Hesperotestudo orthopygia ( Cope, 1878 )

Biewer, Jacob, Sankey, Julia, Hutchison, Howard & Garber, Dennis, 2016, A fossil giant tortoise from the Mehrten Formation of Northern California, PaleoBios 33, pp. 1-13 : 6-11

Hesperotestudo orthopygia ( Cope, 1878) : Holotype — AMNH 5868 .

Referred specimens — UCMP: 232060, 232061 ( V 65711);

132084 ( V 6878); 136086, 134831, 137148, 95918 ( V 71137);

132055, 134832, 136528, 137147, 95919, 95920 ( V 71138); 131793, 131794, 132087, 134830 ( V 81248); 136527 ( V 90007); 136526 ( V 90008).

Description

Epiplastron— There are epiplastra from two individuals present in the assembled material: UCMP 131794 ( Fig. 6A, B ), a complete left and mostly intact right attached to the adjacent plastral elements, and UCMP 137147 ( Fig. 6D, E ), an isolated left from a smaller individual. From the anterior end to about 60 mm posterior, they thicken dramatically: to 37 mm for the former specimen and 35.5 mm for the latter. At this point there is a sudden drop off forming the epiplastral excavation, a ridge below which there is an anterior excavation that gently slopes down to the relatively uniform thickness of the rest of the plastron. This ridge and midline

T Figure 11. A. Ventral view of juvenile shell with sulci divisions of plastron partly visible, UCMP 131793. B. Dorsal view, suture and sulci visible, UCMP 131793. C. Dorsal view of neural 8, showing sulcus, UCMP 136528. D. Ventral view of UCMP 136528 showing neural arch scar. Scale bar= 4 cm.

thickening taper laterally. These epiplastra would take up 57.5 mm and 72.8 mm of the total midline plastron length respectively. The anterior margin between the anterior tips of the epiplastra forms a shallow v-shaped concave emargination. On both specimens, the sulci dividing the gular and humeral scales are visible and deeply impressed.

Entoplastron— There are also two entoplastra present: one part of an articulated anterior plastral lobe, UCMP 131794 ( Fig. 6A, B ), and the other isolated, UCMP 134832 ( Fig. 6C ). Both are roughly diamond-shaped with a broad and rounded posterior and sharper anterior point. Their inner surfaces are concave and thinner by a few millimeters at the center (scapula attach site) relative to the edges. They are wider than long, 130 mm by 100 mm for UCMP 134832 and 149 mm by 123 mm for UCMP 131794, a trait seen in both H. osborniana and H. orthopygia ( Table 2).

Hyoplastron— UCMP 131794 ( Fig. 6A, B ) includes an intact right hyoplastron, that preserves a portion of the bridge, and anterior half of a left hyoplastron. UCMP 95919 ( Fig. 6F, G ) is a partial left hyoplastron lacking the left bridge. On UCMP 131794, the hyoplastra measure 150 mm at the midline. The distance across the base of the anterior plastral lobe (between the right and left axillary notch) is 308 mm. The hyoplastra are thinnest, 8-9 mm, near the midline and near the suture with the hypoplastron. They are thickest, 26 mm, near the free border at the sutures with the epiplastra. On UCMP 131794 the sulci between the pectoral and abdominal scales are visible on the right ( Fig. 6A, B ). Both these sulci are visible on UCMP 95919 ( Fig. 6F, G ). The pectoral scales narrow at the midline to about 27 mm and 24 mm respectively, and maintain this narrowness until widening markedly near the bridge.

Hypoplastron— There are two pairs of hypoplastra. UCMP 136527 ( Fig. 7A, B ) is an articulated left and right hypoplastra missing a section of the right anterior edge, and UCMP 134830 ( Fig. 7C, D ) consists of disarticulated left and right hypoplastra that are missing varying portions of their anterior margins and bridges. Both are thickest at the bridge and then thin to 7 - 10 mm at the thinnest part near the midline. There is a linear bulge on each that extends perpendicular to the midline between the bridges that are 14 mm and 16 mm thick respectively. The hypoplastra measure roughly 159 mm and 136 mm along the midline respectively. The sulci separating the abdominal, femoral, and inguinal scales are visible on the ventral sides ( Fig. 7B, D ). There is extensive contact between the femoral and inguinal scales ( Fig. 7B, D ).

Xiphiplastron— There is a partial pair of xiphiplastra attached to the previously mentioned disarticulated hypoplastra, UCMP 134830 ( Fig. 7C, D ), as well as two isolated elements, UCMP 132086 and UCMP 132084 ( Fig. 8 A-D). The joined pair is intact with the exception of the posterior left side. They are thickened dorsally on the free margin to about 25 mm forming a ridge which then quickly curves down and thins toward the midline to 9 mm. On the ventral side of the disarticulated specimens the sulci between the femoral and anal scales are distinctly visible ( Fig. 8B, C ). This sulcus is obscure on UCMP 134830.

