Hoyasemys jimenezi, Pérez-García & De La Fuente & Ortega, 2012

Pérez-García, Adán, De La Fuente, Marcelo S. & Ortega, Francisco, 2012, A new freshwater basal eucryptodiran turtle from the Early Cretaceous of Spain, Acta Palaeontologica Polonica 57 (2), pp. 285-298 : 287-292

Hoyasemys jimenezi sp. nov.

Figs. 2–4 .

Etymology: In honour of Dr. Emiliano Jiménez Fuentes for his contribution to the knowledge of Spanish fossil turtles.

Holotype: MCCM−LH 84 , an almost complete skeleton, missing the dorsal portion of the skull. The specimen is preserved on two slabs. One shows the ventral view of the skull, the five proximal cervical vertebrae, some dorsal vertebrae, some fragments of the peripherals and of the lateral part of the costal plates, the plastron, sacral and caudal vertebrae and the appendicular skeleton. The other slab preserves the partial carapace and some bones of the hindlimb.

Type locality: Las Hoyas site, La Cierva township, Cuenca Province, Spain.

Type horizon: Calizas de La Huérguina Formation, Upper Barremian, Lower Cretaceous ( Sanz et al. 1988; Diéguez et al. 1995).

Diagnosis.—Small basal eucryptodiran turtle with three pairs of oblique depressions on ventral surface of the basisphenoid. Differing from the rest of basal Eucryptodira in the following character combination: rear contact of the pterygoids with the basioccipital; basisphenoid large and narrower than basioccipital; basioccipital wider than long; cervicals two to four opisthocoelous; single transverse processes on the anterior edge of the centrum of the cervical vertebrae; cervical ribs absent; caudal vertebrae amphicoelous; small chevrons poorly developed, along the posterior caudals; shell low, longer than wide; nuchal plate relatively broad, without anterior or posterior notch; first neural plate longer than the second; two suprapygal plates; anterior pairs of peripherals guttered; mediolaterally expanded posterior peripherals; marginals overlapping onto the whole surface of the peripherals; plastral buttresses reaching only peripherals; posteriorly elongated inguinal process; anals scutes near the border of the hypo−xiphiplastra; most digits of manus and pes with three elongated phalanges.

Description

The anatomical terminology of the cranium follows Gaffney (1972), and of the bones and scutes of the carapace and plastron follows Zangerl (1969).

Skull.—All available cranial information was seen in ventral view ( Fig. 2 ). The cranial length is about a third of the plastral length.

Rostral elements: In ventral view, the labial ridge of both premaxillae and maxillae are visible. Hoyasemys jimenezi lacks a medial hook along the labial margin of the premaxillae. On the left maxilla the labial ridge extends posteriorly forming a straight and sharp edge. The triturating surface and lingual ridge could be preserved, but they are hidden by the left branch of the lower jaws. A part of the upper edge of the maxilla can be observed delimiting the left orbit. A partial left jugal is preserved.

Palatoquadrate elements: The long processus articularis of the right and particularly the left quadrate are well preserved. The right condylus mandibularis is not fully visible, while the left one is partly hidden by the posterior area articulis mandibularis of the lower jaw, showing the lateralmost articular facet. The quadrate−pterygoid suture is hidden by the hyoid elements. Only a part of the left quadratojugal can be seen in ventral view. It contacts the squamosal posteriorly.

The anterior portions of both pterygoids are missing or are at least represented only by minute fragments. The posterior portion of the left pterygoid and especially the right one are broken in fragments that are slightly displaced from their original position. On the right side, the pterygoid–basisphenoid suture runs between bony fragments. Likewise, the left pterygoid bone is sagittally displaced and its medial margin overlaps the lateral edge of the basisphenoid and basioccipital.

Braincase elements: The basisphenoid lies between the pterygoids. It lacks the anteriormost portion. The available ventral surface of the basisphenoid suggests it was subtriangular in shape. The preserved ventral surface of the basisphenoid is ornamented by three pairs of oblique and parasagittal depressions. The two posterior pairs of depressions have a prominent and crenulated ridge on the anterior edge. The anterior depression is the deepest. A blunt crest is situated anterior to the rear depressions. At least two posterior pairs of grooves, and likely also one other, are blind depressions, not related to the development of any cranial foramina.

The basioccipital presents an anterior, transverse contact with the basisphenoid and an anterolateral contact with the pterygoids. The basioccipital is notably broader than the posterior end of the basisphenoid. The basioccipital has a slight midventral ridge. This bone forms a short and robust con−

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dylus occipitalis, likely with lateral contributions from the exoccipitals.

