Gracilisuchus stipanicicorum Romer, 1972

LECUONA Agustina, DESOJO Julia Brenda & CERDA Ignacio Alejandro, 2020, New information on the anatomy and histology of Gracilisuchus stipanicicorum (Archosauria: Pseudosuchia) from the Chañares Formation (early Carnian), Argentina, Comptes Rendus Palevol 19 (3), pp. 40-62 : 43-55

Gracilisuchus stipanicicorum Romer, 1972

( Figs 2-7)

HOLOTYPE. — PULR 08 , incomplete cranium, one disarticulated mandibular ramus; incomplete series of presacral vertebrae and incomplete first sacral vertebra, incomplete paramedian osteoderms, distal ends of the right scapula and humerus (see Lecuona et al. 2017 for full details).

REFERRED MATERIAL. — CRILAR PV 480 , one and a half incomplete cervical centra articulated with each other and with two incomplete ribs, a series of three incomplete vertebrae articulated with one rib, dorsally in contact with the left row of the paramedian osteoderms, and posteriorly four ribs with no articulating vertebrae but in anatomical position ( Fig. 2A, B), a series of six incomplete cervicodorsal vertebrae with some of their ribs preserved and half of a centrum attached posteriorly ( Fig. 2C, D), and moulds of two short fragments of paramedian osteoderms. This specimen was found associated with a scapulocoracoid and an osteoderm series of a proterochampsid archosauriform and several indeterminate remains . CRILAR PV 490 , two articulated cervical vertebra in contact with a short paramedian osteoderm series, one isolated dorsal vertebrae, left humerus, right ulna, right radius, left femur, and six histological sections of the femoral diaphysis and osteoderms ( Figs 3-10). See Lecuona et al. (2017) for a complete list of the hypodigm of Gracilisuchus stipanicicorum .

HORIZON, LOCALITY AND AGE. — Chañares Formation, Ischigualasto–Villa Unión Basin , La Rioja Province, northwestern Argentina ( Fig. 1). Specimens deposited in PVL, PULR, and MCZVP collections come from the type locality of the Chañares Formation ( Fig. 1; Sereno & Arcucci 1994; Lecuona et al. 2017), whereas CRILAR PV 480 and CRILAR PV 490 come from two different localities. CRILAR-PV 480 was collected from Campo de Córdoba locality, and, stratigraphically, from the lower member of the Chañares Formation, at the concretionary level that characterizes this member and where most of the previously reported fossil tetrapods from the Massetognathus - Chanaresuchus assemblage zone (AZ) were found ( Rogers et al. 2001; Fiorelli et al. 2013; Marsicano et al. 2016; Gouiric-Cavalli et al. 2017). CRILAR PV 490 was found at the El Torcido locality, in the lowermost levels of the Massetognathus - Chanaresuchus AZ of the Chañares Formation, approximately 4 km east of the “Chañares type” locality (= Far East Pocket locality of A. S. Romer). CRILAR PV 490 was found in a fine-grained grey sandstone, in levels lacking concretions, and was associated with a cynodont ilum and the skeleton of a rhadinosuchine proterochampsid CRILAR PV 488 ( Ezcurra et al. 2019). The Chañares Formation has been traditionally interpreted as Ladinian in age (Middle Triassic; Stipanicic 1983; Rogers et al. 2001; Morel et al. 2003), but recent U-Pb zircon analyses of samples from the lower and middle levels of the Chañares Formation ( Marsicano et al. 2016) constrained the age of the fossiliferous horizon to 236.1 ±0.6 to 233.7 ±0. 4 Ma, 5-10 Mya younger than the Middle Triassic, representing an early Carnian (early Late Triassic) age. Additional new analyses from detrital zircons taken from two volcanoclastic deposits of the Chañares Formation ( Ezcurra et al. 2017), recovered maximum depositional ages consistent with those previously determined (236.2±1.1 Ma and 233.1±1.1 Ma), confirming the earlier work ( Desojo et al. 2011).