Peripherals— Of the many peripheral fragments present in the collection, only three specimens can be placed as to carapace position. UCMP 97986 (Fig. 9A) is a right peripheral 5 . UCMP 95919 (Fig. 9B) consists of the pygal, the peripheral pairs 10 and 11, and the left peripheral 9. The third , UCMP 95918 (Fig. 9C) is a large segment of two left peripherals missing the dorsal edge. While the pygal has a relatively flat outer surface, the free margins of the peripherals progressively flare anterolaterally, giving them a curved surface. Each of the peripherals has intermarginal sulci that cross them from dorsal to ventral margins and curve anteriorly near the lateral margin. The pygal lacks intermarginal sulci, has a relatively rectangular shape that is taller than wide, 71 x 45 mm at the free margin and 53 mm dorsally. There is a difference in thickness among individual specimens measured at their thickest point . UCMP 95918 is 37 mm thick and nearly twice as thick as UCMP 95919 (23.3 mm) . UCMP 95919 has a width of 54 mm measured along the free margin while UCMP 95918 is 146 mm wide .

Costals— There are five costals in the material from the Mehrten Formation: UCMP 136526 ( Fig. 10A ),UCMP 134831 ( Fig. 10B ), UCMP 137148 ( Fig. 10C ), UCMP 95918 (dorsal part) ( Fig. 10D ), and UCMP 95919 ( Fig. 10E ). The costals in Figure 10 A-C and E have the same elongate shape and about the same thickness, 15 mm, with the exception of the smaller specimen that is only 6.7 mm thick. The fifth ( Fig. 10D ), which belongs to the same animal as the large peripherals, has a maximum thickness of 27 mm before thinning to 15 mm. The origin of the rib head on the four complete specimens (UCMP 134831, UCMP 137148, UCMP 136526, UCMP 95919) occurs about 19 mm (15 mm for the smaller specimen) away from the dorsal suture. The sulci separating the pleural and vertebral scales are visible on all four, in some places lightly and in others deeply impressed.

Juvenile— UCMP 131793 (Fig. 11A, B) is the crushed carapace and plastron of a young tortoise, lacking most or all of the free margins. It is 103 mm long and 92 mm wide as preserved. The bones, inferred scales, and sulci between them are generally visible on each side.

Neural— There is a single small neural, UCMP 136528 (Fig. 11C, D). On the underside the neural arch is visible. An intervertebral sulcus traverses the dorsal surface scales. The greater length than width, lateral sutural projections indicating intercostal articulation and presence of an intervertebral sulcus suggest neural 8.

Figures

Figure 6A-G. Elements of the anterior plastron. A. Ventral view of UCMP 131794, anterior plastral lobe including left and right epiplastra, entoplastron, right and partial left hyoplastron. B. Locations of sutures, dark grey and sulci, light grey. C. Entoplastron, UCMP 134832. Ventral view of sulci, separating humeral scales and gular scales visible. D. Dorsal View of left epiplastron, UCMP 137147. E. Ventral view showing sulcus between gular and humeral scales. F. Ventral view of Partial left hyoplastron, UCMP 95919, showing sulci between humeral, pectoral, and abdominal scales. G. Location of sulci. Scale bars=4 cm.

Figure 7. A. Ventral view of articulated right and partial left hypoplastron, UCMP 136527. B. Locations of sutures and of sulci between abdominal, femoral, and inguinal scales, UCMP 136527. C. Ventral view of posterior plastron including most of left and right hypoplastron and xiphiplastron, UCMP 134830. D. Locations of sutures and sulci between abdominal and femoral scales, UCMP 134830. Scale bars=4 cm.

Figure 8A-D. Two xiphiplastra. A. Dorsal view of UCMP 132086, a left Figure 9A-C. Peripherals. A. Right peripheral 5, lateral view, UCMP xiphiplastra. B. Ventral view of UCMP 132086 showing sulcus between 97986. B. Pygal, left and right peripheral pairs 10-11, and left periphfemoral and anal scales. C. Ventral view of UCMP 132084, right xiphi- eral 9, posterior view showing sulci separating marginal scales, UCMP plastron showing sulci. D. Dorsal view of UCMP 132084. Scale bars=4 95919. C. Lateral view, two articulated partial left peripherals, UCMP cm 95918. Scale bars=4 cm.

Figure 10A-E. Identified costals in external and visceral views. A. UCMP 136526. B. UCMP 134831. C. UCMP 137148. D. UCMP 95918. E. UCMP 95919. Scale bars=4 cm.