Friedman et al.  redescribed a fossil species of sharksucker or remora, †Opisthomyzon glaronensis, first studied in some detail by Wettstein . They (, p. 6) stated that (emphasis is ours) ‘The conclusion that †Opisthomyzon is a stem-group remora, combined with novel anatomical information arising from the discovery and preparation of additional specimens, provides first clues about the sequence of evolutionary changes that led to the assembly of the adhesion disc’.
Friedman et al. , however, failed to cite and apparently did not consult Houy's 1910 monograph , where extensive comments were made on the differences between the fossil and recent remora species and on the evolutionary changes that one must assume to have happened. Houy's publication  was considered the ‘most comprehensive anatomical study of the adult sucking disc of remoras’ in Britz & Johnson's  recent ontogenetic study, a paper that was cited 10 times by Friedman et al. .
Houy  studied the adult anatomy of the sucking disc in great detail and included not only its skeleton, but also the muscles, nerves and sense organs associated with it. In the context of his investigation, he commented on the significance of Wettstein's fossil remora. A full citation of his remarks is provided here (translation and emphases are ours):
Of Echeneis only a single fossil specimen is known from the Glarner Fischschiefer (fish slate). Wettstein (1887) described this specimen as E. glaronensis. Cope (1890) subsequently erected the genus name Opisthomyzon for it. Regarding the differences between the fossil and recent representatives I quote from Wettstein (1887) the following most important facts: ‘The main difference between the recent representatives and the fossil form is the size of the disc and the number of lamellae. Of the latter there are only six in the fossil species, while recent representatives have 12–27. The entire length of the disc is contained 13 times in the total length of the fish, but only 4.75–2.33 times in recent species. The entire disc was positioned behind the skull roof, while it starts in the recent species behind the nostrils.’ These statements by Wettstein and his illustration of E. glaronensis would suggest that a shift of the sucking disc happened in a postero-anterior direction and that the change from a dorsal fin into the disc occurred before the disc moved onto the skull. From the low number of the lamellae, which are comparatively wide longitudinally, one would conclude that there has been a continuous increase in the number of plates, which would have become comparatively narrower. One may have to be a little cautious with these conclusions, because it can hardly be missed in the illustration in Wettstein, which is based on a photo, that 1. The disc is not complete and a small piece is missing anteriorly, 2. that the skull is broken, which may have caused a shift of the disc from its original position. Accepting that a part of the disc is missing and that it has shifted from its original position one thing is certain: it could never have extended onto the anterior part of the skull in the fossil. The disc may have been a little longer but not so much so that it would have covered the entire head. In addition the roughness of the frontals, which are perfectly smooth in recent members, as are all their skull bones covered by the disc, suggests that the disc did not reach the anterior end of the head. It may be possible that the disc of the fossil species extended beyond the posterior limit of the skull, but it nevertheless must have been restricted to the posterior part of the head.
[1, pp. 106–107]
Contrasting Houy's  statement with Friedman et al.'s  conclusions makes it clear that it was actually Houy's 1910  analysis of the condition of the disc in †Opisthomyzon that ‘provides the first clues about the sequence of evolutionary changes that led to the assembly of the adhesion disc’, and not Friedman et al's  much more recent study (p. 6), which for the most part just repeated Houy's  conclusions: ‘It is clear that many key transformations, most notably modification of fin spines into laterally expanded lamellae, took place while the disc occupied a postcranial position. At this stage, lamellae were still joined along the midline, comparable with the condition of dorsal fin spines in generalized percomorphs. The second stage was characterized by anterior migration of the disc, the separation of lamellae into paired ossifications, the development of pectination along the posterior margins of the lamellae and an increase in the number of segments in the disc’. The slight difference between Houy's  view that the disc possibly extended onto the posterior skull in †Opisthomyzon and Friedman et al.'s  conclusion that it was in a postcranial position in this fossil may be explained by Friedman et al.'s  misidentification of the large frontals (identified correctly in [2,3], see also figure 1a) as parietals in their fig. 1b. Friedman et al.'s  conclusion that the posterior position of the disc in †Opisthomyzon is a ‘primitive character not found in living remoras’ is actually not supported by the parsimony optimizations of their anatomical characters onto the trees based on the total evidence datasets and illustrated in their figs S5 and S6. Fig. S5 does not show character 88, which is the one that relates to the position of the first dorsal fin pterygiophore, and fig. S6 shows its position as ‘anterior to the preneural space lying over the cranium’  at the level of Echeneoidei and not at the level of Recent echeneids. This would mean that Friedman et al.'s  hypothesized posterior position of the disc behind the head in †Opisthomyzon is secondary within echeneoids and the anterior position on the head in recent remoras is primitive. We are unable to explain the discrepancy between Friedman et al.'s  mapped character 88 and their conclusions.
While Friedman et al.'s study  provides the first total evidence analysis of the relationships of †Opisthomyzon among echeneoids, the only evolutionary transformations identified by them  and not covered previously by Houy  then are the ‘separation of lamellae into paired ossifications’ during development in recent remoras and ‘the development of pectination’ on these lamellae. The latter point is, however, still debateable. †Opisthomyzon (figure 1b) shows no signs of the series of spinules (Friedman et al.'s ‘pectination’) that characterizes the lamellae of recent remoras and is partly responsible for their astounding adhesion capabilities . As first pointed out more than 100 years ago [3,6] and reiterated recently , the spinules in adult remoras are autogenous from the supporting lamella. Individual spinules easily fall off during maceration  or can be removed, with little force, with forceps in cleared and stained specimens and leave no trace on the supporting lamella (figure 1c). Although speculative, this separation of lamellae and spinules could potentially have led to a loss of the spinules during the process of fossilization, and hence their absence in †Opisthomyzon may be an artefact of preservation, a hypothesis not touched upon by Friedman et al. .
One of the cornerstones of scientific progress is that we are able to rely on what researchers of previous generations have discovered and made available to the scientific community through publications. As current researchers, we can stand on the shoulders of past giants; this is a huge advantage. This advantage, however, comes with a duty that requires us to scrutinize the historical literature concerning the subject we study. We thus give due credit to those who came before and avoid duplicating their efforts.
We thank Ursula Menkveld-Gfeller (NMBE) for providing access to the type of †Opisthomyzon and Hannes Baur (NMBE) and Eike Neubert (NMBE) for their help with figure 1a,b. We are grateful to Eleanor Adamson and Oliver Crimmen (both NHM), and two anonymous reviewers for their comments on the manuscript.
The accompanying reply can be viewed at http://dx.doi.org/10.1098/rspb.2014.0115.
- Received November 8, 2013.
- Accepted December 15, 2013.
- © 2014 The Author(s) Published by the Royal Society. All rights reserved.