4.a.1. Preorbital plate

The preorbital plates (PrO; Figs 2a, b, 3a–h) are relatively broad and flat, having a rectangular shape which differs from the flat triangular-shaped PrO plate of Materpiscis (WAM 07.12.1). WAM 96.4.8 (Fig. 3a) is the largest plate recovered (107 mm) and WAM 99.8.4 is the smallest plate recovered (51 mm), indicating some variation in the bone proportions. The presence of developed overlap margins (ov) on the medial edge of the right PrO plate (WAM 96.4.7, Fig. 3f; WAM 99.8.5, Fig. 3h) indicates that unlike Austroptyctodus, Campbellodus and Rhynchodus, there was no median gap between them. They show a weakly concave lateral margin that curves around the top of the orbits and differs from the almost straight lateral margin present in Austroptyctodus (WAM 70.4.253, 86.9.662) and Materpiscis; however, as in Austroptyctodus and Materpiscis, the PrO does not extend around the upper anterior margin of the eye as in Campbellodus (WAM 86.9.672). In dorsal view the path of the supraorbital sensory canal (soc) can be recognized as a series of minute pores running diagonally across the plate (Fig. 3a). In visceral view the canal is visible as a bony tube (Fig. 3b, d, e, g). The visceral surface is dominated by the supraorbital vault (suov, Fig. 3d), which extends to the anterolateral corner of the plate anteriorly (and posteriorly continues onto the postorbital plate), and is delineated by the preorbital (rpro) and supraorbital (rsuo) ridges (Fig. 3b, e), although these are not as prominent as those in Austroptyctodus.

4.a.2. Postorbital plate

The postorbital plate (PtO; Figs 2a, b, 3i–l) has a smooth concave lateral margin for the orbit (WAM 99.8.16; om, Fig. 3k) and irregular interdigitating margins between the paranuchal and central plates. It is large, compared to other ptyctodonts, being almost twice the size of the paranuchal plate (Fig. 2a). A small notch (nt, Fig. 3i, k) in the posterolateral margin of the postorbital plate formed a margin for the postorbital fenestra (fen, Fig. 2b), which is high and narrow like that in Materpiscis (Long et al. Reference Long, Trinajstic, Young and Senden2008, Fig. 1h). Unlike Campbellodus there are no linear thickenings associated with the postorbital fenestra. A marked embayment (dp, Fig. 3j, l) on the visceral surface of the PtO corresponds to the downwards folding of the plate to form the lateral face of the head shield as an external angulation between the dorsal and lateral faces of the skull; however, this is not as strongly arched as in Austroptyctodus, Campbellodus, Ctenurella and Rhynchodus. In visceral view, it shows a prominent ridge (rpto) delineating the supraorbital vault (Fig. 3l).

4.a.3. Nuchal plate

The nuchal plate (Nu, Figs 2a, b; 4a–d) in Kimbryanodus williamburyensis nov. forms the median posterior margin of the head shield (Fig. 2a). In this respect it is similar to Austroptyctodus, Campbellodus, Materpiscis and Rhynchodus. The margins of the nuchal plate are complete and gently concave with a narrow underlap area to meet the surrounding bones with which it connects. WAM 99.8.1 (its delicate condition precluded photography) is approximately half the size of WAM 96.4.9 (Fig. 4a, b). An anterior point projects between the posterior margins of the preorbital plates but does not separate them (Fig. 2a). The sensory lines are not clearly visible in dorsal view, but are prominent X-shaped raised tubes of bone in ventral view (soc, pp, Fig. 4b). Unlike Austroptyctodus, Campbellodus and Denisonodus, where the bony tubes end at the posterior corners of the nuchal plate, in Kimbryanodus williamburyensis nov. and Materpiscis the bony tubes end immediately posterior to the centre of the plate (WAM 96.4.9; Fig. 4b, d); however, Materpiscis has a shorter and broader nuchal plate. Along the midline of the posterior portion of the nuchal plate is a central groove (gr) with two small lateral grooves (lgr) at the posterior margin of the nuchal plate (Fig. 4b, d). This character is unique to the genus, being present on both nuchal plates recovered (WAM 96.4.9, 99.8.1).