DIAGNOSIS. — Lightly built pseudosuchian with a total skull length that slightly exceeded 90 mm (in the largest specimen, PVL 4612, with incomplete anterior end of the snout) and the length of the largest preserved femur being 80 mm (PVL 4597, one of the largest specimens), diagnosed by the following unique combination of characters (autapomorphies marked with an asterisk): relatively large cranial openings: antorbital fenestra occupying between 30% and 36%, antorbital fossa between 36% and 40%, and orbit between 35% and 42% of the anteroposterior length of skull roof (measured back to the posterior end of the parietals); supratemporal fenestra wider than long*; sclerotic ring with ossicles that not overlap in lateral contact; straight posterior edge of postorbital process of jugal*; presence of postfrontal, with a long lateral process that projects over the postorbital bar*; presence of a small triangular postparietal; large postemportal foramen in proportion to the width of the occipital region; laterally extending anterior process of squamosal; premaxillary teeth with no mesial carina, faint distal carina, and lacking mesial or distal denticles; postzygapophyseal facet of the axis in horizontal plane and with the longitudinal axis posteriorly directed*; high, vertical anterior border of the axial neural spine*; longitudinal median ventral keel on the axial centrum; poorly developed ventral keels on cervical vertebrae; mid-dorsal region of cervical vertebral neural arch with a circular anterolateral depression; spine tables on posterior cervical vertebrae (at least on cervicals 4 of PVL 4597, and 6 and 7 of MCZVP 4118); distal (lateral) end of first sacral rib slightly expanded relative to the proximal (medial) portion*; lack of a well-defined acetabular surface on the pubis; thin L-shaped lamina on proximal pubic apron; lateroventral corner of pubic peduncle with short bony tongue; ischiadic symphysis proximally located*; proximodistally elongated and poorly developed iliofibular trochanter on fibula; staggered paramedian osteoderms; longitudinal keels on dorsal surfaces of laterally bent osteoderms slightly medially located with respect to the centre of the element and ( Lecuona et al. 2017).

OSTEOLOGY

Vertebrae and ribs

The specimen CRILAR PV 480 presents a series of three incomplete cervical vertebrae, the last of which articulates with one rib, and posteriorly four cervico-dorsal ribs in natural position but missing their corresponding vertebrae ( Fig. 2A, B), a series of six fragmentary vertebrae spanning the cervicodorsal transition with several articulated ribs ( Fig. 2C, D), two incomplete articulated cervical centra with two incomplete cervical ribs, two isolated probable vertebrae (not figured), and one isolated dorsal rib. CRILAR PV 490 preserves two articulated mid cervical vertebrae ( Fig. 3) and one isolated posterior cervical vertebra; no ribs are preserved in this specimen. As the axial skeleton of Gracilisuchus has been described recently ( Lecuona et al. 2017), the present description will focus on differences from the previously known specimens and from other taxa.

