Morphology and histology of dorsal spines of the xenacanthid shark Orthacanthus platypternus from the Lower Permian of Texas, USA: Palaeobiological and palaeoenvironmental implications Author Beck, Kimberly G. Author Soler-Gijón, Rodrigo Author Carlucci, Jesse R. Author Willis, Ray E. text Acta Palaeontologica Polonica 2016 2014-12-18 61 1 97 117 http://dx.doi.org/10.4202/app.00126.2014 journal article 294717 10.4202/app.00126.2014 79b7ea3c-7374-401f-8429-01493ad010ab 1732-2421 10980746

Table 1. Orthacanthus platypternus dorsal spines sampled. Measurements are given for the most proximal point available and are rounded to the nearest millimeter. The measured cross-sections are from non-denticulated regions, except for specimen HMNS-J3, cut at the level of the denticulated region. With the exception of HMNS-T1 and HMNS-J3, dentine layer 2 is assumed to be the older layer recorded in the cross-sections of the spines because dentine layer 1, representing the juvenile stage SP1, becomes restricted to the distal part of the Orthacanthus spines during growth (cf. Soler-Gijón 1999) and it is already absent in the non-denticulated region of the small spine HMNS-T2 (see longitudinal section in Fig. 4). Abbreviation: dl, dentine layer of centrifugally growing dentine.

Specimen abbreviation	HMNS catalogue code	Spine width (mm)	Spine height (mm)	Number dentine layers (dl)	Estimated age (years)
HMNS-T1	CBB TUF X Spn 1	4	4	1 (dl1)	1
HMNS-T2	CBB TUF X Spn 2	6	6	1 (dl2)	2
HMNS-J1	CBB JON X Spn 1	9	10	3 (dl2–4)	4
HMNS-J2	CBB JON X Spn 2	8	8	?	?
HMNS-J3	CBB JON X Spn 3	7	7	2 (dl1–2)	2
HMNS-J4	CBB JON X Spn 4	7	6	2 (dl2–3)	3
HMNS-J5	CBB JON X Spn 5	6	6	?	?
HMNS-J6	CBB JON X Spn 6	8	6	2 (dl2–3)	3
HMNS-J7	CBB JON X Spn 7	10	8	>3 (dl2–n)	>4
HMNS-J8	CBB JON X Spn 8	7	7	>3 (dl2–n)	>4
HMNS-J9	CBB JON X Spn 9	7	8	?	?
HMNS-J10	CBB JON X Spn 10	8	8	3 (dl2–4)	4