Helochelydra nopcsai Lapparent de Broin & Murelaga, 1999

Joyce, Walter G., Evers, Serjoscha W., Ren, Sara, Rollot, Yann & Schwermann, Achim H., 2023, The helochelydrid turtle Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) fissure fills of Balve, North Rhine-Westphalia, Germany, including a large sample of granicones, Fossil Record 26 (1), pp. 117-133 : 117

Helochelydra nopcsai Lapparent de Broin & Murelaga, 1999

Type material.

NHMUK R171 (holotype), a fragmentary shell associated with limb and girdle elements ( Nopcsa 1928).

Type locality.

Isle of Wight, United Kingdom (Nopcsa, 1928).

Type stratum.

Wessex Formation, Wealden Group, Barremian, Early Cretaceous ( Joyce et al. 2011).

Diagnosis.

Helochelydra nopcsai can most easily be diagnosed as a helochelydrid by the presence of a secondary pair of occipital tubercles that are formed by the pterygoids, a triangular fossa that is formed by the squamosal at the posterior margin of the skull, a shell covered by a surface texture consisting of distinct tubercles, expansion of the visceral cavity to the front and back of the peripheral series (also present in some compsemydids and pleurosternids), and presence of an entoplastral scute. Helochelydra nopcsai can be distinguished from other helochelydrids by a shell surface texture consisting of evenly spaced tubercles that easily dislocate (also present in Helochelys danubina ), cone-, hook-, and plate-shaped osteoderms (also present in Naomichelys speciosa ), and pterygoids that fully cover the parabasisphenoid ventrally. No other helochelydrid is known to possess compound granicones. Helochelydra nopcsai currently cannot be distinguished from Helochelys danubina using morphological features, because the latter is poorly known and its holotype lost. Stratigraphic criteria (i.e., an age difference of more than 20 Ma) nevertheless suggest that they represent distinct taxa.

Referred material.

Early Cretaceous (Barremian), Wessex Formation, Isle of Wight, United Kingdom (material referred by Lapparent de Broin and Murelaga 1999 and Joyce et al. 2011; Pleurosternidae of Sweetman and Insole 2010); Early Cretaceous (Hauterivian-Barremian), Nouvelle-Aquitaine, France ( Solemydidae of Néraudeau et al. 2012); Early Cretaceous (Barremian), Aragon, Castile and León, La Rioja, and Valencia, Spain ( Solemydidae of Pérez García 2009; Pérez-García et al. 2012, 2013; Scheyer et al. 2015); Early Cretaceous (late Barremian - early Aptian), Balve, Germany (all material described herein) .

Description.

Although the available sample consists of more than 250 specimens, including more than 150 granicones, only a small number of specimens provide useful anatomical information. This description is therefore focused on the most informative specimens.

Surface texture. The surface of all available shell material and granicones is evenly covered by distinct tubercles (Figs 2 - 5 ). Each tubercle is columnar and clearly separated from neighboring tubercles. The tubercles of the shell easily dislocate at their base, where they leave behind a circular scar, which can easily be recognized under the microscope. Although some variation is apparent across the shell (e.g., tubercles are less densely spaced over the neurals), most are about 0.5 to 1 mm wide, about 0.5 to 1.5 mm tall, and separated from neighboring tubercles by about 1.5 to 2 mm. The tubercles are less distinct on the available cranial fragment and all granicones, but similar in dimensions. In all regards, the surface texture closely approximates to that of Helochelydra nopcsai material from the Wessex Formation of England (NHMUK R171, R5195-5199, R5201, R5205, R5238).

Carapace. The available material (Figs 3 , 4 ) suggests that the carapace had a length of up to 70 cm (see Costals below), a nuchal notch extending laterally up to peripheral II (see Nuchal below), and that deep grooves decorated the neural-costal contacts along the posterior half of the shell (see Costals and Neurals below). The carapace is 5 to 6 mm thin along the peripherals and costals, but thickens to 8 mm at the neurals. All parts of the carapace are evenly covered by distinct tubercles that disarticulate easily (Figs 3 , 4 ).