The vertebrae of the series of three cervicals (CRILAR PV 480, Fig. 2A, B) are incomplete and poorly preserved, with the last two preserving only the neural arch and spine and probably from the left side. The morphology of the five ribs preserved in this series suggests a transitional cervicodorsal position. They contrast with the ribs of this region in other specimens of Gracilisuchus (e.g. PVL 4597, see below), but the poor preservation as a thin layer of bone in some regions in CRILAR PV 480, suggests they may be incomplete. The first rib has a short shaft and a strongly expanded, triangular distal end, almost equal in size anteriorly and posteriorly. The second rib has a thicker shaft and is distally triangular, but narrower anteroposteriorly than the first. The remaining ribs have lost their proximal articular ends. The third rib is distally expanded and triangular, being more expanded posteriorly than anteriorly. The fourth and fifth ribs are more like dorsal ribs, where the posterior surface is straight and the anterior is convex. The fourth rib has the anterior projection that extends more anteriorly and is more distally located than in the fifth. The ribs of this series ( Fig. 2A, B) differ from those of other Gracilisuchus specimens (e.g. PULR 08, PVL 4597) in being distally less expanded anteroposteriorly and thus not contacting with each other as occurs in PULR 08 and PVL 4597. This difference could be interpreted as intraspecific variation but may also be due simply to the poor preservation of the ribs of CRILAR PV 480, as only a thin layer of bone in some regions suggests they may be incomplete. The series of six cervico-dorsal vertebrae ( Fig. 2C, D) and the fragmentary series with one and a half vertebral (not figured) of CRILAR PV 480, and the three cervicals of CRILAR PV 490 ( Fig. 3) are quite similar to those in other specimens of Gracilisuchus . The vertebrae of specimen CRILAR PV 490 have a neural spine inclined anteriorly to the same degree as PVL 4597 ( Lecuona et al. 2017), an anterolateral depression anterior to the neural spine in the neural arch, as well as an infrapostzygapophyseal depression below the postzygapophysis ( Fig. 3A, B; Lecuona et al. 2017: fig. 7). They have long longitudinal lateral fossae on the vertebral centra, and the parapophysis continues posteriorly as a longitudinal crest ( Fig. 3B, D). Anterior cervical ribs are only present in CRILAR PV 480 ( Fig. 2C, D); they are posteriorly elongated and contact the immediately posterior rib dorsally through a dorsal groove. The dorsal ribs each have a short capitulum and tuberculum and expand anteriorly at the proximal end of the shaft ( Fig. 2C, D).

Humerus

A complete humerus was found in specimen CRILAR PV 490 ( Fig. 4). It is a slender bone, 41.3 mm long, 4.9 mm wide at the midshaft, and 10 and 9.7 mm wide at its proximal and distal ends, respectively. This general gracile morphology resembles that of Erpetosuchus granti Newton, 1894 ( Benton & Walker 2002) or the crocodylomorphs CM 29894 ( Clark et al. 2000) and Dromicosuchus grallator Simmons, 1965 ( Sues et al. 2003), but contrasts with the robust element of Nundasuchus songeaensis Nesbitt et al., 2014 , Batrachotomus kupferzellensis Gower, 1999 (SMNS 80275), aetosaurs (e.g. Aetosauroides scagliai Casamiquela, 1960 , PVL 2073), phytosaurs (e.g. Smilosuchus gregorii Camp, 1930 sensu Long & Murry 1995 , USNM 18313), and basal loricatans. In anterior and/ or posterior view, the proximal end has a triangular outline, with the proximal surface rather flat and obliquely inclined, with its lateral corner being proximally projected with respect to the medial corner. The humeral head is poorly defined on the proximal end. It is nearly flat and forms an angle with the proximal surface of the humerus. This morphology is similar to Euparkeria capensis Ewer, 1965 (SAM PK 5867), but contrast with some other pseudosuchians (e.g. Erpetosuchus granti [ Benton & Walker 2002]; Batrachotomus kupferzellensis, SMNS 92042, SMNS 80276 [ Gower & Schoch 2009]; Nundasuchus songeaensis [ Nesbitt et al. 2014]; Terrestrisuchus gracilis Crush, 1984 , NHM UK P 47/22ii), which have a rounded, clearly defined protuberance located at some distance from the medial margin, and the proximal surface between the head and the medial margin is distally inclined. The lateral and medial margins of the shaft are gently curved, contrasting with the highly concave medial margin of Nundasuchus or both margins of Erythrosuchus africanus Broom, 1905 ( Gower 2003). The long axes of the proximal and distal ends in transverse view are nearly parallel to one another ( Fig. 4C, F), differing from the offset condition in Erythrosuchus africanus ( Gower 2003) , Euparkeria capensis ( Ewer 1965) , Nundasuchus songeaensis ( Nesbitt et al. 2014) , Postosuchus kirkpatricki Chatterjee, 1985 ( Weinbaum 2013), Aetobarbakinoides brasiliensis Desojo et al., 2012 , or Batrachotomus kupferzellensis (SMNS 80275), for example. The deltopectoral crest is slightly raised from the diaphysis, merging with the centre of the shaft at 40% from the proximal end. The medial surface of the crest has a shallow fossa ( Fig. 4D: f).

