Supplementary Info. Brazeau and Ahlberg MS-2005-05-06098A
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Panderichthys sp. LDM 60/123
The specimen consists of a single individual from the tip of the snout to
approximately the mid-trunk in ventral view. The ventral half of the specimen is missing,
exposing the braincase (Ahlberg et al. 1996), dorsal hyoid arch, palatal surface in ventral
view and the internal surfaces of the dorsal operco-gular series and squamation. The
posterior part of the (anatomical) left lower jaw ramus is preserved in articulation with
the quadrate. The right quadrate is missing.
The specimen is well-preserved, but is substantially fractured as the bone and
surrounding matrix is quite brittle. It is preserved in a light-coloured clay matrix,
resulting in a low visual contrast with the bone that makes it difficult to distinguish the
two in photographs. However, all structures described are well represented in the actual
specimen and detailed in the specimen drawings.
The specimen has been slightly flattened dorsoventrally. This has compacted the
suspensorium and forced the quadrates laterally. However, the orientation, position, and
connectivity of elements does not appear to be greatly affected. All elements appear to
have approximate positional correspondence with their homologues in Eusthenopteron
(Jarvik 1980).
One significant difference from the description by Ahlberg et al. (1996) is the
interpretation of the hyomandibula. The previous work was conducted prior to more
complete preparation in the region of the suspensorium, which revealed that the
“hyomandibula” of Ahlberg et al. was in fact a pair of epibranchials (1 and 2) that partly
concealed the real hyomandibula. Ahlberg et al. (1996) mistakenly concluded the
hyomandibula to be generally like that of Eusthenopteron, in spite of some significant
Supplementary Info. Brazeau and Ahlberg MS-2005-05-06098A
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differences pointed out by Vorobyeva and Schultze (1991). Our interpretation, however,
differs from both these accounts in demonstrating a modified relationship to the opercular
bone and the epipterygoid region of the palatoquadrate.
On the anatomical left side of the specimen, the distal tip of the hyomandibula
rests at the position of the opercular facet. The hyomandibula measures approximately
40 mm long (however the ends appear to have been unossified). This corresponds to the
approximate distance between the opercular facet and the lateral commissure. In the
reconstructed skull (main text Fig. 3) The measurements do not greatly alter given that
the reorientation of the opercular is commensurate with the restoration of the quadrates to
a more ventromedial position. We conclude that the hyomandibular ossification
terminates at the position of the opercular facet, representing the location of the
ligamentous attachment between the two.
References
Ahlberg, P. E., J. A. Clack, et al. (1996). "Rapid braincase evolution between
Panderichthys and the earliest tetrapods." Nature 381: 61-64.
Jarvik, E. (1980). Basic Structure and Evolution of Vertebrates. London, Academic Press.
Vorobyeva, E. and H.-P. Schultze (1991). Description of panderichthyid fishes with
comments on their relationship to tetrapods. Origins of the Higher Groups of
Tetrapods: Controversy and Consensus. H.-P. Schultze and L. Trueb. Ithaca, NY,
Cornell University Press (Canstock): 68-109.