4.a.4. Central plate

The central plates (Ce; Figs 2a, b, 5a–f) are approximately the same size as the preorbital plates and in this respect similar to Austroptyctodus and Ctenurella. However, they lack the anterior-medial embayment for the posterior processes present on the preorbital plates of Austroptyctodus and Campbellodus. A small anterior underlap is present for the overlap of the preorbital plate and extensive lateral contact margins for the postorbital and paranuchal plates (WAM 99.8.18, Fig. 5a; WAM 99.8.13, Fig 5b). The margin in contact with the paranuchal plate is weakly convex in dorsal view where the skull roof bends down (Fig. 2b). A smooth process of dermal bone, which continues onto the paranuchal plate, forms the posterior process of the skull roof (ppr; Fig. 5a, b, e), which is similar to Austroptyctodus, Ctenurella and Campbellodus.

4.a.5. Paranuchal plate

The paranuchal plate (PNu; Figs 2a, b, 5g–l) is short (WAM 96.4.12, 40 mm), being approximately half the length of the postorbital plate (WAM 99.8.15, 75 mm), whereas in Austroptyctodus and Materpiscis it is the same length as the postorbital plate and in Ctenurella, Rhynchodus, and Rhamphodopsis it is longer. Compared with other ptyctodonts the paranuchal plate is relatively flat. A deep notch (nt) is present in WAM 96.4.12 (Fig. 5k), corresponding to a notch on the postorbital plate for the postorbital fenestra (fen, Fig. 2b), as in Austroptyctodus and Campbellodus. The anterior process (WAM 96.4.12; pr, Fig. 5i) surrounding the notch is longer and narrower than in either of these taxa, resembling instead the process of Materpiscis. The median part of the posterior pit line canal is visible externally as a series of pores and in visceral view as a well-developed tube of bone (pp; Fig. 5l).

4.a.6. Marginal plate

Two marginal plates (M) have been preserved and the straight dorsal margin of this plate forms a contact margin for the postorbital plate (Figs 2b, 6a, e, g, j). The overall shape of the marginal plate most closely resembles those of Austroptyctodus, Ctenurella and Materpiscis with its deeply convex posterior margin; unlike Austroptyctodus and Materpiscis, the denticulated, ridged ornament on the postocular division is absent. On one marginal plate (WAM 96.4.19) there is a well-developed square-ended process on the ventral border (pr, Fig. 6g–j), but on the other plate the ventral border is incomplete (WAM 99.8.1; Fig. 6a, e). Austroptyctodus (WAM 86.9.662, MNH P57637), Ctenurella (MNHN ARD 229, MNH P 53343, 53344) and Materpiscis also have a process on the ventral border of the marginal plate but it is smaller. The orbital division of the M plate is longer than the postorbital division in Kimbryanodus williamburyensis nov., whereas in Ctenurella and Materpiscis it is shorter and in Austroptyctodus they are equal. As in Austroptyctodus there is a well-developed, thick, inwardly projecting mesial lamina on the smooth visceral surface (ml; Fig. 6b–d, f, h, i), comprising subocular (sol) and postocular (pol) divisions (Fig. 6b, c, h).

4.b.1. Supragnathal plate

The supragnathal (u.tpl, Figs 2b, 7h–1) is approximately one-third high as it is long, has a gently curved biting margin with a ‘beak’-like extension that fits into a corresponding depression on the anterior margin of the infragnathal. A small anterior dorsal process forms the attachment to the autopalatine (WAM 96.4.14; adp, Fig. 7i), which when compared to Campbellodus appears quite low, resembling more the condition seen in Austroptyctodus. A prominent anterior ridge (rd) extends from the biting surface to the dorsal margin, a character unique to Kimbryanodus williamburyensis nov. (WAM 96.4.16; Fig. 7h). The occlusal surface is slightly concave, pitted and transversely grooved.

