Orthacanthus platypternus
Creators: name: Beck, Kimberly G.
name: Soler-Gijón, Rodrigo
name: Carlucci, Jesse R.
name: Willis, Ray E.
Publisher: name: Zenodo
Year: 2014
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Chondrichthyes
Xenacanthiformes
Xenacanthidae
Orthacanthus
Orthacanthus platypternus

description: Orthacanthus platypternus (Cope, 1884)   
Figs. 3–7, 8A, 9.  
 Referred material.—15 isolated dorsal spines (HMNS-T1–2, HMNS-J1–10, SMU 68799–68801) from the Craddock Bone Bed (northern Baylor County, Texas, USA), lower Clear Fork Formation, Early Permian (Leonardian age; see Nelson et al. 2013).   
 Description   
 General morphology.—The external morphology of a dorsal spine of Orthacanthus platypternus is well represented by the specimen SMU 68801 (Fig. 3D), 114 mm long, and lacking the most distal region. Two posterior rows of denticles covered about 48 mm of the complete length of the spine, indicating that the denticulation could be extended for the distal half of the complete spine. The cross-section was oval near the opening of the pulp cavity but became circular to subtriangular in the distal part of the non-denticulated region and circular in the denticulated region. The spine was 8.4 mm wide at the proximal end of the denticulated region and reached a maximum width of 9.4 mm proximally, near the basal opening of the pulp cavity. The robustness index maximum width to total length ratio) of the preserved spine is 1:12. The denticle sizes (base length) ranged 1.7–2.3 mm. A density of denticulation of 0.40 denticles/mm remained uniform through the spine SMU 68801 and in the much smaller fragmentary specimens SMU 68799 and 68800 (juveniles) that showed the most proximal denticles (Fig. 3B, C).   
 The most distal denticulation (Fig. 3A) of a juvenile spine was preserved on the smallest spine sampled (HMNS-T1). Thirteen denticles covered approximately 23.5 mm of the total spine fragment length (40 mm). These denticles increased in base length and denticle height proximally except for the most proximal denticle, which was approximately 1 mm shorter in length than the denticle before (distal to) it. Spacing between denticles on HMNS-T1 increased proximally until the 6 th denticle, after which spacing between denticles fluctuated. The denticle sizes ranged from 0.4 mm (first denticle, distally) to 2 mm (denticle 13). The density of denticulation dramatically increased from 1.1 denticles/mm in the distal part to 0.32 denticles/mm in the proximal part.   
 Histology.—The general histology of spines thin-sectioned is shown in serial longitudinal sections (Fig. 4) and serial cross-sections (Fig. 5). The pulp cavity of many spines was filled with calcite and quartz, as well as opaque minerals such as iron oxides. The pulp cavity was located slightly off-center in the spine (as seen in cross-sections) at the proximal end of the denticulated region (Fig. 5), but became closer to the posterior side of the spine in the non-denticulated region (Figs. 6, 7). The wall of the spine was composed of two structural components—an outer layer of centrifugally growing dentine (growing outward from the periphery of the spine), and a layer of centripetally deposited dentine (growing inward) lining the pulp cavity.   
The centrifugal dentine was composed primarily of highly vascular trabecular dentinal layers. Centrifugal lamellar dentine was in the most distal parts of the dentinal layers and at the periphery of large spines. Most spines had occasional enlarged vascular canals in the outer half of the spine wall (Fig. 6B, see also Fig. 5A, C). Dentinal tubules radiated out from the outermost lamellae of denteons, although some vascular canals were simple and did not have distinct lamellae. When a major growth line was present, denteons below the growth line were more developed than those outside of the line. Colour banding was apparent in most spines (Figs. 5A–D, 6A, 7).  
The centripetal dentine mainly exhibited a lamellar structure with incremental deposition of hard tissue (Figs. 4, 5). Centripetal lamellar dentine thickened distally, and filled the pulp cavity of the most distal sections. Spines HMNS-T2 and HMNS-J3, shown in Figs. 4 and 5, respectively, each had a region of centripetal trabecular dentine in the pulp cavity near the distal end.  
Though the smallest spine sampled (HMNS-T1) was sectioned just proximal to the denticulate region, only one dentine layer (SP1) was present. This layer contained no growth lines, changes in colour or vascular patterns, or lamellar dentine (Fig. 6A). However, in thin-sections made near or through the denticulate region of larger spines, centrifugal and centripetal lamellar dentine were present, and in some cases, the dentine could be divided into two depositional layers, separated by a major growth line (see Figs. 4–6). Dentine layer SP1 was thickest in the distal portion of spines, gradually became thinner proximally, and was not present in most fragments proximal to the denticulate region. Dentine layer SP2 was thickest proximally and was not present in the most distal portions of denticulate regions (see Fig. 5). Where both dentine layers were present, a major growth line separated them.  
