On fossils, phylogenies and sequences of evolutionary change.

We thank Britz et al. [1] for introducing Houy's [2] prescient interpretation of the significance of †Opisthomyzon for the evolution of modern remora anatomy , and regret not having discovered his elegant account ourselves [3]. Many prominent ichthyologists have considered the relevance of †Opisthomyzon since its discovery more than a century ago [4– 7], but none of their observations approaches Houy's clear explication of the sequence of evolutionary transformations associated with the origin of the adhesion disc or so closely foreshadow our own conclusions. We therefore welcome the translation provided by our co-workers and the exchange arising from their comment [1], which together give Houy the long-delayed recognition he richly deserves. However, we disagree with their claims concerning other aspects of our study. A central theme of our co-workers' comment [1] is that the evolutionary sequence we proposed [3] is rendered largely redundant when confronted with Houy's [2] proposal. While we agree that our study provides corroboration for his hypothesized trajectory, it is unclear how robust phylogenetic placement of †Opisthomyzon and discovery of additional primitive features of its disc represent a duplication of past efforts. Although Houy clearly implies †Opisthomyzon is a stem remora [2], competing alignments with Phtheirichthys [6,8] and Echeneis [9] that postdate his account highlight past uncertainty surrounding the affinities of this fossil. At best, these alternative placements within the remora crown render its implications for the evolution of the disc ambiguous. Recognition of past efforts is a cornerstone of science, but so too is the critical testing of old ideas using new data and techniques. Our study supports and adds to Houy's work, because we further documented the anatomy of †Opisthomyzon and constrained its systematic position. Without a clear phylogenetic framework, †Opisthomyzon remains mute with respect to the evolution of the remora adhesion disc. There is no doubt that the paired bones constituting the majority of the visible skull in the holotype of †Opisthomyzon are frontals. The structures labelled as par-ietals in this fossil [3] are depressed features at the rear of the frontals (rather than the middle of these ossifications, indicated by arrows in reference [1]) that appear to bear horn-like projections possibly representing crests. We are willing to consider an alternative interpretation as posterior extensions of the frontals, but what is agreed between previous authors [2], our co-workers [1] and us [3] is that the ornamented frontals preclude anterior extension of the disc over …