Technical Comments

Comment on "The Early Evolution of the Tetrapod Humerus"

Science  17 Sep 2004:
Vol. 305, Issue 5691, pp. 1715
DOI: 10.1126/science.1099606

In their analysis of humeral evolution in early tetrapods (taken here as meaning vertebrates with limbs), Shubin et al. (1) described a suite of derived characters that represents a significant advance on previous analyses (24). This makes their negative assessment of GSM 104536, the humerus provisionally attributed to Elginerpeton pancheni (2, 4, 6), all the more surprising. The specimen (see Fig. 1, A to F) derives from the late Frasnian (mid-Late Devonian) locality of Scat Craig in Scotland, and is thus about 10 million years older than the Catskill Formation humerus ANSP 21350 that forms the centerpiece of the Shubin et al. study (1, 7). It is associated with tetrapod lower jaws, snout fragments, shoulder girdle fragments, ilia, a femur, a tibia, and a probable tetrapod neural arch (2, 4, 6, 8). The Scat Craig specimens form the earliest known group of postcranial tetrapod elements, and GSM 104536—if correctly interpreted—is the earliest known tetrapod humerus.

Fig. 1.

GSM 104536, tetrapod humerus from the Late Devonian of Scotland. (A) Dorsal, (B) ventral, (C) distal, (D) proximal, (E) cranial, and (F) caudal views. Drawing from (4), highlighted with color. The same drawing was figured in (2), but at that time the radial facet (still partly obscured by matrix) was interpreted as a broken surface. GSM denotes British Geological Survey, Keyworth, UK. Thick outline indicates true edge, thin outline indicates broken edge, and vertical hatching indicates broken surface. The crest of the ectepicondyle and the distal margin of the bone have been eroded away, but most other surfaces and edges are perfectly preserved. Character numbers as in text: scapulohumeral muscle insertion (blue), muscle insertion above radial facet (pink), recess and crest at junction of ectepicondyle and entepicondyle (purple), articular head (green), radial facet (yellow). Note the flatness of the humeral shaft and entepicondyle in (C) and (D). (G) Comparison between GSM 104536 (left) and an immature Ichthyostega humerus collected in Greenland in 1998 (right, image courtesy of J. A. Clack), both in ventral view. Radial facet (yellow); incomplete margin (eroded in GSM 104536, covered by matrix in Ichthyostega) (red). Dashed outline in Ichthyostega indicates articular head reconstructed, from an associated specimen. Note that the radial facet is partly covered by the radius in Ichthyostega. Not to precise scale, but the specimens are of comparable size.

After presenting their list of derived humeral characters, Shubin et al. (1) state, in the caption of figure 2, that “[t]he putative humerus of Elginerpeton... becomes difficult to interpret in light of these observations, because it has none of the synapomorphies shown in Fig. 2. By lacking the shared characters present in the most basal node, the identification of Elginerpeton as a tetrapod humerus would require both the evolution of unique features and the loss of all the apomorphies shared by tetrapods and their successive two outgroups.” They reference the provisional description of GSM 104536 from 1991 (2), but the full description from 1998 (4) is not cited. The Shubin et al. statement that GSM 104536 has no tetrapod characteristics whatsoever begs the question of why I ever identified it as a tetrapod, and their claim is a contradiction of the published descriptions of the specimen (2, 4).

The derived characters described by Shubin et al. for Panderichthys + tetrapods include a dorsoventrally flattened shaft; an enlarged and posteriorly inclined ectepicondylar ridge; a broad, shallow proximal depression for scapulohumeral muscle insertion; and an enlarged area for muscle insertion above the radial condyle. The characters described for tetrapods alone include a distally expanded ectepicondylar process, a distinct recess and incipient crest at the proximal union of ectepicondylar and entepicondylar processes, an enlarged and proximodistally expanded entepicondylar process, and a ventral crest confluent with the posteromedial rim of the entepicondylar process. Shubin et al. claim that all of these features are absent in GSM 104536.

