Solemys vermiculata, Lapparent de Broin and Murelaga, 1996

Scheyer, T. M., Pérez-García, A. & Murelaga, X., 2015, Shell bone histology of solemydid turtles (stem Testudines): palaeoecological implications, Organisms Diversity & Evolution (New York, N. Y.) 15 (1), pp. 199-212 : 203-206

publication ID	https://doi.org/ 10.1007/s13127-014-0188-0
persistent identifier	https://treatment.plazi.org/id/03F1333E-FFF7-FFD0-FC83-BD214976F929
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scientific name	Solemys vermiculata
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Solemys vermiculata and Solemys sp. from the Late Cretaceous of Spain

The sampled specimens of Solemys vermiculata from Laño and Solemys sp. from Armuña share the same histological features, thus they are described in one section, with variation linked to differences in plate shape and size class being pointed out as necessary. All specimens present a diploe structure framed by internal and external compact cortices ( Fig. 3a and b View Fig ). The thickness of the internal cortex ranges from being roughly sub-equal to the external cortex in some elements (e.g. costals MCNA-15047 and UPUAM-14001; neural MCNA-15044) to being greatly reduced in others (e.g. neural MCNA-15054, costal MCNA-15043).

External cortex—The external cortex comprises a more external zone, the ornamental zone, and a more internal zone. The former zone consists of parallel-fibred bone grading into lamellar bone, which is vascularised by a reticular network of primary vascular canals. The characteristic surface ornamentation of valleys and protrusions is well visible in the thin-sections as well, with growth marks in the more external zone extending parallel to the external bone surface. The deeper ornamental trenches seen in some of the specimens (e.g. neural MCNA-15054) are visible as deep incisions between adjacent protrusions in the sections. The more internal zone is composed of interwoven structural fibre bundles (ISF sensu Scheyer and Sánchez-Villagra 2007; Scheyer and Sander 2007; Fig. 3c View Fig ), extensively vascularized by primary osteons and primary vascular canals. The length and thickness of the fibre bundles varies within the thin-sections. Growth marks are less traceable in this more internally situated zone. There is no distinct transition towards the cancellous bone but a gradual change, indicated by an increasing number of erosion cavities and secondary osteons.

Cancellous bone—Many samples show rather dense interior cancellous bone ( Fig. 3a and b View Fig ), with wide-meshed trabecular bone being restricted to thick interior-most core areas of the shell bones (e.g. neural fragment MCNA-15044, costal MCNA-15047) or adjacent only to the internal cortex (e.g. indeterminate shell fragment UPUAM-14003). In the thicker elements (i.e. neural MCNA-15054, peripherals MCNA-15048, UPUAM-14000) the cancellous bone is generally more extensive, in case of the neural (MCNA-15054) almost reaching the internal bone surface. Even in these thicker shell bones, the majority of the trabeculae are primary, with secondary lamellar bone only lining the vascular spaces. The bone trabeculae are short and thick, whereas the intertrabecular spaces are round to ovoid. In the longitudinally sectioned costals MCNA-15043 and UPUAM-14001, the trabecular structure indicating the extension of the rib within the costal plate is faintly distinct from the surrounding cancellous bone.

Internal cortex—The internal cortex consists of parallel-fibred bone, with Sharpey’ s fibres inserting frequently into the cortical tissue. The transition between interior cancellous core and internal cortex can be quite distinct ( Fig. 3a and b View Fig ), where few scattered larger secondary osteons invade the cortical tissue (e.g. costal UPUAM-14001, neural MCNA-15044, shell fragment UPUAM-14002), or there can be a gradual transition, with almost the complete cortex being pervaded by secondary osteons or erosion cavities (e.g. neural MCNA-15054, shell fragment UPUAM-14004, costal MCNA-15043). The cortical tissue is otherwise vascularised by scattered simple primary vascular canals or primary osteons.

Sutures—Suture zones can be extensive in some samples (e.g. costals UPUAM-14001, MCNA-15047) showing a strong relief composed of well interdigitating, elongated pegs and sockets (see Fig. 2b View Fig ). In peripheral UPUAM-14000, a deep socket (2 mm wide and 5 mm deep) is present to accommodate the distal rib end of the associated costal plate.

Solemydidae aff. Helochelydra sp. from the Early Cretaceous of Spain

Additional sections of some specimens of aff. Helochelydra sp. from Morella (MNCN 59503) are used herein to confirm the histological data gained previously from the Lower Barremian material of the Camarillas Formation of the site of Poca, in Galve ( Pérez-García et al. 2013). For this reason, we here provide a short summary of the previous findings and add details on the new material where appropriate. The bone samples from both, the Galve and Morella localities reveal diploe structures framed by cortical bone layers ( Fig. 4 View Fig ).

External cortex—In all samples the external cortex could be divided into a parallel-fibered external ornamental zone and a thin internal zone in which longitudinally and transversely sectioned fibre bundles form a meshwork. The internal zone of the external cortex is vascularized by reticularly arranged, primary vascular canals, whereas the external zone is mostly avascular ( Fig. 4a and b View Fig ).