Neurals. We are able to identify six elements as neurals: WMNM P88808 (Fig. 3a ), P49476 (Fig. 3b ), P49351 (Fig. 3c ), P88809 (Fig. 3h ), and P88807 (Fig. 3e ). WMNM P88807 is a larger shell fragment that consists of two neural elements in addition to costal remains including two neural elements that are up to 8 mm thick (Fig. 3e ). The posteriorly angled orientation of the associated rib heads relative to the neural column suggest that the elements represent neurals V, VI, or VII. Their outline is hexagonal with short anterolateral sides. The anterior of the two neural shows faint traces of a midline keel, as has also been reported for neurals of Naomichelys speciosa ( Joyce et al. 2011). WMNM P88808 (Fig. 3a ) is the largest available neural, flat, about 5 mm thick, and has a hexagonal outline (i.e., contacts with six surrounding bones are apparent). Its dorsal surface is badly eroded: there are faint traces of an intervertebral sulcus, but only the bases remain of the surface tubercles. The orientation of this element is not clear, but its size suggests that it originates from the anterior half of the shell. WMNM P49351 (Fig. 3c ) is only half as long anteroposteriorly as the previous element, but about 7 mm thick. A sharp median depression crosses its surface. WMNM P49476 (Fig. 3b ) has similar dimensions to WMNM P49351, but it is only about 4 mm thick and flat. The equilateral, hexagonal outline of the last two elements suggests an origin from the back of the shell. WMNM P88809 (Fig. 3d ) is the smallest available neural and only about 2 mm thin. Only the right two thirds are preserved. Its small size and equilateral, hexagonal outline suggest that it originates from the posterior portions of the shell of a juvenile.

Costals. A number of elements can be diagnosed as costals, of which the following seven are the most informative: WMNM P88807 (Fig. 3e ), a shell fragment that includes remains of three costals, WMNM P88810, a partial left costal I (Fig. 3f ), P88811 (Fig. 3h ) and P88812 (Fig. 3g ), two large costal fragments, and P49470, the median portion of a midsized costal (Fig. 3i ). The dermal portion of these costals is about 4 mm thick. The ribs, when present, are broad, about a quarter of the anteroposterior width of the costal, but only add about 2-3 mm to the thickness of each element. The three costal remains preserved in WMNM P88807 (Fig. 3e ) are in articulation with two neurals and, therefore, best document the medial aspect of these elements. The slight posteriorly angled orientation of the ribs suggest that they originate from the mid-posterior part of the shell. All elements have two contacts with the adjacent neurals. The free, medial portion of one costal rib is well preserved. It is relatively narrow, about 2 mm across, and delimits a relatively low costovertebral tunnel. WMNM P88810 (Fig. 3f ) can be identified as a partial left costal I by the presence of the articular scar for dorsal rib I. The sulcus between vertebral I and pleural I diagonally traverses the dorsal surface of this element. A rounded anteromedial facet for articulation with the nuchal suggests that the nuchal deeply inserted between the anterior costals. WMNM P88811 (Fig. 3h ) is significant, as it possesses intact anterior and posterior margins. The greatest anteroposterior “width” of this element is 56 mm, which is a little greater than the greatest costal width that can be observed for the only known complete skeleton of the helochelydrid Naomichelys speciosa ( Joyce et al. 2014). Assuming similar proportions, the full carapace may therefore have reached a length of over 70 cm. WMNM P49470 (Fig. 3i ) only has an anteroposterior “width” of 3 cm and, therefore, likely originates from the back of the shell of a smaller individual. It is notable for exhibiting a concavity just above the rib head, near its medial articulation with the neurals. Together with the grooved neurals, this suggests some meso-scale sculpturing of the shell surface towards the back of the shell.

Nuchal. Two specimens preserve remains of the nuchal: WMNM P88813, the right half of a large nuchal (Fig. 4a ), and WMNM P88814, a smaller nuchal fragment, which remains articulated with remnants of the adjacent right costal I and peripherals I and II (Fig. 4b ). The two specimens suggest that a broad nuchal notch was present that was framed by peripherals II, as is typical for helochelydrids ( Joyce et al. 2014; Pérez-García et al. 2020a). The oblique anterolateral contact of the nuchal with peripheral I is oriented more to the anterior than the side. There is no evidence for costiform processes. The right portion of a cervical scute is preserved in WMNM P88813 (Fig. 4a ). As this specimen broke near the midline, we estimate that the complete cervical was at least twice as wide than long. Lateral to the cervical scute, a sulcus is apparent that delimits the contact between marginal I and vertebral I. There is no evidence for the presence of a prepleural on the nuchal. The skin-scute sulcus runs on the underside of the nuchal just behind the anterior margin of the bone.

