JOURNAL OF MORPHOLOGY 274:627–644 (2013) Histology of ‘‘Placoderm’’ Dermal Skeletons: Implications for the Nature of the Ancestral Gnathostome Sam Giles, Martin Ru¨ cklin, and Philip C.J. Donoghue* School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK ABSTRACT The vertebrate dermal skeleton has long 2002). These lineages include the jawed acantho- been interpreted to have evolved from a primitive dians and placoderms, and the jawless ostraco- condition exemplified by chondrichthyans. However, derms (osteostracans, galeaspids, heterostracans, chondrichthyans and osteichthyans evolved from an an- thelodonts, and anaspids), all of which possessed cestral gnathostome stem-lineage in which the dermal an extensively developed mineralized dermal skele- skeleton was more extensively developed. To elucidate the histology and skeletal structure of the gnathostome ton composed of bone and, in many lineages, der- crown-ancestor we conducted a histological survey of the mal tubercles comprising enameloid and dentine diversity of the dermal skeleton among the placoderms, (Donoghue and Sansom, 2002). With respect to the a diverse clade or grade of early jawed vertebrates. The histological structure of the dermal skeleton in the dermal skeleton of all placoderms is composed largely of gnathostome crown ancestor, the most poorly a cancellar architecture of cellular dermal bone, sur- known and yet most significant of these extinct lin- mounted by dermal tubercles in the most ancestral eages are the placoderms, the evolutionary rela- clades, including antiarchs. Acanthothoracids retain an tionships of which remain the focus of vigorous ancestral condition for the dermal skeleton, and we re- debate (Fig. 1; e.g., Johanson 2002; Brazeau, 2009; cord its secondary reduction in antiarchs. We also find Young, 2010; Davis et al., 2012). Perceived as a that mechanisms for remodeling bone and facilitating different growth rates between adjoining plates are clade, Placodermi has been most widely considered widespread throughout the placoderms. J. Morphol. a sister lineage to crown-gnathostomes (Fig. 1A; 274:627–644, 2013. Ó 2013 Wiley Periodicals, Inc. Young, 2008,2010). Perceived as an evolutionary grade, placoderms comprise a series of successive KEY WORDS: histology; placoderm; dermoskeleton; sister-lineages to crown-gnathostomes (Fig. 1B; gnathostome; evolution; bone Johanson, 2002; Brazeau, 2009; Davis et al., 2012). In either instance, placoderms remain integral to understanding the histological structure of the der- mal skeleton in the ancestral crown gnathostome, the condition from which both chondrichthyans INTRODUCTION and osteichthyans are derived. Thus, we set out to survey the histology of the dermoskeleton across Among living vertebrates, only the gnathostomes placoderm diversity. In providing an insight into (the clade of living jawed vertebrates) possess a the tissues present in the placoderms, we mineralized skeleton. The gnathostome clade com- attempted to resolve the plesiomorphic condition of prises chondrichthyans (sharks, rays, and holoce- phalans) and osteichthyans (all other living jawed vertebrates). Naturally, therefore, chondrichthyans Sam Giles is currently at Department of Earth Sciences, Univer- have been pivotal in attempts to understand skele- sity of Oxford, Oxford, UK tal evolution among osteichthyans (Williamson, 1849,1851; Hertwig, 1874a,b,1876,1879,1882; Contract grant sponsor: EU FP7 Marie-Curie Intra-European Fellowship (to MR and PCJD); NERC grant NE/G016623/1 (to Stensio¨, 1961; Ørvig, 1951,1968,1977; Reif, 1976, PCJD). 1978a,b,1980,1982). To be sure, chondrichthyans are a natural outgroup to osteichthyans, but they *Correspondence to: Philip C.J. Donoghue, School of Earth Scien- do not necessarily reflect an ancestral gnathostome ces, University of Bristol, Wills Memorial Building, Queen’s Road, state. This expectation is true of the chon- Bristol BS8 1RJ, UK. E-mail: [email protected] drichthyan skeleton, and most especially of the micromeric dermal skeleton, which is reduced with Received 23 October 2012; Revised 21 November 2012; Accepted 3 December 2012 respect to the primitive gnathostome condition, as evidenced by successive extinct sister lineages to Published online 31 January 2013 in the chondrichthyans, osteichthyans and, indeed, to Wiley Online Library (wileyonlinelibrary.com) crown gnathostomes (Donoghue and Sansom, DOI: 10.1002/jmor.20119 Ó 2013 WILEY PERIODICALS, INC. 628 S. GILES ET AL. widespread in arthrodires. These may be composed of bone or semidentine, the latter of which is a putative synapomorphy of placoderms (Goujet and Young, 2004; Young, 2010) defined by its unipolar cell lacunae. Semidentine cell lacunae were first identified in placoderms by Gross (1935) and were initially referred to as ‘‘Unipolare Knochenzellen’’ (unipolar bone cells), reclassified as the tissue ‘‘semidentine’’ by Ørvig (1951). Ørvig’s reinterpre- tation was not initially accepted universally and the term was rejected by some authors, including Bystrow (1957) who preferred to interpret the tis- sue as bone, and Kulczycki (1957) who thought it transitional between bone and dentine. Enamel has not been found in the arthrodires; the thin layer of enamel identified in Sedowichthys by Fig. 1. Evolutionary relationships of principal groups of ver- Bystrow (1957) has been widely disregarded, and tebrates. Phylogenetic relationships among the principal groups Stensio¨’s (1925) enamel-coated ‘‘coccosteid’’ jaw has of stem- and crown-gnathostomes. The more recent hypothesis been reinterpreted as acanthodian (Miles and of placoderm paraphyly (A) vs. the traditional view of placo- Young, 1977). Bystrow (1957) identified an appa- derm monophyly (B). rent evolutionary transition within arthrodires, from an ancestral state of thin plates bearing the gnathostome dermal skeleton and, in conse- ‘‘dermal teeth’’ (with dentine and enamel) to an quence, provide insight into the subsequent evolu- increasing thickness of plates and filling in of tion of this skeletal system in extant vertebrates. ‘‘teeth’’ with bone. Stensio¨ (1934) identified Haver- sian canals in the basal layer of Phyllolepis. Antiarchi. Antiarchs also exhibit a three-lay- ered structure, with a superficial lamellar layer Synthesis of Previous Research into the (termed the ‘‘Tuberkelschicht’’ by Gross, 1931), a Histological Structure of the Placoderm cancellous spongiosa, and a compact basal lamellar Dermal Skeleton layer (Grundlamellenschicht after Gross, 1931). Research into the microstructure of placoderm This structure is typified by Asterolepis ornata dermal armour has been ongoing since the 19th (Heintz, 1929). Juvenile specimens of A. ornata century (e.g., Agassiz, 1844; Gu¨ rich, 1891). How- and Bothriolepis canadensis exhibit three layers ever, there has been no systematic attempt to sur- even in early ontogeny (Gross, 1931; Downs and vey skeletal composition and reports are disparate Donoghue, 2009), with the compact layers being both in their taxonomic and anatomical coverage, the first to develop. Antiarchs differ from Arthro- with little comprehensive understanding of how dira in the presence of a basal cancellous compo- tissues and structures are distributed across differ- nent in the superficial layer. In addition, the su- ent placoderm groups. Here, we review the sum of perficial layer as a whole is separated from the published knowledge. middle spongiosa by a ‘‘planar discontinuity’’ Arthrodira. The dermal skeleton is perceived (Goodrich, 1909; Gross, 1935). This was inter- generally to consist of a superficial compact lamel- preted by Downs and Donoghue (2009) as a lar layer (referred to as the ‘‘Oberfla¨chenschicht’’ cementing line, and also as a plane of overlap by Heintz, 1929, or, within phyllolepids, as the between adjacent scarf joints. The superficial layer ‘‘Skulpturschicht’’ by Gross, 1934), a cancellous may possess tubercles that, in derived forms at spongiosa, and a compact lamellar base. The pres- least, are composed of cellular bone. It has been ence of distinct layers and their relative thickness speculated that semidentine is present in ancestral varies from one account to another; in Phyllolepis, antiarchs (Young, 2008), but this has not been sub- Stensio¨ (1934) notes that the superficial layer is stantiated. Both the superficial and middle layers confined to the ornament, and Hills (1931) identi- contain evidence of extensive resorption and sec- fies a multilayered basal layer. Heintz (1929) rec- ondary bone growth (Downs and Donoghue, 2009). ognized two distinct layers of spongiosa within the The presence of spheritic mineralization in all dermal armour of Heterogaspis (Monaspis; contra three layers of the dermoskeleton has led to a Downs and Donoghue, 2009): a superficial-most number of different interpretations. Stensio¨ (1931) ‘‘Maschenschicht’’ with irregular interconnected considered the tissue to be an intermediary cavities, and a basal-most ‘‘Kanalschicht’’ with cav- between bone and cartilage, whereas Ørvig (1968) ities typically parallel to the basal surface. The classified it as ‘‘globular bone.’’ Burrow (2005) sug- spongiosa is also purported to contain Haversian gested the tissue was cartilage and thus part of canals (Bystrow, 1957). Superficial tubercles are the endoskeleton. Downs and Donoghue (2009), Journal of Morphology HISTOLOGY OF ‘‘PLACODERM’’ DERMAL SKELETONS 629 demonstrated that the topographic position of the ventrolateral plate MNHN HISTPAL 2800 and Yunnanolepis