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Orthacanthus Platypternus Acta Geologica Polonica, Vol. 68 (2018), No. 3, pp. 421–436 DOI: 10.1515/agp-2018-0025 Orthacanthus platypternus (Cope, 1883) (Chondrichthyes: Xenacanthiformes) teeth and other isolated vertebrate remains from a single horizon in the early Permian (Artinskian) Craddock Bonebed, lower Clear Fork Group, Baylor County, Texas, USA GARY D. JOHNSON Shuler Museum of Paleontology, Institute for the Study of Earth and Man, Southern Methodist University; PO Box 750274, Dallas, Texas, 75275-0274 USA. E-mail: [email protected] ABSTRACT: Johnson, G.D. 2018. Orthacanthus platypternus (Cope, 1883) (Chondrichthyes: Xenacanthiformes) teeth and other isolated vertebrate remains from a single horizon in the early Permian (Artinskian) Craddock Bonebed, lower Clear Fork Group, Baylor County, Texas, USA. Acta Geologica Polonica, 68 (3), 421–436. Warszawa. An unusual 6–8 cm layer of prismatic cartilage and matrix containing some 8,800 teeth, coprolites, incom- plete occipital spines, and denticles of Orthacanthus platypternus (Cope, 1883) occurs in the lower Permian (Artinskian) Craddock Bonebed in Texas, USA. It is the only species of shark present in the Clear Fork Group except for three worn Xenacanthus Beyrich, 1848 occipital spine fragments and two teeth of ?Lissodus (Polyacrodus) zideki (Johnson, 1981) (Hybodontoidei), both being the first occurrences in this unit. Analysis of measurements of teeth with complete bases randomly selected from 3,050 initially available teeth failed to reveal the presence of sexual dimorphism or the discrete presence of juveniles as expected, based on an indepen- dent study which identified the presence of Orthacanthus juvenile occipital spines. A few highly symmetrical small teeth are present, which had not been previously observed in the Texas lower Permian. They may be symphyseals and restricted only to juveniles. Other unusual teeth include germinal teeth and deformed teeth, both of which occur in the Clear Fork and underlying Wichita groups. One tooth displays an apparent example of the equivalent of an “enamel pearl” on one of its cusps. The most unusual teeth are those that appear to have undergone various stages of resorption. Only the lingual margin of the base is affected in which the apical but- ton is resorbed to varying degrees until only the labial margin with the basal tubercle and the three cusps are all that remain. If the teeth were undergoing resorption, then the perplexing problem is why the apical button is resorbed and not the superjacent basal tubercle. Other vertebrate remains include palaeoniscoid scales and teeth and unidentified tetrapod bone fragments, jaw fragments, and teeth. Rare fragments of bones (scales?) bear a “comb edge” which have not been previously observed in the Texas lower Permian. Key words: Orthacanthus platypternus; Early Permian; Craddock Bonebed; Texas. INTRODUCTION al. 2015). It is located in the lower Clear Fork Group south of Lake Kemp in north-central Baylor County, The Craddock Bonebed is best known for its Texas, USA (Text-fig. 1; a complete stratigraphic wealth of amphibian and reptilian remains (Bakker et section is given in Johnson 2013; see also Beck et 422 GARY D. JOHNSON Text-fig. 2. Appearance of the Craddock Bonebed site following excavation in 1996. Bed of concentrated shark remains occurs at level of shovel handle (50 cm long) spines (Text-fig. 3) predominated, and that the larger Text-fig. 1. Location of the Craddock Bonebed, Texas, USA. The spines could be separated by gender. A much more formations below (east of) the Clear Fork Group (undifferentiated) complete analysis by Beck et al. (2015) supports this occur in the underlying Wichita Group (Hentz 1988) conclusion. The smallest teeth (≤1 mm in greatest dimension) resemble the larger teeth and are unlike those described by Zidek (1993). Murry and Johnson al. 2016). A bed of unknown size containing shark (1987) reported only O. platypternus from the Clear prismatic cartilage, which is mostly disseminated in Fork Group except at the base where one other xe- reddish-brown calcareous mudstone that contains xe- nacanthid species occurs (Johnson 2003). Schneider nacanthid teeth and occipital spine fragments, as well and Zajic (1994) regarded the species as being a rep- as isolated vertebrate remains of other taxa, near the resentative of Xenacanthus Beyrich, 1848; this opin- top of the Bonebed. About two square meters of this ion, however, is not followed here, based on Agassiz 6–8 cm thick bed was excavated in 1996 (Text-fig. 2). (1843) and Lund (1970). Intact pieces of cartilage represent unknown skeletal portions; none show evidence of palatoquadrate or meckelian cartilage with in situ teeth. This study rep- MATERIAL AND METHODS resents a more comprehensive conclusion than given earlier by Johnson (2015a, b). Nine samples of the shark layer were screen- Williston (1911) reported the presence of two washed, with some fractions treated with ≤10% ace- types