The midshaft of the humerus is nearly circular in section and becomes flattened anteroposteriorly toward the distal end. The distal end has a poorly defined entepicondyle and ectepicondyle, forming simple corners at the medial and lateral sides ( Fig. 4), instead of rounded articular condyles, as in some other taxa (e.g. Turfanosuchus dabanensis, IVPP V 3237; Postosuchus kirkpatricki [ Weinbaum 2013]; P. alisonae Peyer et al., 2008 , NCSM 13731; Batrachotomus kupferzellensis, SMNS 80275, SMNS 80276, SMNS 92042; Terrestrisuchus gracilis, NHM UK P 47/22).

The distal half of the humerus of CRILAR PV 490 can be compared with the only known humerus of Gracilisuchus ( PULR 08), which is very similar in size, with the mid-diaphysis width of the former being 1.11 times larger than PULR 08. The humerus of PULR 08 preserves the distal end partially as a mould, where a trochlear recess can be seen to be similar in width to that of CRILAR PV 490, whereas the morphology of the distal condyles differs, as these are barely rounded and separated distally in PULR 08. Additionally, the medial condyle of PULR 08 is more narrow than the lateral, whereas in CRILAR PV 490 the lateral one is narrower ( Fig. 4D). The medial distal condyle of CRILAR PV 490 is slightly more distally projected than the lateral one. On the anterior distal surface, there is a shallow trochlear recess that fades proximally and disappears at 25% from the distal end ( Fig. 4D: tr). The lateral margin has a low bump at 7.6 mm (almost 20%) from the ectepicondyle ( Fig. 4B), which might be homologous to the ectepicondylar flange of Nundasuchus ( Nesbitt et al. 2014) and the supinator process of Batrachotomus (SMNS 80275, Gower & Schoch 2009). However, the latter two taxa differ in that they show elongated ridges, whereas CRILAR PV 490 merely has a bump ( Fig. 4B: lb). Lateral to this structure, CRILAR PV 490 presents a small and shallow circular depression ( Fig. 4B: cd), that may be homologous to the ectepicondylar groove for the passage of the radial nerve; however, this depression is very different in shape from the groove morphology (being longer than wide) of the ectepicondylar groove in taxa such as Postosuchus kirkpatricki ( Weinbaum 2013) , Nundasuchus songeaensis ( Nesbitt et al. 2014) , and Batrachotomus kupferzellensis (SMNS 80275, SMNS 80276, SMNS 92042).

Ulna

Most part of the diaphysis including the proximal end of a right ulna is preserved in CRILAR PV 490 ( Fig. 5). The preserved element is 36.7 mm long, 3.7 mm wide dorsoventrally at the diaphysis, and 8.5 mm wide dorsoventrally at the incomplete proximal end. The ulna is estimated to have lost around 3.5 mm, based on comparison with the gracilisuchid Turfanosuchus dabanensis (IVPP V3237), the ulna of which is around 0.97 times the length of the humerus.

The incomplete proximal end is flared in medial or lateral view, exhibiting only part of the olecranon processes and a slightly raised bump on the lateral proximal surface. In proximal view, it has a teardrop-shaped outline, tapering dorsally ( Fig. 5C). The proximal lateral bump probably represents a lateral tuber (=radius tuber, Nesbitt 2011; Fig. 5D). This tuber is present in most paracrocodylomorphs (e.g. Batrachotomus, SMNS 80275; Fasolasuchus tenax Bonaparte, 1978 , PVL 3850; Postosuchus alisonae [ Peyer et al. 2008]), aetosaurs, and basal dinosauriforms, and absent in most archosaurian outgroups ( Nesbitt 2011), although it may be poorly developed in the euparkeriid Halazhaisuchus qiaoensis Wu, 1982 ( Sookias et al. 2014). In the proximal region, the medial surface has a wide and long depression, and the dorsal surface has a narrower and shorter depression for contact with the radius ( Fig. 5D: ra). The dorsal surface of the shaft has a medially faint ridge ( Fig. 5D: r) starting near the proximal end and extending all the way down the preserved portion of the shaft, probably for the insertion of the flexor ulnaris muscle on the dorsal surface ( Meers 2003; Otero 2018). This ridge is also present in other taxa, such as the euparkeriid Halazhaisuchus qiaoensis ( Sookias et al. 2014) , and the loricatans Batrachotomus kupferzellensis (SMNS 80275), Postosuchus kirkpatricki ( Weinbaum 2013) , and P. alisonae (NCSM 13731), among others.

