Orthacanthus platypternus ( Cope, 1884 )

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, Acta Palaeontologica Polonica 61 (1), pp. 97-117 : 102-105

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”).

Figures

Fig.3. External morphology of dorsal spines of Orthacanthus platypternus (Cope, 1884), Lower Permian, Craddock Bone Bed, Texas, USA. A. HMNS-T1, juvenile,lateral view.B. SMU 68799,juvenile, posterior view.C. SMU 68800, juvenile, posterior (C1) and postero-lateral (C2) views.D. SMU 68801, adult, denticulated, postero-lateral view (D1) and non-denticulated, posterior view (D2) regions. Numbers 1 to 17 point to the positions of the denticles along the posterior sides of the spines. Numbers 1’ to 17’ correspond to the right row of denticles.Grey areas in B–D2 represent sedimentary matrix. Scale bars 5 mm.

Fig. 4. Serial longitudinal sections of dorsal spine of Orthacanthus platypternus (Cope, 1884), Lower Permian, Craddock Bone Bed, Texas, USA; specimen HMNS-T2, where A represents the most proximal section and E is the most distal. Samples C–E include the denticulate region. Photos of the sections (A1–E1), interpretative drawings (A2–E2). Scale bars 1 mm.

Fig. 5. Serial cross-sections (A, C, D) of dorsal spine of Orthacanthus platypternus (Cope, 1884), Lower Permian, Craddock Bone Bed, Texas, USA; specimen HMNS-J3, where A represents the most proximal section and D is the most distal. B. Detail of section in A showing three minor growth lines close to the base of the right denticle in the figure (arrows).

Fig. 6. Cross-sections of dorsal spines (non-denticulated region) of Orthacanthus platypternus (Cope, 1884), Lower Permian, Craddock Bone Bed, Texas, USA. Comparison between the smallest, specimen HMNS-T1 (A), and the largest Orthacanthus dorsal spine sampled, specimen HMNS-J1 (B). Posterior sides of the spines are pointing to the top of the figure.

Fig. 7. Cross-section of dorsal spine of Orthacanthus platypternus (Cope, 1884), Lower Permian, Craddock Bone Bed, Texas, USA; specimen HMNS-J7 non-denticulated region) showing color-banding in centrifugal trabecular dentine and growth lines in lamellar dentine. A. General view (posterior side of spine towards the top of the figure). B, C. Details of periphery showing pairs of growth lines (marked by arrows) in the centrifugal lamellar dentine. D. Detail of interglobular spaces within the centripetal lamellar dentine (arrows denote interglobular spaces).

Fig.8. A.Reconstruction ofthe dorsal spine(mainly the denticulated region) of Orthacanthus platypternus, basedon specimens HMNS-T1 and HMNS-T2. B, C. Reconstructions of the dorsal spine of Orthacanthus meridionalis (modified from Soler-Gijón 1999: fig.10); PU-XE76 (B) PU-XE19 (C). Note that distal denticles belong to the first spine (juvenile).The intersection of the first major growth line and the denticulated border of the spine indicate the proximal end of the denticulated region of the first spine; the following “spines” exhibit the distal denticles and those corresponding to each new growth stage.

Fig. 9. Bivariate plots of relationship between width and height in cross sections of Orthacanthus dorsal spines. A. Orthacanthus platypternus (Lower Permian, Craddock Bone Bed, Texas, USA). B. Comparison of O. platypternus with O. meridionalis (Upper Carboniferous, Puertollano, Spain) and Orthacanthus sp. (Upper Carboniferous, Robinson, Kansas, USA). Note the linear regression (y = 0.91x + 0.15; n = 65, r2 = 0.98) calculated by Donelan and Johnson (1997) for the isolated dorsal spines of O. platypternus. The maximum and minimal values are also included here for a comparative reference; values of the rest of the specimens are not included for clarity. Grey shaded areas indicate approximate intervals of the three size clusters corresponding to juveniles and adults according the biometric analysis by Donelan and Johnson (size intervals are based on unpublished data presented in a poster at 57th Annual Meeting of the Society of Vertebrate Paleontology, Chicago, October 1997). Note the ontogenetic trajectories of several individuals of O. platypternus (HMNS-T2 and HMNS-J3), O. meridionalis (PU-XE19, 20, 74 and 76) and Orthacanthus sp. (KUVP-72324) showing stages SP1–n spine proper). Data from O. meridionalis and Orthacanthus sp. after Soler-Gijón (1999: table 1 and figs. 4–8). Vertical grey arrows point to the position in the linear regression for SMU specimens according the maximum width values at the proximal end of denticulated regions: 3.5 mm (SMU 68799), 4.5 mm (SMU 68800) and 8.4 mm (SMU 68801). Abbreviations: SP1–5, spines 1–5; SpPr, spine proper.