Peripherals. A number of peripherals are preserved, of which we present the six most informative. WMNM P88814 includes a nearly complete right peripheral I articulated with remnants of the adjacent element (Fig. 4b ). WMNM P88815 is a first peripheral I as well, but lacks much of its dorsal surface (Fig. 4c ). These two elements document that peripheral series articulated with the nuchal laterally along an oblique (WMNM P88814) to straight (WMNM P88815) suture, costal I posteriorly, and peripheral II laterally. The scute-skin sulcus runs diagonally across the underside of the peripheral, thereby bridging the location of this sulcus near the margin of the shell at the nuchal with the deep location on the second peripheral. Although damaged, these bones also document that the visceral cavity extends all the way from the bridge region to the medial fifth of peripheral I, as had previously been documented for other helochelydrids ( Joyce et al, 2014; Pérez-Garcia et al, 2020a). WMNM P88816 is a relatively large peripheral element that we tentatively interpret as the right peripheral XI, as it displays a small, but elongate facet on its visceral side best interpreted by reference to Naomichelys speciosa ( Joyce et al. 2014) as the articulation site for thoracic rib IX (Fig. 4h ). This element shows faint traces of an intermarginal scute sulcus and documents that the visceral cavity extends posteriorly from the bridge region to reach the pygal. The visceral cavity of Helochelydra nopcsai , therefore, extended all the way from the medial fifth of peripheral I to the pygal, much as in Aragochersis lignitesta ( Pérez-Garcia et al, 2020a), but also Pleurosternon bullockii ( Joyce et al. 2022). However, three finger-like pockets within the cavity of WMNM P88816 suggests that the visceral cavity is not developed evenly across the shell (see arrows in Fig. 4h ). The waning visceral cavity combined with minor asymmetries suggest that WMNM P41690 is a damaged right peripheral X as well (Fig. 4f ). As the most anterior and most posterior peripherals show evidence of a visceral cavity, it is not possible to identify the serial identity of the damaged, remaining peripheral elements. Three notable remains, WMNM P47259 (Fig. 4g ), P49051 (Fig. 4d ), and P88817 (Fig. 4e ) nevertheless document variable development of a low gutter. WMNM P49051 and P88817 furthermore highlight that the dorsal and ventral branches that make up each peripheral are thin-walled and meet each other just medial to the edge of the carapace. The peripherals of the bridge region are therefore extremely fragile.

Epiplastron. The only available epiplastron, WMNM P88818, is near complete, but mineral precipitations on the external surface partially conceal its sulci (Fig. 5b ). The epiplastron formed the anterior aspects of the transversely oriented anterior plastral lobe. The epiplastron has a convex posteromedial contact with the entoplastron. The epiplastron hereby slightly underlaps the entoplastron, which, as a result, would have appeared smaller in ventral than in dorsal view. An equally broad contact is present posterolaterally with the hyoplastron. Though damaged, deep pockets along the posterolateral margin of the epiplastron suggest that at least two peg-like processes of the hyoplastron inserted into the epiplastron. A short median contact is furthermore apparent with its counterpart. A notch along the anterior margin of the epiplastron signifies the location of the gular-extragular sulcus. A faint trace of this sulcus can be gleaned on the ventral surface that runs posteriorly from the notch (Fig. 5b ). A similar notch for the extragular-humeral sulcus is apparent on the lateral margin of the epiplastron (Fig. 5b ). On the dorsal side, the extragular-humeral sulcus veers posteriorly to cross the lateral side of the epiplastron (see arrows in Fig. 5b ). Its posterior end is not preserved, as this was likely located on the hyoplastron. As a result of this arrangement, the extragular covered only part of the epiplastron in ventral view, but most of it in dorsal view. The dorsal side of the epiplastron is characterized by a thickened epiplastral lip.

Entoplastron. Two partial entoplastra are available, WMNM P48296 (Fig. 5a ) and P88819 (Fig. 5c ), of which the former is better preserved. The posterior half of the entoplastron is lacking in either specimen, but comparison with Helochelydra nopcsai (NHMUK R171) suggests that the entoplastron was slightly longer than wide. The entoplastron slightly overlaps the epiplastron along its slightly convex anterolateral contact. The posterolateral contact, as far as preserved, broadly underlapped the hyoplastron. Traces of the embryonic interclavicle are apparent on the dorsal side of both available elements, much as in other helochelydrids. The gulars only cover a small triangular surface on the ventral side of WMNM P48296, but about a third of the ventral surface of P88819. The extragulars minutely overlapped both elements. Only a minute entoplastral scute is apparent in WMNM P48296. An entoplastral scute seems to be absent in WMNM P88819, in which the humeral-gular junction is just about preserved.