Portions of the opisthotics are present on both sides. The contact of the opisthotic with the exoccipital is visible posteromedially and the contact of the opisthotic with the quadrate anterolaterally.

The exoccipitals are preserved on both sides, the left being more complete than the right. Two foramina nervi hypoglossi are located in a unique depression close to the base of the condylus occipitalis. The foramen jugulare posterius is not entirely formed by bone and confluent with the fenestra postotica.

Lower jaw.—Both mandibular rami are preserved and articulated with the skull ( Fig. 2 ). The dentary is the dominant element of the lower jaw. A dentary−articular sutural line is not evident in the posteroventral view of mandible. There is no evidence of a retroarticular process. The jaw joint is located well anterior relative to the basioccipital condyle.

Hyoid apparatus.—The hyoid apparatus is represented by a single pair of branchial horns, which consist of ossified rods extending from the middle of the palate to the posterolateral corner of the skull ( Fig. 2 ).

Vertebral column.—It is possible to identify some cervical and thoracic vertebrae, in addition to all sacral and some caudal vertebrae.

Cervical vertebrae: Remains of the atlas, axis, third, fourth and the anterior portion of the fifth cervical vertebrae are preserved ( Fig. 2 ). The atlas preserves its right neural arch, part of the left one and the crescentic atlantal intercentrum. The axis, and third and fourth cervical vertebrae are relatively short. The third and fourth cervicals are slightly larger than the axis. The transverse processes are well−developed, single, and placed slightly cranially. The neurocentral suture divides the transverse processes. The vertebrae are located in lateral view. A low ridge represents the neural spine. The centra of the axis, third, and fourth cervical vertebrae are opisthocoelous, having a well−formed anterior condyle. The fifth cervical vertebra is caudally hidden by the left epiplastron, but its badly preserved anterior end seems to be convex too. There are well developed midventral keels from the axis to the fifth centrum. These keels are more developed cranially, decreasing to the rear part, showing a convex ventral outline. This convexity gradually increases from the second to the fifth vertebrae.

The base of the high neural arch of the second cervical vertebra is broken and is partially displaced. The prezygapophysis of the third cervical vertebra is situated near the broken base of the second neural arch. The zygapophyses are long; particularly the postzygapophyses are well developed. In the third cervical vertebra, the peduncle that bears the postzygapophyses is as longer as the vertebral centrum. No ribs are present along the preserved cervical series.

Dorsal, sacral, and caudal vertebrae: Three dorsal centra are exposed throughout the central fontanelle of the plastron ( Fig. 3 ), but just one of them is antero−posteriorly complete. Its centrum is elongate and possesses a ventral sagittal ridge. All available articular surfaces are slightly concave. The cranial articular surface of the most caudal of these vertebrae is exposed. It is dorso−ventrally depressed and has a subtriangular outline.

Four precaudal vertebrae are visible in the sacral area, of which two or three may be sacral vertebrae. The two anterior preserved precaudal vertebrae have low and wide centra with a strong midventral keel. The anterior articular surface of the first vertebra is platycoelous and mediolaterally elongated. The posterior one is not visible. The two posterior precaudal (sacral) vertebrae have low, wide centra with a low midventral keel.

The preserved portion of the tail is about a quarter of the plastral length. Twelve caudal vertebrae with amphicoelous centra are visible. The first four caudal vertebrae have transverse processes that are as long as their centra. The length of the transverse processes decreases posteriorly and disappears at the level of the eighth caudal vertebra. Starting with the fourth caudal vertebra, slightly developed articular surfaces for haemal arches are apparent. Small chevrons are attached to the sixth−seventh, seventh−eighth and eighth−ninth caudal vertebrae. The caudal vertebrae 9 to 12 are rotated with respect to the anterior ones and their haemal arches,

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when present, are slightly displaced from their natural positions. The neural arches of these vertebrae are strongly modified and its neural spines taper forwardly and overlap the prezygapophyses.

Carapace.—The carapace is oval and low, and elongate in shape ( Fig. 4 ). Unfortunately most of its right side is not preserved and the anterior left side is slightly displaced. The dorsal surface of the vertebral scutes is weakly sculptured by grooves radiating forwards and laterally from a midpoint on the posterior edge of the vertebral scutes.

