Placoderm muscles and chordate interrelationships.

Trinajstic et al. (2007) reported exceptionally preserved soft tissues in two placoderm fish from the Devonian of Australia. This exciting finding presents fresh morphological data with a bearing on an old phylogenetic problem: deep branching structure within the vertebrate tree. Trinajstic et al. (2007) show that axial muscle blocks in the placoderm Austroptyctodus more closely resemble those of living lampreys than those of extant gnathostomes, and argue that this is consistent with the prevailing hypothesis that placoderms are the sister group of all remaining jawed vertebrates. Unfortunately, the comparative framework that provides context for these exceptional fossils contains a number of errors that we address here. It is stated that ‘comparing muscle structure and pattern of the Gogo placoderms with extant taxa indicates that the closest extant relative to the placoderm is the lamprey (fig. 2, table 1)’ (Trinajstic et al. 2007, p. 199). This conclusion is incorrect and contradicted by the cladogram that it references: placoderms are more closely related to living jawed vertebrates than they are to any ‘agnathan’. Lampreys and placoderms share primitive characters that have no bearing on their relationships to each other. It is also stated that lampreys have seven extrinsic eye muscles, as it appears to have been the case in some placoderms. This carries with it the implication that placoderms retain a primitive condition, but lampreys have six extrinsic eye muscles (Fritzsch et al. 1990). The caption of fig. 2 implies that the cladogram it depicts is derived from the matrix given in table 1, but no description of methods is provided. We reproduced this dataset and analysed it using the exhaustive search algorithm in PAUP v. 4.0b (Swofford 2002). We find a single shortest tree of 21 steps, but it is not the one given in fig. 2. Our analysis places acanthodians as the sister group of osteichthyans, not chondrichthyans. However, placoderms remain the sister group of all other jawed vertebrates. A more serious matter is that more than half of the characters (9 out of 17) in the matrix contain coding mistakes, and just less than 18 per cent (18 out of 102) of all entries are wrong. We have corrected these based on the following arguments. Character 1 is re-scored ‘1’ for chondrichthyans, acanthodians and osteichthyans because all have endoskeletal mineralization (Janvier 1996; Coates et al. 1998). Character 3 is re-scored as ‘0/1’ for ‘ostracoderms’, some of which possess paired fins (Janvier 1996). For character 7, we take ‘paired nasal sacs’ to refer to broadly separated nasal capsules. This character must be re-scored as 1 for placoderms because paired nasal capsules are present in this group (Janvier 1996). Character 9 is re-scored as 1 for all taxa