Radius

CRILAR PV 490 also preserves the distal portion of the right radius, probably representing less than the half of the distal region ( Fig. 6). The total preserved length is 34.1 mm, the mid-diaphysis is 3.1 mm wide lateromedially, and the distal end is 6.2 mm wide lateromedially. The distal articular surface is almost flat and faces posterodistally. It has a shallow, lateromedially elongated depression in the middle of the facet when observed in posterior view ( Fig. 6E). A subcircular outline is observed in distal view ( Fig. 6E). This distal end is somewhat similar to the bevelled surface present in Halazhaisuchus qiaoensis ( Sookias et al. 2014: fig. 7R) and differs from the directly distally facing surface of some paracrocodylomorphs (e.g. Batrachotomus kuferzellensis, SMNS 80275; Fasolasuchus tenax, PVL 3850; Postosuchus kirkpatricki, Weinbaum 2013 ). Additionally, paracrocodylomorph taxa also contrast with CRILAR PV 490 in having elongated distal outlines, whereas CRILAR PV 490 has a subcircular one. The dorsomedial surface of the shaft shows an elongate scar near the distal end, which is nearly twice as long as wide ( Fig. 6D: sc) and probably represents the origin of the extensor digitorum superficialis muscle ( Otero 2018). This scar was not observed in other taxa used for comparison. The lateral surface of the shaft shows a very faint ridge ( Fig. 6D: r) that slightly twists posteriorly near the distal end.

Femur

The approximate distal half of a left femur is preserved in CRILAR PV 490 ( Fig. 7). The femur is described with its longitudinal axis oriented dorsoventrally, and the axis through its distal condyles perpendicular to the axial plane of the animal. This fragment is 31 mm long, 6.8 mm wide at its distal end, and has a very narrow diaphysis (3.4 mm wide) that is almost circular in cross-section. It is slightly posteriorly bowed in lateral view but straight in anterior view ( Fig. 7 A-D). The posterolateral surface shows a well developed ridge that extends nearly one third of the preserved length of the shaft from the distal end ( Fig. 7A: lip). This ridge might represent the adductor ridge (ar, sensu Hutchinson 2001), the linea intermuscularis caudalis (lip), or the merging of both structures, because sometimes they join together ( Hutchinson 2001). This ridge is particularly sharp in CRILAR PV 490, contrasting with the low ridge in taxa such as Erythrosuchus africanus ( Gower 2003) , Pseudochampsa ischigualastensis Trotteyn et al., 2012 ( Trotteyn & Ezcurra 2014), Turfanosuchus dabanensis (IVPP V3237), Arganasuchus dutuiti Jalil & Peyer, 2007 (AMNH.F AZA 900), and Effigia okeeffeae Nesbitt & Norell, 2006 ( Nesbitt 2007: fig. 44, r). At the distal end, the lateral and medial condyles are separated anteriorly by a shallow intercondylar groove ( Fig. 7C, E). The medial condyle is more distally projected and is lateromedially narrower than the lateral one; it has a posteriorly tapering distal end and is slightly inclined medially. The lateral condyle is damaged on its posterior surface and a distally incomplete ridge arise from the proximal end of this condyle, which is interpreted as the crista tibiofibularis ( Fig. 7D: ctf). This crista is present in other early archosaurian and archosauriform taxa including Vancleavea campi Long & Murry, 1995 ( Nesbitt et al. 2009: fig. 16), probably Aetosauroides scagliai (PVL 2073) , Batrachotomus kupferzellensis (SMNS 80278), Arganasuchus dutuiti (AMNH.F AZA 900), and Nundasuchus songeaensis ( Nesbitt et al. 2014) , and is probably homologous with the lateral condyle of the crocodylomorph Dromicosuchus grallator (NCSM 13733). The morphology of this bone is very similar to the single previously known femur of Gracilisuchus stipanicicorum ( PVL 4597), sharing a sigmoid curvature, a narrow diaphysis compared to the width of the distal end, and the presence of a medially inclined medial condyle. However, CRILAR PV 490 differs from PVL 4597 in the stronger development of the adductor ridge and crista tibiofibularis (“lateral condyle”; Lecuona & Desojo 2011).