Hyoplastron. Two partial hypoplastral remains can be identified with confidence. WMNM P47721 (Fig. 5d ), the more complete of the two, documents the lateral margin of the anterior plastral lobe, just posterior to the articulation of the hyoplastron with the epiplastron, while WMNM P88820 (Fig. 5e ) documents a segment slightly further to the back. The two fragments combined indicate that the skin-scute sulcus is barely located on the visceral side of the hyoplastron, but converges with its margin towards the back. Indeed, the sulcus terminates in WMNM P88820 near the margin, which suggests that this element broke at the humeral-pectoral sulcus. The resulting lip is only lightly developed towards the back, as seen in WMNM P88820, but becomes pronounced anteriorly to form a finger-like process that inserted anteromedially into the epiplastron, as seen in WMNM P47721.

Two additional, fragmentary specimens, WMNM P48226 (Fig. 5f ) and P88821 (Fig. 5h ), exhibit radiating, pin-like tubercles on their visceral sides that are reminiscent of those found on the hyoplastron of Naomichelys speciosa , but they may also represent remains of the mesoplastra or hypoplastra. A third fragment, WMNM P88822 (Fig. 5g ), is a fragment of the bridge that may similarly represent part of the hyo- or hypoplastron. A rib-like thickening on the visceral side of this fragment is the remnants of a buttress. A series of clearly delineated sulci form the outlines of at least two inframarginals, two marginals, and the neighboring plastral scutes.

Mesoplastron. No mesoplastral remains could be identified as such.

Hypoplastron. We are able to identify at least three remains as hypoplastra, WMNM P88823 (Fig. 5i ), P88825 (Fig. 5j ), and P88824 (Fig. 5k ), all of which preserve the thickened inguinal notch, but can be recognized as hypoplastra by remnants of the xiphiplastral articulation. WMNM P88823, the best of the three preserved elements, documents the transverse contact with the mesoplastron and the slightly oblique posterior contact with the xiphiplastron, which is dorsally stabilized by two xiphiplastral processes that dorsally cover the hypoplastron and a hypoplastral process that dorsally covers the xiphiplastron (Fig. 5i ). The relatively intact medial margin suggests that central and posterior plastral fontanelles are present (see arrows in Fig. 5i ) and that the hypoplastra only contacted one another along rib-like processes that radiate from the inguinal notch, features otherwise found in Naomichelys speciosa ( Joyce et al. 2014), but not in the holotype of Helochelydra nopcsai (NHMUK R171). The relatively small size of this specimen opens the possibility, however, that these are juvenile features. As preserved, the abdominal/femoral sulcus crosses the hypoplastron diagonally from the inguinal notch to the central plastral fontanelle, but otherwise runs posterior from the notch to meet with the skin-scute sulcus on the dorsal side (Fig. 5i ). In this area, the hypoplastron is thickened to nearly 2 cm in the smallest available specimen (WMNM P88823), but reaches 2.5 cm in the largest (WMNM P88825). In the area of the notch, the skin-scute sulcus runs onto the dorsal side of the hypoplastron to form a semi-lunate lip, but runs near parallel to the margin once it reaches the xiphiplastron (Fig. 5j ).

Xiphiplastron. The remnants of a xiphiplastron are preserved in WMNM P88823 attached to the hypoplastron (Fig. 5i ). Its contact with the hypoplastron is described above, as is the development of the skin-scute sulcus. A comparison with Naomichelys speciosa suggests that the xiphiplastron broke close to the femoral-anal sulcus. The anal scute, therefore, did not cross the hypoplastra-xiphiplastral suture.

Granicones. More than 150 granicones are available for study (Fig. 6 ). With few exceptions (see below), all granicones are singular, conical elements that consist of a flat base, which anchored the element into the skin, and an apex, which protruded from the body ( Joyce et al. 2014). The external surface of all elements is covered by surface texture that closely mirrors that of the shell. A qualitative assessment suggests that three morphotypes are present: cones (ca. 35%), hooks (ca. 43%), and plates (ca. 22%). As the name implies, cones are conical elements where the apex is situated above the base (e.g., Fig. 6 : P88864, P88873). These elements tend to be smaller. Hooks greatly resemble cones, but the apex is offset beyond the surface of the base (e.g., Fig. 6 : P88922, P88856). Plates, finally, resemble hooks in that the base is offset from the base, but that are notably flat (e.g., Fig. 6 : P88939, P88921). These tend to be the largest available elements. Our morphometric analysis (see below) suggests that these morphotypes do not populate discrete parts of the morphospace, but rather grade into each other. We nonetheless retain this terminology for convenience. As all elements were found in isolation, we do not know which parts of the body they once covered. A complete skeleton of Naomichelys speciosa that includes numerous articulated granicones, however, suggests that plates covered the external surfaces of the lower limbs while the spur-like hooks were located on the walking surfaces of the limbs ( Joyce et al. 2014).