4.b.2. Infragnathal plate

The infragnathal plate (l.tpl, Figs 2b, 7j–l) is narrow and anteriorly sectorial, similar to the condition seen in Rhynchodus and Ctenurella, whereas the lower toothplates of Austroptyctodus, Campbellodus and Materpiscis are higher and wider. The narrow, posterior division of the biting surface is triturating and the trabecular inner body of the gnathal is revealed in an acid-etched specimen (WAM 99.8.1, Fig. 7l). Although a triturating posterior surface is more common in Ptyctodus, Denison (Reference Denison1985) notes that in Rhynchodus the gnathals may have a triturating surface posteriorly. The lower tooth plates have a well-developed ventral process (vp, WAM 96.4.15, 7j, k), which is also present in Ctenurella (MNHP ARD 230A).

4.c.1. Autopalatine

The single perichondrally ossified autopalatine WAM 96.4.13 (Figs 2b, 7f, g) is sub-rectangular in shape, resembling that of Ctenurella (MNHN ARD 229) rather than the triangular-shaped autopalatine of Austroptyctodus (WAM 70.4.253). A deep embayment directly posterior to the anteroventral corner received the broad triangular dorsal process of the supragnathal (mdep; Fig. 7g) and is positioned more anteriorly in both Austroptyctodus and Materpiscis. A ridge is developed along the anterior margin of the autopalatine from which a lamina of strongly folded bone projects on the anterodorsal corner. This ridge of bone is absent in Austroptyctodus (WAM 70.4.253) and very narrow in Materpiscis. Two further embayments, one median and one anterior (dep; Fig. 7g), are present, and all three embayments are separated by thickened perichondrial bone elevated above the surrounding surface. A strong fold is present on the mesial perichondrial lining which results in the ventral portion of the bone flaring outward (fl, Fig. 7f).

4.c.2. Endocranium

Long (Reference Long1997) gave a detailed description and reconstruction of the endocranium in ptyctodonts based on Chelyophorus, Austroptyctodus, Campbellodus, and the then-undescribed Kimbryanodus williamburyensis nov. The endocranium comprises three paired ossifications, the orbital, occipital and ethmoid, and of these, only the occipital (WAM 96.4.18) ossification is well preserved with the margins intact, although it is not in situ (Fig. 7a–e). This material does not significantly differ from that described by Miles & Young (Reference Miles and Young1977) and Long (Reference Long1997), with the exception that the occipital process of the endocranium (pr.oc) is shorter in Kimbryanodus williamburyensis nov. and does not extend past the glenoid process, whereas in Austroptyctodus (NHM P57664 and NHM P50910) the occipital process of the endocranium flares posteromedially.

4.d.1. Median dorsal plate

Only one complete median dorsal plate is preserved (MD; WAM 99.8.22; Fig. 8a–c); it is broad, and shield-like in shape, with concave anterior and posterior margins. In Austroptyctodus and Materpiscis the posterior margin is acutely pointed. A low median crest (mcr; Fig. 8b) is present on the posterior dermal surface. Unlike Austroptyctodus (WAM 70.4.254, NHM P50908), the median dorsal plate has very little ornament on the external surface of the plate and is weakly arched. The overlap area for the median dorsal plate on the anterior dorsolateral plate (Mdoa; Fig. 9d) suggests that the median dorsal plate was steeply inclined in profile (Fig. 2b). There are no overlap areas to suggest the presence of a median dorsal spine, but a deep posterior groove (gr) on the ventral surface suggests an articulation with a posterior basal plate (Fig. 8c). In this respect it is similar to Austroptyctodus (Long, Reference Long1997) and Chelyophorus (D. Goujet, pers. comm.), Ctenurella and Materpiscis.