My 1991 description (see Table 1) explicitly compared GSM 104536 with the humeri of Eusthenopteron and early tetrapods and identified equivalents of a number of these synapomorphies. The “thin, flat entepicondyle, continuous with flat body of humerus” and “sharp preaxial margin” in that description are collectively equivalent to the Shubin et al. “dorsoventrally flattened shaft” plus “enlarged and proximodistaly expanded entepicondylar process” [see figure 2 in (1)]. My “narrow, tall ectepicondyle” overlaps with their “enlarged and posteriorly inclined ectepicondylar ridge” and “distally expanded ectepicondylar process.” The narrow, linear, sharp-edged quality of the tetrapod ectepicondyle is not specifically mentioned by Shubin et al., but is shared by all Devonian representatives of the group and is clearly visible in figure 2 in (1).

Table 1.

Comparison of humeral character states [a reproduction of table 1 in (2), the preliminary description of GSM 104536].

Humeral character state Eusthenopteron GSM 104536 Tetrapods
Entepicondyle Thick, curved, rounded end; distinct from cylindrical body of humerus Thin, flat, round end; continuous with flat body of humerus Thin, flat, rectangular; continuous with flat body of humerus
Transverse humeral ridge Present, strong Absent Present
Preaxial margin Rounded Sharp Sharp
Pectoral process Weak Strong Strong
Proximal region Short Short Long
Ectepicondyle Broad, low Narrow, tall Narrow, tall
Ectepicondylar foramen Present Absent Present*
Supinator process Close to ectepicondyle Close to ectepicondyle On preaxial margin
Size of caput Large Small Large
  • The Scat Craig humerus GSM 104536 was compared with those of the osteolepiform Eusthenopteron and the early and primitive tetrapods Ichthyostega, Acanthostega, Crassigyrinus, Proterogyrinus, Eoherpeton, and Greererpeton. The tetrapod humeri agree with respect to all the characters compared in the table, except (*) that an ectepicondylar foramen is absent in Proterogyrinus, Eoherpeton, and Greererpeton (I regard this character state as derived within the Tetrapoda). The humerus of Eusthenopteron compares well with those of other osteolepiforms and rhizodonts, although there are minor differences in the shapes and positions of the processes; this humeral morphology is assumed to be primitive relative to that of tetrapods. `Body' refers to the central core or shaft of the humerus, running from the caput to the radial and ulnar facets. The “proximal region” is that part of the body of the humerus which lies proximal to the entepicondyle and pectoral process. Bold typeface indicates shared derived character state; italics indicate unique character state.

  • The description of GSM 104536 cited by Shubin et al. thus directly contradicts their assertion that this specimen lacks tetrapod characteristics. No explanation for this anomaly is given in their report (1, 2). Furthermore, the specimen drawings in the 1991 description show several of the new derived characters defined by Shubin et al. The depression for scapulohumeral muscle insertion, the enlarged area for muscle insertion above the radial condyle, and the recess and crest at the junction of ectepicondyle and entepicondyle can all be easily identified (Fig. 1, A to F). Again, these similarities are not discussed in (1).

    GSM 104536 is incomplete and has some unusual features for a tetrapod humerus. For example, it lacks a ventral crest and has a very short proximal shaft portion with a small head. However, the latter feature is shared with subadult individuals of Ichthyostega (9) and may be a juvenile feature (Fig. 1G). Given these points, it is unfortunate that Shubin et al. did not examine the specimen GSM 104536 itself before embarking on their analysis. It is also regrettable that they did not cite the full description (4), which shows that it has an elongate, anteroventrally positioned radial facet (Fig. 1)—a feature otherwise only known in Ichthyostega (9, 10). By denying the existence of the described and figured tetrapod characters of GSM 104536, rather than subjecting them to constructive criticism, Shubin et al. missed a valuable opportunity to investigate the affinities of what is still, probably, the earliest known tetrapod humerus. We now need to move on from assertion to explicit debate.

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