ELASMOBRANCHII

Brazeau, Martin D. & Friedman, Matt, 2014, The characters of Palaeozoic jawed vertebrates, Zoological Journal of the Linnean Society 170 (4), pp. 779-821 : 781-782

CYCLOSTOMATA ELASMOBRANCHII HOLOCEPHALI SARCOPTERYGII ACTINOPTERYGII

1969; Jarvik, 1980), but the concept of Placodermi that stabilized prior to the Second World War remains effectively indistinguishable from that which has persisted to the present day. It is important to recognize that workers of the time were comfortable with the notion of paraphyletic assemblages as legitimate taxonomic groups, and many authors understood that placoderms might be ‘united’ by shared primitive characters rather than any specializations of their own. This sentiment was clearly stated by Moy-Thomas (1939: 29) in his influential Palaeozoic Fishes: ‘the more knowledge of them [placoderms] has increased the more certain it has become, that they represent a large early gnathostome group, probably containing the ancestors of all modern fishes’.

The modern era of placoderm systematics began with Denison’s (1975: 9) character-based review of placoderm intrarelationships. He concluded that the group could be recognized on the basis of: an anteriorly placed gill chamber, lying beneath the neurocranium; a neck joint between the neurocranium and synarcual; and dermal bones covering the head and shoulder girdle. The first and last of these features are demonstrably plesiomorphic based on outgroup comparison, whereas the second is, as Denison himself admitted, possibly homoplastic. Miles & Young (1977) built upon Denison’s (1975) efforts at inferring placoderm intrarelationships, but the placement of placoderms as a whole to other groups of gnathostomes was their primary preoccupation. Like Denison, Miles & Young (1977) did not focus on the question of placoderm monophyly, and instead worked under the assumption that placoderms formed a clade to the exclusion of other gnathostomes. These formative works set the agenda for subsequent investigation: the relationships within placoderms (e.g. Young, 1980, 1986; Gardiner, 1984a; Goujet, 1984b; Forey & Gardiner, 1986; Goujet & Young, 1995), and the relationships of placoderms to other vertebrates were topics for research (e.g. Goujet, 1982, 1984b; Gardiner, 1984a; Young, 1986), but placoderm monophyly was not subject to critical testing.

The most explicit arguments for placoderm monophyly ( Goujet, 1984b, 2001; Young, 2010) share a common pedigree, tracing their ancestry to Goujet’s (1982: 29) list of synapomorphies. These are summarized here based on our own translation:

presence of semidentine, a specific kind of hard tissue; [5] an omega-shaped palatoquadrate, with adductor muscles inserting on the ventral surface of this structure and the internal face of the suborbital plate; [6] fusion between dorsal elements of the mandibular and hyoid arches with dermal plates of the cheek.

Goujet (1984b: 237) was later able to expand his list of placoderm synapomorphies, which had grown from six to 11. These additional characters were:

[7] endocranium composed of two ossifications (rhinocapsular and postethmo-occipital) separated by a fissure, unless secondarily fused; [8] long ethmoid region of the endocranium with terminal nasal capsules and a long subnasal shelf; [9] lateral orbits; [10] variable skull pattern, with numerous plates; [11] cheek covered by three plates, including a large submarginal.

Subsequent studies regarded many of these characters as uninformative for the usual reasons: they were either primitive, or their polarity could not be determined through outgroup comparison. Not long after Goujet’s argument for placoderm monophyly, Maisey (1986: 225) concluded that support for Placodermi might be more apparent than real, and that ‘[c]haracterization of placoderms as a monophyletic group... is problematical’.

Goujet (2001: 210) later produced a list of five synapomorphies [their equivalent(s) from Goujet, 1984b are given in parentheses]: a dermal shoulder girdle encircling the trunk and making an articulation with the skull through a joint (a partial combination of characters 1 and 2); a distinctive pattern of dermal bones contributing to the skull roof and cheek (a combination of characters 3 and 11); simple jaws bearing two or three pairs of bony plates (a new character); direct connection between the dermal operculum and braincase via a hyoid arch cartilage (a subset of character 6); and the presence of semidentine (character 4). In response to cladistic analyses rooted on jawless vertebrates that can test – but have failed to support – placoderm monophyly ( Friedman, 2007a; Brazeau, 2009; a similar pattern has been found subsequently by Davis et al., 2012 and Zhu et al., 2013), Young (2010) more than trebled Goujet’s most recent list. As the most recent argument favouring a classical Placodermi, we provide a detailed review of this synapomorphy scheme in a later section.