Osteoderms

Paramedian osteoderms have been recovered in both specimens, CRILAR PV 480 and 490. They are somewhat damaged and incomplete but exhibit the known morphological characters of Gracilisuchus (i.e., PULR 08, PVL 4597; Lecuona et al. 2017). CRILAR PV 480 preserves a sequence of nine osteoderms of the left paramedian row ( Fig. 2B), and CRILAR PV 490 preserves a sequence of five fragmentary paramedian osteoderms ( Fig. 3). The osteoderms are asymmetrical with a well developed dorsal crest located closer to the medial than to the lateral margin ( Fig. 3C, E: cr); thus, the lateral surface (“wing”) is larger than the medial one.The dorsal surface medial to the crest is slightly deeper right next to the crest than the surface lateral to the crest. The paired osteoderms are arranged in a staggered fashion and are longer than wide. In CRILAR PV 490, the osteoderms are somewhat larger than those in CRILAR PV 480 and their edges are faintly crenulated; the edges of CRILAR PV 480 are too poorly preserved to show any ornamentation.

HISTOLOGY

Femur

The femoral diaphysis of CRILAR PV 490 was sectioned for histological analysis at two points approximately at the mid-diaphysis, lacking the fourth trochanter or other superficial features. Both sections are almost identical. The medullary cavity is large and empty, occupying approximately 63% of the total diameter measured through the longest dimension, and 37.5% of the total surface area ( Fig. 8A). The cortex is composed of primary tissue of parallel-fibered bone (PFB), which is more organized near the outer border and decreases in organization toward the inner cortex. In the external cortex, the osteocyte lacunae are oblong in shape, ordered, and parallel to the surface ( Fig. 8B). Toward the inner cortex, the cell lacunae tend to be circular but still with the longest axis parallel to the surface and somewhat disorganized with respect to the outer cortex. The primary vascular canals are simple, and one single primary osteon is observed. The vascular organization is mostly longitudinal, but a few radial and several irregular vermiform canals are present ( Fig. 8C). The inner cortex presents more vascular canals than the mid- and external cortex, which is consistent with the better organization of the external cortex mentioned above.

A thin layer of secondarily formed lamellar tissue is observed in a short portion of the perimedullar border, interpreted as a probable fragment of the inner circumferential layer (ICL) ( Fig. 8C). There are no clear growth marks in the studied samples (e.g. lines of arrested growth [LAGs], zones and annuli), and thus the minimal age of the specimen cannot be estimated. However, close to the medulla, a fragment of inner cortex is separated from the remaining cortex by a clear and sharp border, suggesting that this fracture might have occurred along a line of weakness, such as a LAG ( Fig. 8A, C). However, this interpretation is speculative and therefore discarded because the line of fracture does not continue through all or most part of the circumference.

Osteoderms

The histological and microanatomical characters of each section of the osteoderms are similar to each other and thus described together. The osteoderm samples consist of two isolated elements and an incomplete series of two continuous rows, probably corresponding to the left series of contiguous osteoderm rows (sensu Ross & Mayer 1983). The paramedian osteoderms of Gracilisuchus stipanicicorum have a dorsal longitudinal crest located slightly medial to the midline of the element; they present an inverted-V shape in transverse section and the lateral part (or “wing”) is somewhat laterally bent. The outer surface is slightly rugose, with some radial and very shallow grooves ( Fig. 3A, B: gr).

