Determinación de áreas críticas de estabilidad

This character statement was modified from Coates and Sequeira 2001a (C76); 2001b (C76) where the emphasis was placed on the shark condition; simplified character statement also in Maisey 2001 (C4); character statement possibly duplicated in (C23); it may be possible to multi code this statement based on the shark condition.

A general statement, at this time, about whether state [2] represents the acanthodian condition (i.e. the condition seen in Acanthodes bronni), is not possible, but this condition is mirrored in the primitive osteichthyan Ligulalepis (AMF 101607) based on the location o f the hyomandibular facet in the braincase o f this example. Articulation o f the hyomandibula with the braincase above the jugular canal may be proved to be a primitive condition of gnathostomes or a condition shared between osteichthyan-like fishes dmdi Acanthodes.

(53) Interhyal (‘Interhyal accessory elem ent’ sensu Miles 1973b): absent (0), or present (1).

See Coates and Sequeira 2001a (C59); 2001b (C59); see also Lauder and Liem 1983: 97 (C6 not part o f a matrix; presence o f which is attributed to Gardiner 1973: 219).

This bony element is observed clearly in only one Acanthodes specimen (NHM P.49990), a cast o f HU MB23 from the Lebach shales, Germany. Unfortunately, the original is now lost (Miles 1973b: 103; personal observation). A second more questionable example of this element may be observed in HU MB 18b, where the interhyal accessory element is

directed antero-posteriorly (perhaps likely as a result o f post-mortem disruption).

Although the primitive condition o f this element has undergone considerable debate (see Patterson 1994: 72-3 for discussion; see also Gardiner 1973: 129, 1984: 357; Patterson 1982; Olsen 1984, and Véran 1988 for comments on the orientation o f the symplectic and interhyal in select osteichthyans), it is seen in NHM P.49990 and its presence, although not observable in other examples oiAcanthodes bronni, it is supported by these further

observations, i.e. the length o f the posterior arm o f the hyomandibula (Hm.v, Miles 1973b) ofyf.. bronni is consistently shorter (taking into consideration the cartilaginous spaces between the two bony elements o f the hyomandibula) than that o f the palatoquadrate region (as seen in HU MB 3, MB 16, MB 18b).

Therefore if an accessory element was absent, this would create an awkward articulation point forward of the articulation centre between the upper and lower jaw articulation points.

This arrangement o f short hyomandibula and long ceratohyal is inconsistent with other observations o f the branchial skeleton o îAcanthodes bronni (Nelson 1968, Schultze

1993), where the articulation centre o f the dorsal and ventral rami meet posterior to the articulation centre o f the branchial arch in front it. The putatively primitive condition o f the acanthodian hyomandibula can be seen in three acanthodian genera, including two examples o f Climatius (namely GSM 49785 and an uncatalogued NMS 2001.7.5) at least two examples

0Ï Ischnacanthus from the MOTH Formation, Canada (e.g. UAL VP 32401 and 42215), and

in one specimen o f Howittacanthus (NMV P. 179586) from Victoria, Australia, where the hyomandibula has been preserved.

In all o f the above specimens, the posterior arm o f the hyomandibula terminates just behind and level with the mandibular jaw joint, therefore, the existence of an accessory ossification linking the hyomandibula to the ceratohyal is highly unlikely.

The posterior ramus o f the hyomandibula also ends posteriorly in a large socket. This suggests that it was connected by cartilage to another bony element. In the aforementioned UAL VP specimens o f Ischnacanthus, the posterior end o f the hyomandibula lacks a socket and is capped by a smooth sheath of perichondral bone, thus demarcating the end o f the postero-dorsal arm o f the hyoid arch. In NHM P.49990 the ‘interhyal’ as described by Gardiner (1973) and Miles (1973b), is small enough to fit inside the hyomandibular socket while the solid, convex terminal end would articulate with the socket o f the ceratohyal which is constructed to accept a solid, round ossicle.

The presence o f an interhyal accessory element seems to be an apomorphy shared by A. bronni with extinct and extant osteichthyans (i.e. teleostomes sensu Miles 1973b). This

feature does not, however, characterise a relationship between the acanthodians in toto with the osteichthyans, but assists in clarifying a relationships between o. Acanthodes bronni with primitive bony fishes. This feature was also used to separate the teleostomes (osteichthyans + acanthodians) from the elasmobranchs which appear to lack this endoskeletal element (Jollie

1971: 93).

DENTITION

Remarks: Denison 1979: 6 reviewed acanthodian teeth as consisting o f only three types: 1) single teeth; 2) tooth spirals or whorls; and 3) teeth fused to dermal jaw bones. Except for the presence o f pharyngeal denticles in Brochoadmones milesi, his general statement about acanthodian tooth types appears to be too simplistic, i.e. the combination and morphology o f tooth structures in acanthodians is in fact much broader.

CHARACTER EVALUATION

For example Climatius, was described by Denison (1979: 25) and others as possessing tooth whorls, multicuspidate, transversely flattened teeth with the possibility that some teeth could be single. This combination has been confirmed in this study with some exceptions. The cusp morphology of these single teeth is variable and does not necessary follow a tricuspidate structure (i.e. principal cusp followed laterally by single smaller cusps) This is demonstrated by a tetracuspidate tooth (Fig. 5.9B) found in NMS (Powrie) 1891.92.195. Furthermore, some Climatius examples possess tooth whorls (e.g. see Fig. 5.9C) which also depart from the traditional semi-spiral structure where the centre teeth are replaced with a gap. Most notably for Climatius is the presence o f a structure interpreted here as a rugose, posteriorly-mounted tooth plate (rug.dent.jp, Fig. 5.7). A similar convex, rugose bulge stemming from the posterior portion of the Meckel’s cartilage (mk, PI. 3, Miles 1973a) is present in the only other example o f the endoskeletal jaws o f Climatius (see Miles 1973b), however, closer inspection o f the original material is necessary before the presence o f a tooth plate can be confirmed.

Maisey (1986: 225) (character L4) used the absence o f teeth or dentigerous jaws to unite the acanthodida (see cladogram 9A on p. 226). He also used the presence o f single teeth and tooth whorls (character L7) as the only synapomorphy to unite the Climatiidae (see cladogram 9A, p. 226). Pharyngeal denticles were not used as a character in my analysis based on the absence o f this feature in all known acanthodians (contra Smith and Coates 2001 : 228) except for Brochoadmones.

(54) Posterior hinge o f dentigerous gnathal bone* (sensu Long 1986b) of mandibular arcb: absent (0), or present (1).

FIGURE 5.6A

This character is the result o f observations based mainly on the jaws o f the ORS and MOTH examples o f Ischnacanthus gracilis. However, this feature is now known to be present in the problematic diplacanthid, Uraniacanthus spinosis (see above figure).This character statement was constructed to collapse two character statements, i.e. presence- absence o f dentigerous gnathal bone and presence-absence dentigerous gnathal bone hinge, into a single combined character based on the fact that both character conditions are either present or absent together. This character statement is strengthened further by the evidence presented from a recent jaw example o f Climatius sp. (see rug.dnt.jp. Fig. 5.7). It possesses a rugose (non dentigerous) gnathal bone without a posterior hinge.

Long (1986:334) used ‘dentigerous gnathal bone’ as his ‘synapomorphic character’ for his node 3, but only associated this feature with ischnacanthid acanthodians.