of sharks in the Craddock Bonebed, on the tic acid, and concentrate sorted, but only completely basis of occipital spines, but did not mention teeth. A so for Areas I, III, and V (Text-fig. 4). The remaining preliminary study by Donelan and Johnson (1997) of areas continue to be processed for small remains, teeth recovered by bulk sampling suggested only one but to date have not added significant information. species of shark, Orthacanthus platypternus (Cope, Area IV yielded the largest (11 cm) piece of carti- 1883), is present. Analysis of the occipital spines by lage. Compared to the numbers of O. platypternus Donelan and Johnson (1997) suggested that juvenile teeth obtained by bulk sampling from other Clear Text-fig. 3. Three views of the distal end of a presumed juvenile occipital spine of Orthacanthus platypternus (Cope, 1883) (SMU 77016) from Area V in the Craddock Bonebed shark layer, Texas, USA (see Text-fig. 4); six pairs of ventral denticles are present; compare with Beck et al. (2016, fig. 3A, D). Scale bar equals to 1 mm ORTHACANTHUS PLATYPTERNUS FROM THE PERMIAN CRADDOCK BONEBED, TEXAS 423 Area Area Area I III V Teeth with incomplete bases 1618 875 964 Teeth with complete(?) bases in matrix 398 296* 104 Teeth with complete bases 1441 923 1088 Measured teeth 130 89 124 Teeth with resorbed(?) lingual margins 237 166 109 Deformed teeth 20 17 17 Text-fig. 4. Sketch of the excavated portion of the Craddock Germinal teeth 120 56 45 Bonebed shark layer (see Text-fig. 2) covering about two square Totals 3964 2422 2451 meters showing the nine sample areas Table 1. Numbers of Orthacanthus platypternus (Cope, 1883) teeth collected from three of the nine sample areas (Text-fig. 4) in the shark layer (Text-fig. 2) at the Craddock Bonebed, Texas, USA; total number of teeth = 8,837. *30 teeth are instead associated with cartilage Area Area Area I III V Approximate number of broken tooth cusps 1680 1225 1890 Text-fig. 5. Two examples of Orthacanthus platypternus (Cope, Approximate number of tooth fragments 1249 563 753 1883) coprolites from Area I (left, SMU 77314) and from Area Approximate number of cartilage fragments* 400 475 550 V (right, SMU 77315), from the Craddock Bonebed shark layer, Mucous-membrane denticles 837 689** 828 Texas, USA. Fragments of palaeoniscoid scales and bone fragments Dermal denticles 60 23 48 occur in SMU 77314 and a scale (lower left) and prismatic cartilage Discrete occipital spine fragments 284 13 44 occur in SMU 77315 Approximate number of coprolite fragments 180 72 400 Table 2. Additional remains of Orthacanthus platypternus (Cope, 1883) collected from three of the nine sample areas (Text-fig. 4) in Fork localities (Murry and Johnson 1987; Johnson the shark layer (Text-fig. 2) at the Craddock Bonebed, Texas, USA. * after Sampson and Johnson 2004 (some of the analyzed samples 1999), the total number from those three samples is are from the Craddock Bonebed); ** includes 209 donated to another staggering (Table 1). Descriptive terminology used institution (Märss 2006, but without reference to these specimens) in describing the teeth is provided by Johnson (1999). All specimens are cataloged in the Shuler Museum of Paleontology at Southern Methodist University in Dallas, Texas (abbreviated SMU). brane denticles as opposed to essentially no change in Additional remains associated with O. platypter- the dermal denticles. nus are presented in Table 2. Occipital spine frag- ments in Table 2 do not include “chips” and spine denticles; it is therefore impossible to know how many ORTHACANTHUS PLATYPTERNUS TEETH spines are actually represented in these Areas. Their IN THE CRADDOCK BONEBED numbers do not include those studied by Donelan and Johnson (1997), i.e., 23, 11, and 21 from Areas I, III, The original purpose of this report was to demon- and V, respectively. Also, coprolite fragments present strate that only teeth of one species of xenacanth shark a similar problem; but when observed in total, the is present in the shark layer, and to try to distinguish collection is quite remarkable besides those few that juvenile teeth from adult teeth. Nearly complete teeth are complete (Text-fig. 5; worth noting is their small are extremely rare; nearly all available are illustrated size). They presumably could have been produced (Text-figs 7 and 8), a condition which seems to be uni- only by juveniles. versal based on illustrations in Johnson (1999) from Denticles attributed to O. platypternus (Table 2) much smaller samples. Whether these are juvenile include mucous-membrane denticles (see Zidek et al. or adult teeth is largely uncertain, as no reasonably 2003 for discussion regarding a different xenacanth larger teeth are available. Based on Johnson (1999), genus; Peyer 1968, pp. 54–60) and presumed dermal there was little doubt that only one species is present denticles (Peyer 1968, p. 54, pl. 8b), both shown in (Text-figs 7 and 8). Johnson (1999, pp. 242, 243) noted Text-fig.
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