The osteoderms are relatively thin, with an inner cancellous core, or spongy bone, surrounded by two layers of compact bone, the superficial cortex and the deep cortex ( Fig. 9A). The inner core is thick, occupying most of the central region of the osteoderm, or most of the superficial half of the osteoderm in different samples, leaving a low superficial cortex. The resorption cavities are large and separated by thin trabeculae in the centre of the osteoderm lateral “wing” (sample G2a, Fig. 9A) and in the dorsal longitudinal crest (samples G2d and G3, Fig. 9D). Here the trabeculae are almost completely formed by thick layers of lamellar bone of secondary origin, and some of them show small remains of the primary parallel-fibered bone in the centre. The resorption cavities decrease in size toward the margins of the osteoderm, where they are separated by thick trabeculae bordered by small layers of lamellar bone. These trabeculae are also formed by abundant remains of primary bone, formed by parallel-fibered tissue. The birefringent layers of secondary lamellar bone have elongate osteocyte lacunae in line with the fibres.These layers were deposited due to remodel - ling of the bone that had already begun when the animal died.

The deep cortex is thicker than the superficial one, presenting a primary matrix of parallel-fibered bone. The birefringence of this tissue reveals fibre bundles oriented diagonal and parallel to the cortical surface ( Fig. 9A). Each bundle has intrinsic fibres extending parallel to each other and fusiform bone cell lacunae aligned with the intrinsic fibre orientation. The fibre bundles extend into the trabeculae of the inner core in various directions, but the osteocyte lacunae are less elongate in this region. This difference is more evident in some samples where the cell lacunae are small and elongate near the surface, but larger, less elongate, and more abundant in the inner core ( Fig. 9C). Near the lateral and medial margins of the osteoderms, the fibre bundles follow the same curvature as the osteoderm surface, having been sectioned longitudinally. Some fibre bundles are perpendicular to all the surfaces of the osteoderm and others perpendicular to the section plane. Those perpendicular to the plane of section appear as minute (ca. 20 Μm) circular monorefringent “dots” circled by a thin birefringent “line”, creating an overall structure resembling “chicken wire” ( Fig. 9B: cwl). These transverse bundles are present in different regions in the samples, such as in a long single layer next to the ventral surface around the midpoint of the lateral “wing” ( Fig. 9B), in the ventralmost part of the inner core, as well as in the superficial cortex. In contrast, they are absent in the deep cortex.

The superficial cortex is thin compared with the inner core and the deep cortex. Near the surface it shows numerous birefringent fibre bundles extending parallel to the surface, and, toward the inner core, the bundles start to take different directions and to be smaller in size. The vascularization of the osteoderms is poor, showing small simple canals of longitudinal orientation. They are more abundant in the deep cortex than in the superficial one, which are distinguished under normal light ( Fig. 9A, C). Sharpey’s fibres are observed in some samples. In two of them, these extrinsic fibres are present in the deep cortex next to the surface, being oriented in several directions.Another sample shows several Sharpey’s fibres in the longitudinal dorsal crest, which orientation near the surface is perpendicular to the surface and further from it they take a diagonal direction ( Fig. 9D). Growth marks are not evident in the osteoderm samples, with the possible exception of a very faint and short line near the ventral surface of the deep cortex of the G3 sample. This could be interpreted as a LAG; however, due to its short and faint condition, this is a dubious interpretation and therefore discarded.

In longitudinal section, there is an anterior region with three layers ( Fig. 10), the correspondence of which to the cortices and the inner core cannot be ruled out because of the partially obscured microanatomy. These layers are recognized based on the general orientation of the fibres. The osseous matrix of the superficial layer is formed by fibres oriented parallel to the surface and several elongated osteocyte lacunae oriented almost parallel to the surface. Fibre orientation in the inner layer is difficult to assess, but some longitudinal fibres are recognized next to the deep layer. Finally, the deep layer contains abundant Sharpey’s fibres oriented nearly perpendicular to the surface, which become more diagonal toward the middle region of the osteoderm and then horizontal at its posterior end.