4.d.2. Anterior dorsolateral plate

The anterior dorsolateral plate (ADL; Figs 2b, 9a–e) has a distinct, dorsolateral angular inflexion (WAM 99.8.20; Fig. 9c) similar to Chelyophorus and Rhynchodus. It is broader along the dorsal margin than the ventral margin and in this respect is similar to Austroptyctodus and Ctenurella, whereas in Materpiscis the dorsal and ventral margins are of similar dimensions and in Campbellodus the ventral margin is broader. A small, thin ventral process of ornamented bone meets the anterior lateral plate (Fig. 9d) and defines a V-shaped insertion for the anterior lateral plate, which is similar to that described for Austroptyctodus, Ctenurella, Chelyophorus and Materpiscis.

4.d.3. Anterior lateral plate

The narrow dorsal margin of the anterior lateral plate (AL, WAM 99.8.12, Fig. 9f–h) forms an acute apex at the contact with the anterior dorsolateral plate. An unornamented groove (gr, Fig. 9f) separates the exterior of the plate from the thin mesial lamina, which has four rows of stellate tubercles. There are a variable number of stellate tubercular rows in Austroptyctodus, with WAM 70.4.253 having seven rows, whereas NHM P50908 has only four rows. A second AL plate (WAM 99.8.17) does not have the mesial lamina preserved so it is impossible to determine if there is any variation in row number present in Kimbryanodus williamburyensis nov.

4.d.4. Interolateral plate

The interolateral plate (IL; anterior ventrolateral plate of Miles & Young, Reference Miles and Young1977; Johnson & Elliott, Reference Johnson and Elliott1996) is similar to that of Denisonodus and Rhamphodopsis, being strongly arched in a dorsoventral direction and lacking a developed postbranchial lamina (WAM 99.8.7; Fig. 9k, l). The reticulate ornament on the external surface is sparse and confined to the central region of the plate. Three rows of small stellate denticles are present on the anterior face of the plate (Fig. 2b). A larger postbranchial lamina is present in Austroptyctodus with seven rows of denticles present and Campbellodus (WAM 86.9.672) with five rows of denticles present.

4.d.5. Spinal plate

The narrow, elongate spinal plate (Sp; Fig. 9i, j) is slightly wider at the posterior end. The dorsal and ventral margins are thickened into ridges and the central region of the plate has a reticulated ornament (Fig. 9i). There is no distinct projection of a spinal process as seen in Rhamphodopsis. The visceral surface is concave and smooth (Fig. 9j). The spinal plate is smaller compared to all other ptyctodonts with the exception of Campbellodus.

4.d.6. Anterior median ventral plate

The anterior median ventral plate (AMV) is not preserved in anatomic position, being adhered to the visceral surface of the anterior dorsolateral plate (WAM 99.8.12) so that only the exterior face is visible (Fig. 9h). The symmetrical plate is extensively covered by the interolateral plates, except for a narrow waisted ornamented region along the midline of the plate. It differs from Austroptyctodus, Campbellodus and Materpiscis in having an acute point on the anterior medial margin. In the other taxa the anterior margin is straight or concave.

4.d.7. Pelvic claspers of males

Dermal pelvic elements comprising hook-shaped ‘claspers’ have been preserved with four hooked tubercles on the posterior end of each laterally curving clasper (WAM 96.4.17, Fig. m–o). This differs from Austroptyctodus, in which there are three pointed tubercles (Miles & Young, Reference Miles and Young1977). There are three to four additional low tubercles on the posterior surface and the rest of the bone is unornamented. Each element is expanded anteriorly with a deep ventral concavity as in Austroptyctodus (Miles & Young, Reference Miles and Young1977).