Plates: The nuchal is a large plate slightly displaced from the midline of the carapace. It is trapezoidal in shape, lacking the posterior notch for articulation with the first neural. This plate also lacks an anterior emargination.

The specimen preserves the first five plates of the neural series. The first two neurals are rectangular, while from the third to fifth they become hexagonal with short anterolateral edges. The first neural is the largest in the preserved series and despite its rectangular shape, it has contacts with two pairs of costals. The second neural plate by contrast contacts only with one costal (costal 2).

Hoyasemys jimenezi has two suprapygals. The specimen shows half of the left side of both suprapygal plates, a small part of the right region of this later, and the proximal margin of the pygal, quadrangular in shape. A cast of the pygal plate suggests that no posterior notch is present. The complete left costal series is preserved. The right costal series is composed of the lateral ends of the four posterior plates, slightly displaced and overlapping onto the peripheral bones.

The posterior costal plates form lateral fontanelles with the peripheral bones. These fontanelles are apparently larger than they probably were due to compaction and disarticulation of the specimen and the relative displacement of some of the plates of the carapace.

As it is typical in other turtles, Hoyasemys jimenezi likely had eleven pairs of peripherals. However, only two left peripheral plates of the anterolateral margin and the last five pairs of peripherals from both sides of the posterolateral margin are preserved. Among these bones is a fragment of a trapezoidal and elongated first left peripheral plate. It is laterally displaced from its contact with the nuchal. The anterolateral margin of this peripheral is upturned. Several fragments of the second left peripheral, also guttered, represent the remainder antero−lateral margin. Peripheral plates from eight to eleven are mediolaterally expanded.

Scutes: In the anterior region of the nuchal plate the limits of the scutes are not visible, so it is unknown if Hoyasemys jimenezi has one or more cervical scutes on this plate. The first vertebral scute covers the posterior portion of this plate. Most of the first, second, third, fourth and fifth vertebral scutes can be recognized. They are hexagonal, wider than long, and with lateral angled edges.

The contact between the first and second vertebral scutes is located on the first costal and neural plates, the contact between the second and third vertebral scutes on the third costal and neural plates, the contacts between the third and fourth on the sixth costal and probably sixth neural, and the contact of the last vertebral scutes over the last pair of costals and the first suprapygal.

Apparently, the marginal scutes are at least as wide as peripheral plates, but the contact between pleurals and marginals is not evident.

Plastron.—The length of the plastron is about the 70% of the carapace. It has a narrow bridge, with deeply concaves axillary and inguinal notches, and with a subtriangular posterior lobe ( Fig. 3 ). The plastron is slightly shifted in relation to its original position. The distal regions of the right hyoplastra or hypoplastra are not preserved and the lateral left margin is located below the peripheral plates. There therefore is no contact between the plastron and the carapace, and the nature of the plastron/carapace connection is unclear. The plastral buttresses only reach the peripheral plates.

There is a rounded mid−sized central plastral fontanelle between the hyoplastra and hypoplastra. Preservation does not allow identification of lateral bridge fontanelles, but if these existed they would have been very small.

Plates: The epiplastra are large, broad and elongate, giving a rounded outline to the anterior lobe of the plastron. Between the epiplastra and the hyoplastra there is a long, probably ligamentous attachment. The entoplastron is narrow and barely protrudes from between the hyoplastra. The hyoplastra meet each other in the plastral midline and with the entoplastron along an interfingering contact. This anterior half of the plastron is partially disarticulated likely due to compaction.

Mesoplastra are absent. The posterior half of the plastron is also displaced from its natural position. Consequently there is a large gap between the hyoplastra and hypoplastra. Hyoplastra are shorter than hypoplastra. The suture between the hypoplastra and xiphiplastra is medially directed toward the rear, with a small notch laterally. The xiphiplastra are elongate and lack anal notch.

Scutes: The morphology of the scutes of the anterior half of the plastron is not known because the sulci are not discernible. Inframarginal scutes are not preserved. The sulci between the abdomino−femoral and femoro−anal scutes are clearly visible. The abdomino−femoral sulci, restricted to the hypoplastra, articulated along a line that extends from the deepest point of the inguinal notch to the posterior part of the hyo−hypoplastral fenestra. The femoral−anal sulcus is oblique, tends sagittally to reach the hyoplasta−xiphiplastra contact, but remains restricted to the xiphiplastron.

