Series: Bulletin of the British Museum (Natural History) Zoology Volume: 36/3
54 pages, 42 b/w line drawings
Macrochirichthys macrochirus is a highly specialized piscivore derived from an ancestral lineage now represented by the genera Salmostoma and Securicula. These taxa are members of a monophyletic assemblage (the cheline group) in which three lineages can be recognized, viz. the cheline, represented by Chela; the oxygastrine, by Macrochirichthys, Parachela and Oxygaster, and the salmostomine by Salmostoma and Securicula. Parallelism between Macrochirichthys and Securicula has in the past led to the erroneous assumption of their close relationship (Bănărescu, 1967).
The Cultrinae (Bănărescu, 1967) have been shown to be a non-monophyletic assemblage although two groups of genera – the cultrine and hemicultrine – may be monophyletic. One of these is possibly the sister-group of the chelines.
Macrochirichthys is capable of a great degree of cranial elevation, made possible by the ball and socket connection of the basioccipital with the 1st vertebra and modifications of the epaxial musculature. Cranial elevation is also possible – perhaps to an even greater degree – in Chela. Salmostoma represents the plesiomorph lineage in which there are seen pre-adaptations to this function, namely, a short post-paneiai area with a posteriorly rounded cranium and a reduced supraoccipital process; short ethmoid region overlapped by the frontals; rounding and shortening of the 1st vertebra with reduction of its lateral processes; a caudal inclination of the neural complex and modification of the anterior supraneurals. Such adaptations have resulted, on the one hand, in the evolution of forms such as Macrochirichthys and Securicula where the cranium is compressed and the dorsal musculature has extended across the skull roof; and on the other hand, in Chela, where the cranium has remained broad and there has been little modification to the dorsal muscle elements. Likewise, the protrusibility of the upper jaw is virtually lost in Macrochirichthys and Securicula but retained in Chela.
Further comparison between Macrochirichthys and the characoid Rhaphiodon has highlighted the major anatomical differences involved in achieving similar evolutionary status. It may be added here that a neck-bending mechanism similar to that of Rhaphiodon appears to be present in the siluroid Belodontichthys (pers. obs.).
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