A total of five elements deviate strongly from the three morphotypes described above (Fig. 6 ). Two of these (WMNM P88935 and P88940) resemble large hooks by having an apex that is offset relative to the base, but the base in itself is not flat and round, but rather folded along the long axis of the element. We speculate that this type of element may have covered folds on the outside of the limbs. The three other unusual granicones are remnants of compound granicones. WMNM P88974 consists of a single, conical apex, but open sutures on at least one side of its base suggest that it was laterally sutured to at least one more granicone. WMNM P88964 consists of two conical apices that are broadly sutured to one another at their bases. Although the outline of the jointly formed base is somewhat angular, the element appears to be intact. WMNM P88962 also consists of two conical apices, but numerous angularly arranged sutures on both sides of the jointly formed base suggest that it formally articulated with at least two, but likely up to five additional granicones, to form a compound element with at least four to seven apices.

PCA of all measurements indicates that 65.7% of the overall variance can be explained by size or allometric differences between specimens, explained by PC1 (Table 1 ). No differentiation of morphotypes is apparent based on size differences alone. Our morphospace plot of size-adjusted principal components (Fig. 7 ) shows that PC2, which explains 71.9% of shape variation after correcting for size, is the only principal component along which separate clusters of specimens may be determined, as plated granicones plot at low PC2 values with minimal overlap of specimens pre-assigned to other morphotypes. Cones and hooks have broad overlap across PC3 (20% of variance; Fig. 7a ) and PC4 (8.0% of variance; Fig. 7b ). The absence of separation for these morphotypes and the broad spread of values across PC3 can be explained by shape variation within the pre-assigned hook or cone condition. For example, hooks exist that have large basal measurements but low tip height (Table 1 ), such as WMNM P88873 (Fig. 7a ), the positive extreme point on PC3, but simultaneously hooks exist that have high tip heights compared to relatively small bases (e.g., WMNM P88864, which plots at low PC3 values for the hooked morphotype; Fig. 7a ). Similarly, cones with strongly asymmetrical bases plot at extreme negative PC3 values (e.g., WMNM P88940), whereas cones with similar tip offset but symmetrical bases plot at extreme positive PC3 values (e.g., WMNM P88921). The relatively close association in morphospace of hooks like WMNM P88873 and cones like WMNM P88921 is thus primarily caused by similar base dimensions paired with low tip heights (Table 1 ), regardless of their difference in tip offset which is the primary distinction of cones and hooks based on qualitative comparisons.

Cranium. A three-dimensionally preserved, partial, left parietal (WMNM P88806) is the only skull bone in the sample (Fig. 8 ). The anterolateral and medial margins of its dorsal plate, which formerly contacted the left postorbital and the right parietal, are intact, but the anterior and posterior margins show signs of damage. Striations on the posteromedial side of the element, contained within the area that constituted the cavum cranii, could plausibly represent the articulation area with the supraoccipital, which is usually overlain by the parietal in this area. A short posterolateral facet may represent the former contact with the left squamosal. A comparison with IWCMS 1998.21, a complete Helochelydra nopcsai skull from the Isle of Wight ( Joyce et al. 2011), suggests that only the anterior half of the element is preserved. The two specimens likely originated from individuals of the same size, as WMNM P88806 has an anteroposterior length of about 2.7 cm, while the equivalent area measures about 2.8 cm in IWCMS 1998.21. The two skulls differ notably in that WMNM P88806 exhibits clear cranial sulci which outline at least 12 polygonal cranial scutes. Their great number, however, obscures homology with the cranial scutes of other Cretaceous turtles, as these typically exhibit only four scutes in the equivalent area ( Sterli and de la Fuente 2013; Evers et al. 2021). On the ventral side, traces remain of the heavily damaged descending process. What remains is an outwardly convex crest of bone that documents the greatest width of the cavum cranii.