Major growth lines were most evident in spines HMNS-T2 and HMNS-J3 (Figs. 4, 5). A growth line was evident in four out of five cross-sections of specimen HMNS-J3 (all except for the most distal) and can be seen most clearly in Fig. 5A. Vascular canals to the inside of the growth line were lined with lamellae, whereas vascular canals to the outside of the ring were not. In serial longitudinal sections (Fig. 4), the major growth line gradually projected toward and eventually contacted the pulp cavity, proximally.  
Some of the larger spines also had minor growth lines in the lamellar dentine at the periphery of SP2 (Fig. 5, 7). In specimen HMNS-J3, three minor growth lines were fine, dark and evenly spaced (Fig. 5B). In specimen HMNS-J7, growth lines were arranged in at least three pairs (Fig. 7B), although the spine fragment was not long enough to determine if these growth lines were major or minor growth lines.  
Growth lines, such as those seen in Fig. 5B, were also present in the lamellar dentine around the pulp cavity of four spines (specimens HMNS-J1, J3, J7, J8). In most specimens, colour-banding and growth lines usually circumvented vascular canals.  
Cross-sections of specimen HMNS-J3 show the orthodentine (dark colour) forming the distal part of the denticles see Fig. 5D). The inner region of denticles consisted of white dentine, with many long dentinal tubules radiating outward from at least one vascular canal at the base of or in the innermost region of the denticle. In serial longitudinal sections of HMNS-T2, the denticulate region did not extend to the entire length of SP1 and all denticles present on the available fragment were added to SP2 (Fig. 4).  
Interglobular spaces were found in interdenteonal areas of all spines. These spaces were generally more numerous near the pulp cavity of all spines, and were especially dense in proximal sections, near the pulp cavity of larger spines. In proximal specimen HMNS-J7, interglobular spaces and iron oxides were extremely dense in some regions near the pulp cavity and also markedly decreased in number and size at the border between the innermost layer of white centripetal dentine and the following, darker coloured trabecular dentine of the spine wall (Fig. 7D). Spine HMNS-J8 had a similar distinct decrease in interglobular spaces where the centripetal lamellar dentine lining the pulp cavity met the centrifugal trabecular dentine of the spine wall.  
In several spines, calcospherites on the periphery of the spine wall formed a translucent border (Fig. 5B). Globular calcospherites were most clearly visible on the outer edge of specimen HMNS-J3, and were associated with numerous small interglobular spaces. Interglobular spaces represent poorly mineralized regions between calcospherites, which did not completely fuse during dentinogenesis (Currey 2006). In most cases, interglobular spaces were distributed sporadically through the spine wall, and in the highest numbers and density near the pulp cavity of spines. The high density of interglobular spaces within the centripetal lamellar dentine of specimens HMNS-J7 and HMNS-J8 supports the hypothesis that younger dentine was hypomineralized and became more mineralized as the individual aged (as suggested by Soler-Gijón 1999). Hypomineralized dentine (particularly that near the pulp cavity) was probably more susceptible to diagenesis and degradation following invasion of microorganisms, leading to the deposition of dark, opaque minerals (such as iron oxide) seen associated with interglobular spaces of the centripetal lamellar dentine of some spines (Fig. 7D) (Turner-Walker 2008). Altered dentine and interglobular spaces filled by authigenic minerals are relatively common in fossil chondrichthyan and acanthodian fin-spines (see discussion and references in Botella et al. 2012).  
The colour-banding present in almost all spines was also noted by Maisey (1978) in finspines of hybodont sharks. The colour of fossil dentine can be related to diagenesis, where well-preserved dentine often appears reddish in colour, whereas layers of dentine exposed to the environment may be white-coloured. However, in some cases in this study, a major growth line separated two colour bands as well as two different stages in denteon development (such as in Fig. 5A), suggesting that colour banding can also be related to dentine deposition. The smallest spine sampled here (HMNS-T1) corresponded to a very young juvenile individual as indicated by the short denticulation, and lacked colour banding. The largest spine sampled (HMNS-J1) also showed weaker colour banding than most other spines, probably due to a very fast mineralization during dentinogenesis (see discussion below, in section “Histology and skeletochronology”).
description: Published as part of Beck, Kimberly G., Soler-Gijón, Rodrigo, Carlucci, Jesse R. & Willis, Ray E., 2016, Morphology and histology of dorsal spines of the xenacanthid shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and palaeoenvironmental implications, pp. 97-117 in Acta Palaeontologica Polonica 61 (1) on pages 102-105, DOI: 10.4202/app.00126.2014, http://zenodo.org/record/10980746