6.a.1. Monophyly of the Ptyctodontida

The Ptyctodontida can be defined by the following four synapomorphies (Fig. 10a, b, node 1): possess only one pair of supragnathals, the posterior supragnathals (character 5; 0→1); infragnathal plates without cusps (character 6; 0⇒1); pre-pelvic claspers (males) (character 7; 0⇒1); pineal plate forms part of anterior margin of skull (character 10; 0⇒1);

The upper and lower dentitions of the ptyctodonts are here considered a synapomorphy of the family. All known ptyctodonts possess one pair of supragnathals, considered homologous with the posterior supragnathals because they attach to the autopalatine, whereas in Romundina the single pair of supragnathals is considered homologous with the anterior supragnathals because they attach to the ethmoid ossification. All other placoderms, with the exception of the petalichthyids in which the dentition remains unknown, have both an anterior and posterior pair of supragnathals (Denison, Reference Denison and Schultze1978; Stahl, Reference Stahl, Arratia and Schultze1999; Young, Lelièvre & Goujet, Reference Young, Lelièvre and Goujet2001). The lower dentition of ptyctodontids comprises laterally compressed plates of pleuromic dentine without a growing tritoral surface (Fig. 7l). The dentition therefore differs from the chimearoids, which have numerous hypermineralized tritoral patches growing from a dental lamina and forming a multi-tritored tooth plate. The presence of dental laminae within placoderms remains a contentious issue (Smith & Johanson, Reference Smith and Johanson2003; Burrow, Reference Burrow2003; Young, Reference Young2003).

There has been much discussion as to whether the pelvic claspers of ptyctodontids are homologous with those of the chondrichthyans and holocephalans, particularly the chimaeroids (Miles, Reference Miles1967; Ørvig, Reference Ørvig and Lehman1962), or the result of convergence (Patterson, Reference Patterson1965, Reference Patterson1992; Miles & Young, Reference Miles and Young1977; Denison, Reference Denison1985; Forey & Gardiner, Reference Forey and Gardiner1986; Stahl, Reference Stahl, Arratia and Schultze1999). Goujet & Young (Reference Goujet, Young, Arratia, Wilson and Cloutier2004) cited five characters, which they hypothesized determined the monophyly of the placoderms, although four of the five synapomorphies have been questioned (Zhu, Yu & Janvier, Reference Zhu, Yu and Janvier1999; Zhu & Schultze, Reference Zhu, Schultze and Ahlberg2001; Burrow & Turner, Reference Burrow and Turner1999; Smith & Johanson, Reference Smith and Johanson2003) and their analysis places ptyctodonts within the Placodermi. A connection between the holocephalans and the ptyctodonts is now considered unlikely.

The presence of the pineal plate as the most anterior of the head shield supports the view of Denison (Reference Denison and Schultze1978) that ptyctodonts had an anteriorly unossified cranial roof. It appears there has been a reduction in the number of head shield plates, with the loss of the rostral and post-nasal plates, which are common to other placoderms. A reduction in the number of trunk shield plates compared with other placoderms is also observed.

6.b.1. Clade 1

Rhamphodopsis (Fig.10a, b, node 2) is separated from the other ptyctodonts through the retention of two basal characters: the presence of scales (character 4) which occur on the pelvic fins but are unknown on the tail (Miles Reference Miles1967); a cylindrical condyle on the anterior dorsolateral plate, which forms the trunk articulation of the glenoid joint (character 17; character 9 of Forey & Gardiner, Reference Forey and Gardiner1986). The glenoid condyle in other ptyctodonts is developed as a short shelf, either transversely or longitudinally orientated (Miles & Young, Reference Miles and Young1977; Forey & Gardiner, Reference Forey and Gardiner1986). Two additional characters, the centrals participating in the posterior border of the skull roof (character 8, 0→1) and anterior margin of the nuchal plate square (character 20, 0→1), occur within a number of other taxa.