Figures

Figure 2. Shell remains of Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. Photographs of select specimens highlighting variation in preservation. (a) WMNM P 88818, a right epiplastron (also see Fig. 5 b); (b) WMNM P 88824, a right partial hypoplastron (also see Fig. 5 k); (c) WMNM P 88823, a partial left hypo- and xiphiplastron (also see Fig. 5 i); (d) WMNM P 88808, a partial, anterior neural (also see Fig. 3 a); (e) WMNM P 88813, a partial nuchal (also see Fig. 4 b); (f) WMNM P 88816, a partial left peripheral XI (also see Fig. 4 h). All specimens are to scale.

Figure 3. Neural and costal remains of Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. All images were obtained from CT generated 3 D models. (a) WMNM P 88808, a partial, anterior neural; (b) WMNM P 49476, a partial, posterior neural; (c) WMNM P 49351, a partial, posterior neural; (d) WMNM P 88809, a partial, posterior neural; (e) WMNM P 88807, a carapacial fragment consisting of two neurals and remnants of three adjacent costals; (f) WMNM P 88810, a partial left costal I; (g) WMNM P 88812, a partial costal; (h) WMNM P 88811, a partial costal; (i) WMNM P 49470, a partial right costal. Anatomical direction are placed in quotes for elements with uncertain orientation. With the exception of the illustrations, all specimens are to scale. Abbreviations: co = costal; ne = neural.

Figure 4. Nuchal and peripheral remains of Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. All images were obtained from CT generated 3 D models. (a) WMNM P 88813, a partial nuchal; (b) WMNM P 88814, a carapacial fragment consisting of remnants of a nuchal, rights peripheral I, remnants of right peripheral II, and remnants of right costal I; (c) WMNM P 88815, a partial right peripheral I; (d) WMNM P 49051, a partial bridge peripheral; (e) WMNM P 88817, a partial bridge peripheral; (f) WMNM P 41690, a partial right peripheral XI; (g) WMNM P 47259, a partial peripheral; (h) WMNM P 88816, a partial left peripheral XI. Anatomical direction are placed in quotes for elements with uncertain orientation. With the exception of the illustrations, all specimens are to scale. Abbreviations: CE = cervical scale; co = costal; MA = marginal scale; nu = nuchal; pe = peripheral; VE = vertebral scale.

Figure 5. Plastral remains of Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. All images were obtained from CT generated 3 D models. (a) WMNM P 88819, a partial entoplastron; (b) WMNM P 88818, a right epiplastron; (c) WMNM P 48296, a partial entoplastron; (d) WMNM P 47721, a partial right hyoplastron; (e) WMNM P 88820, a partial left hyoplastron; (f) WMNM P 48226, a plastral fragment; (g) WMNM P 88822, a plastral fragment; (h) WMNM P 88821, a plastral fragment; (i) WMNM P 88823, a partial left hypo- and xiphiplastron; (j) WMNM P 88825, a partial right hypoplastron; (k) WMNM P 88824, a right partial hypoplastron. With exception of the illustrations, all specimens are to scale. Abbreviations: AB = abdominal scale; EG = extragular scale; EN = entoplastral scale; FE = femoral scale; GU = gular scale; HU = humeral scale; hyp = hypoplastron; xi = xiphiplastron.

Figure 6. A selection of granicones, Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. All images were obtained from CT generated 3 D models. All specimens are shown to scale in lateral (top), apical (bottom left), and basal view (bottom right) and associated with their WMNM catalog number. Specimens were in part chosen to highlight the morphological extremely highlighted by the PC plots (see Fig. 7).

Figure 7. Morphospaces of four granicone measurements from PCA ordination (N = 165). (a, b), bivariate morphospaces including size variation. (c, d), morphospaces corrected for size and allometry. A, PC 1 vs. PC 2. B, PC 2 vs. PC 3. C, PC 2 vs. PC 3. D, PC 2 vs. PC 4. The proportion of variance explained by PC axes are indicated in the axis labels, whereby values in A, B represent total variance (including size) and values in B, C represent shape variance (re-scaled to exclude PC 1). An identical plot with unique point labels is provided in Suppl. material 2.

Figure 8. Helochelydra nopcsai from the Early Cretaceous (late Barremian - early Aptian) of Balve, North Rhine-Westphalia, Germany. WMNM P 88806, a partial parietal in dorsal (a), ventral (b), medial (c), and lateral view (d). All images were obtained from CT generated 3 D models.