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What are the acanthomorphs?

With 311 families and more than 16,000 species, acanthomorphs constitute one of the major groups of extant vertebrates. The group is quite recent. The oldest known acanthomorph fossils were found in Cretaceous layers (100 million years ago). In the early Tertiary (50 million years ago), this clade is already diversified, with fossils known from almost all the recent subgroups. The diversification events might have taken place in the Tethys, an ancient ocean that disappeared during the formation of the Alpine chains (Alps, Caucasus, Himalayas…).

Acanthomorphs are so diverse that establishing morphologically based relationships and giving a simple definition of the group is very difficult. In 2010, Wiley and Johnson recognised 9 osteological features as defining the group, but these characters are complex and hard to visualise in a first approach or for a beginner.

Nevertheless, acanthomorphes can be defined relatively easily: they possess hollow and unsegmented spines in the anterior part of the dorsal and anal fins (red arrows on the following image).

Dicentrarchus labrax, Dicentrarchidae
D'après Chanet et al. (2009 [1]).

However, even if this feature can be really useful to identify most of these animals, it is not enough. The analysis of all available data shows that these spines regressed several times within Acanthomorpha… Several acanthomorph subgroups do not possess this type of spine. The following image shows both morphological diversity of acanthomorphes and some of the subgroups (indicated by a red circle) lacking hollow and not segmented spines in the anterior part of dorsal and/or anal fins.

Acanthomorphs' diversity

This heterogeneous distribution considerably obscured the understanding and reconstruction of the evolutionary history of the group. The Acanthomorpha has been often described as a bush at the top of teleost fishes. Its tangled twigs have discouraged many morphologists, leading to confusion and transmission of obsolete classifications by default. At the beginning of the XXIth century, new studies based on molecular data provided new hints. They are based on the comparisons of sequences for both mitochondrial and nuclear genes (Miya et al. 2001, 2003 ; Chen et al. 2003; Dettaï et Lecointre, 2004 [2] [3], 2005 [4], 2008 [5]; Smith et Wheeler, 2004, 2006 ; Smith et Craig, 2007 ; Mabuchi et al. 2007 ; Kawahara et al. 2008 ; Li et al. 2009 [6]). Works dedicated to the phylogeny of this group multiplied and blossomed in collaborations including both molecular and morphological data. Our current classification and understanding of the evolution of the group, although still incomplete, owes a lot tot these recent results.