Cancellous bone—The interior parts of a bones are strongly remodelled, so that the trabeculae are all secondary in nature and consist of secondary lamellar bone ( Fig. 4a and b View Fig ).

Internal cortex—The internal cortex is composed of parallel-fibered bone locally grading into lamellar bone. The bone tissue is vascularised by a few simple vascular canals. Towards the interior cancellous bone, successive remodelling of the compact bone into a trabecular meshwork ( Fig. 4c View Fig ) can be observed in a plastral fragment (probably corresponding to a hyoplastron or to a hypoplastron, MNCN 59503).

Solemydidae aff. Naomichelys sp. from the Cretaceous of North America

In general all elements express a diploe structure and cortices of similar thickness. The external ornamentation consisting of characteristically high and isolated tubercles or columns is seen in all the shell bone sections ( Fig. 5 View Fig ). Depending on the bone element that was sampled and its density of tubercles, the adjacent areas, i.e. ornamental valleys, vary in size and extent. The tubercles seen in the limb ossicles ( Fig. 5b View Fig ) are never as distinct and raised as high above the bone surface as can be the case in the shell bones. Indeed, they are more reminiscent of the ornamentation described for the solemydid limb ossicles, the ‘granicones’, from the Purbeck Limestone Formation ( Barrett et al. 2002). Further microstructural differences between the shell bones and limb ossicles are pointed out where necessary.

External cortex—The cortex of the shell bones consists of two zones ( Fig. 5d and e View Fig ), with an outer, ornamental zone being composed of parallel-fibred bone and an inner zone of thick coarse intervowen structural fibre bundles. The tubercular/columnar ornamentation can be seen to originate at the well delimited junction between the outer and inner zone of the cortex. Apart from a few scattered primary vascular canals, the outer zone is avascular,, whereas the inner zone is vascularised by scattered short primary vascular canals and primary osteons.

Starting as small pillow-like and pustule-like protrusions, the ornamental tubercles/columns have at first a concentric external growth. Adjacent and in between the columns the first layers of parallel-fibred bone are deposited. With continuing growth of the shell plates, the interstitial areas of parallel-fibred bone and the columns grow in external direction until a maximum diameter of the columns is reached. At the margins of the columns, a tight flexure zone develops where the parallel-fibred bone tissue of the columns and the tissue of the adjacent areas meet. The layers of the latter appear to be dragged externally by the columnar growth. The growth of the external zone of the external cortex is generally well observable due to cyclical growth marks that are present within the parallel-fibred bone of both the columns and the interstitial areas. While the growth marks are widely spaced at first, the space decreases with continued growth. In the samples of presumably old individuals, the growth marks are tightly spaced adjacent to the external surface of the bone. Adjacent to the external-most layers of interstitial parallel-fibred bone deposited in the ornamental valleys, the columns grow preferentially externally until they protrude from the external bone surface for several millimetres. The growth marks and other histological details are best visible where the plane of sectioning cuts medially through an ornamental column. Bone cell lacunae are generally round in the parallel-fibred bone of the columns, while they are slightly flattened in the parallel-fibred bone of the adjacent areas. In contrast, the external cortex of the limb ossicles is composed of a single undivided unit consisting of ISF. The ornamental tubercles/ columns are not protruding far above the surrounding bone surface. As such, the cortex does not possess the clear columnlike ornamentation composed of parallel-fibred bone, which is present in the shell bones.

Cancellous bone—In both the shell bones and the limb ossicles, the cancellous bone consists of an irregular arrangement of short thick and longer, more slender trabeculae, with the largest vascular spaces being found in the centre of the interior cancellous bone ( Fig. 5c, f and g View Fig ). Larger secondary osteons are also developed in the more external and more internal regions of the cancellous bone. The trabecular meshwork is primary, however many trabeculae have been secondarily remodelled. The gross of the bone trabeculae constitutes lamellar bone with generally few flattened and elongated bone cell lacunae.

Internal cortex—The internal cortex of the shell bones consists of parallel-fibred bone that can locally grade into lamellar bone ( Fig. 5c and g View Fig ). Bone cell lacunae are slightly flattened and oblong, and distinct growth marks are not observed in the bone matrix. Sharpey’ s fibres are present in some shell fragments, but they insert more numerously adjacent to the rib bulge in the internal cortex of the costal fragment TMP 90.60.07. Coarse fibre bundles, potentially interwoven structural fibres, are present in the samples from the Albian Antlers Formation, but not in the Cenomanian Formemost Formation. The internal cortex of the limb ossicles is of various thickness in the sampled bones. In both bones, however, it is composed of a regular meshwork of longitudially and transversely oriented fibre bundles. Among the latter, the delineations of individual fibre bundles are visible as thin bright lines ( Fig. 5h and i View Fig ). Vascularisation in all elements is generally low, with an occasional scattered secondary osteon or a primary vascular canal pervading the tissue, which often open up to the internal bone surface in the limb ossicles.