Pectoral girdle and forelimbs.—Pectoral girdle: Most of the elements of the left and right scapular girdles lie under the hyoplastra and can only be partially described. Only the glenoid cavity is visible on both sides where the humeri are articulated. The articular surface is slightly convex.

Humerus: Both humeri are exposed in ventral view. The axis of the humeri presents a slightly sigmoidal shape and a strong medial constriction. The medial process is about twice as large as the lateral process. Between them, a shallow intertubercular fossae is present. The proximal end is slightly cranio−caudally wider than the distal one. The articular surface of the distal end is directed ventrally. The radial and ulnar condyles are only lightly developed.

Radius and ulna: These elements are relatively well preserved on both sides. The radius is slightly longer, and more columnar and slender than the ulna. The ulna is about twothirds the length of the humerus and is flattened dorso−ventrally, with a convex proximal end. The olecranon fossa is not well developed.

Carpus and manus: The right and left carpi are partially preserved and the relative position of the elements has suffered some distortion. Both carpi are exposed in dorsal view. The preserved bones of the proximal carpus are difficult to determine because of slightly disarticulation. However, the ulnare, the intermedium and the centrale can be recognized in both forelimbs. The five distal carpals are preserved in both carpi. They are rounded elements. Distal carpals I to III are subequal in size, while distal carpal IV is the largest and the V is the smallest.

The metacarpals are relatively long elements. As usual, the first metacarpal is relatively short and broad in comparison with the other metacarpals. The proximal end of metacarpal I has an expanded base that is broader than the rest of the bone. Metacarpals II to IV are similar in general size and shape, while the V is the shortest. This metacarpal is preserved in bad conditions on both manus.

The phalanges are exposed mainly in lateral or dorsal views. They are preserved on the right manus in digits I to IV and in the left one in all the digits but not complete. The first digit has two phalanges, the second and the third have three. Digit IV probably had three phalanges, but there is no evidence to ensure that this number is not higher. Digit V has two preserved phalanges, the first and the claw.

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Pelvic girdle and hindlimbs.—Pelvic girdle: Partially preserved and slightly distorted pelvic elements are present. Just a small part of the acetabular portion of the right ilium is available. The articular surface is narrow and concave. Most of the pubis lies under the visceral surface of the plastron. The proximal region of both pubes is partially exposed around the acetabulum. The pubis is a thin blade, as can be seen in the available portion of the right one. The ischia are caudally displaced and sagittally articulated, almost completely exposed. The lower end is expanded.

Femur: Both femora are preserved articulated with the acetabulum. They are slightly S−shaped, and slightly longer than the humeri. The rounded femoral head forms an angle of approximately 120 ° from the main axis of the bone. The right femur is exposed in lateral view, while the left one is exposed in latero−ventral view. The femur expands distally forming a large tibial condyle. A small fibular condyle is placed on the posteroventral margin.

Tibia and fibula: The tibia and fibula are equal in size and slightly shorter than the femur. They articulate with one other. The tibia has an expanded proximal end. Its broad proximal articular surface articulates with much of the tibial condyle of the femur. The medial edge of the tibia is concave and the lateral edge is nearly straight. Distally, the tibia expands slightly. The fibula is a slender bone with a head scarcely wider than its shaft, while the distal end is broader and flatter than the proximal end.

Tarsus and pes: Both tarsi are partially preserved. As in the carpi, the proximal elements are somewhat disarticulated. It is not possible to identify the different bones of the right tarsi. Two elements of the left tarsi probably correspond to the astragalus and calcaneum. The element interpreted as the astragalus is larger than the calcaneum, and proximally articulates with the tibia and fibula.

The distal tarsals are oval in shape. In the left hindlimb five metatarsals are recognized, which increase in length from I to IV and decrease in the last one. The metatarsals are slender and long. Metatarsal V is characterized by a flat L−shape.

Digits I to V are relatively well preserved. Digit I has two phalanges. In the digit II there is only one preserved phalanx. Digit III has three phalanxes. The first phalanx and the proximal region of the second phalanx are observed in digits IV and V. Therefore, the total number of phalanges in these two digits is not known.

Stratigraphic and geographic range.—Known only from the type locality.

Figures

Fig. 2. Skull and cervical vertebrae of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM−LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).

Fig. 3. Ventral view of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM−LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).

Fig. 4. Dorsal view of the eucryptodiran turtle Hoyasemys jimenezi gen. et sp. nov. (MCCM−LH 84) from the Early Cretaceous of Las Hoyas, Spain. Photograph (A) and explanatory drawing (B).