6.b.2. Clade 2

This clade, comprising Campbellodus, Denisonodus, Rhynchodus and Ptyctodopsis (Fig. 10, node 4), is characterized by the following two synapomorphies: a complex M-shaped articulation of the anterior dorsolateral plate to the anterior lateral plate (character 15, 0→1) and a short postorbital division of the marginal plate (character 22, 0⇒1). This node can be further subdivided into two: the first containing Campbellodus and Rhynchodus (Fig. 10, node 5), the second containing the two North American genera, Ptyctodopsis and Denisonodus (Fig. 10a, b, node 6). Campbellodus and Rhynchodus are united by the presence of preorbital plates with no mesial contact (character 9, 0⇒1, character 1 of Johnson & Elliott, Reference Johnson and Elliott1996) and it has been independently acquired in Austroptyctodus. This character was coded as present in all ptyctodonts in the analysis of Johnson & Elliott (Reference Johnson and Elliott1996); however, Long (Reference Long1997) reinterpreted the arrangement of the bones forming the skull roof in Rhyncodus, Austroptyctodus and Campbellodus, determining that the preorbital plates were separated either side of the mid-line in these taxa.

Two character reversals occur in Campbellodus: the presence of multiple median dorsal plates (character 3, 1⇒0) and the presence of a scale-covered tail (character 4, 1⇒0). New specimens from the Canadian Arctic have revealed that the median dorsal plate in Romundina cf. stellina was also a composite structure comprising two elements and that this may represent the least derived condition (D. Goujet, pers. comm.). The presence of a scale-covered tail is also thought to be the basal condition, with Rhamphodopsis the only other ptyctodont with scales. Austroptyctodus was thought to also have a covering of scales on the tail (Long, Reference Long1997), but these have since been reinterpreted as the anterior lateral plates of three embryos (Long et al. Reference Long, Trinajstic, Young and Senden2008).

The American taxa are united (Fig. 10a, b, node 6) by four characters: the centrals participating in the posterior border of the skull roof (character 8, 0→1); an elongated nuchal plate (character 23, 1→0, also present in Austroptyctodus, Ctenurella and Kimbryanodus williamburyensis nov.); a median point on the anterior margin of the median dorsal plate, which projects in front of the anterior dorsolateral plates (character 18, 0⇒1). Denison (Reference Denison1985) hypothesized that the median notch in the posterior border of the head shield was for the reception of this point; however, this seems unlikely, as a median notch is present in taxa that lack a median point, such as Austroptyctodus, Campbellodus, Kimbryanodus williamburyensis nov. and Rhynchodus; and absent in Denisonodus, in which the median dorsal has a median point.

The presence of crushing, ‘Ptyctodus’-type lower tooth plates (character 12, 1⇒0) also unites these two taxa. Denison (Reference Denison1985) noted that some ptyctodonts have a hybrid tooth type with sectorial blades anteriorly and a tripartite occlusal area on the posterior portion of the biting surface. It is probable that the structure of the tooth plates, both supragnathals and inferognathals, indicated similar feeding patterns rather than relationships among the ptyctodonts.

6.b.3. Clade 3

The remaining clade (Fig. 10a, b, node 7) contains Kimbryanodus williamburyensis nov., which possesses a small spinal plate, and the four aspinothoracid ptyctodontids Austroptyctodus, Chelyophorus, Ctenurella and Materpiscis. Johnson & Elliott (Reference Johnson and Elliott1996) noted differences in the size of the spinal plates between taxa so in this analysis the character was coded as multistate. A small spinal plate has independently occurred in Campbellodus, and spinal plates are large and projecting in Denisonodus, Ptyctodopsis, Rhamphodopsis and Rhynchodus.

In addition, the genera of node 7 (Fig. 10a, b) all possess an anterior dorsal lateral plate with a simple inverted V-shape articulation to the anterior lateral plate (character 15) and they all lack a dorsal spine on the median dorsal plate (character 13, 0⇒1). Austropytctodus, Ctenurella, Kimbryanodus williamburyensis nov. and Materpiscis are further united (node 8, Fig. 10) by having a straight overlap margin between the interolateral and anterior dorsolateral plates (character 26, 0⇒1); a short broad anterior median ventral plate (character 16, 0⇒1); a marginal plate with a ventral process (character 27, 1⇒0) and a short dorsal process on the upper tooth plate, which is considered a reversal (character 11, 2⇒1).