon herrerai, and some members of the extant subrubrum group. In FAM 12778, the deeply incised anterior musk duct lies just ventral to and along the axillary buttress. The inguinal musk duct orifice is situated on the anterior half of P7 at the juncture where the posterior-most inguinal inframarginal scute contacts the anterior M7. Like the external terminus of the axillary musk duct, the external inguinal musk duct orifice is a deep and relatively broad pock. It is unclear if K. pannekollops possessed a set of caudal musk duct pores at the ventral juncture of M10–11 as this area is badly damaged in FAM 12778. The presence of caudal musk duct pores is a synapomorphy for K. subrubrum, Kinosternon steindachneri, and Kinosternon baurii.
Vertebrals 1–4 are antero-posteriorly long and narrow in
width (Fig. 4.1 and 4.3). V1 is 33.02 mm wide and is ~22% of the carapace length. The anterior-most lateral sulci of V1 are not restricted to the nuchal as in Kinosternon rincon, Kinosternon wakeeniense, and extant species of Sternotherus, but instead overlap onto P1. Vertebral 1 contacts only M1 and does not extend to M2. There is a pronounced constriction at the contact between V1–2, so that PLR1 overlaps N1. The degree of overlap of PLR1 onto N1 in FAM 12778 was not observed in extant kinosternine specimens examined, although PLR1 contact with
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