Re-discovery of a Supposedly Lost Muscle in Palaeognathous Birds and Its Phylogenetic Implications

In four palaeognathous species dissected and in two species with section series, a true homologue of a M. geniohyoideus of other tetrapods was found. This muscle is innervated by the cranial nerve XII and extends from its origin on the mandibular symphysis caudally to its insertion on the ceratobranchiale. The muscle is sandwiched between the floor of the mouth and the M. intermandibularis. In none of the twelve neognathous species dissected, nor in the many descriptions in the literature was a muscle of this topographic configuration found. The presence of a M. geniohyoideus is part of the groundplan of birds in general, and is retained in palaeognaths as a plesiomorph feature. It is therefore not indicative of a monophyly of this group. In neognathous birds, the M. geniohyoideus was lost once or possibly twice. If it were lost once, namely in the stemgroup of the neognaths, this would be an autapomorphy of the group, corroborating its monophyly. However, it is also conceivable that the M. geniohyoideus was lost twice. Before this question can be resolved, more data, especially on the comparative functional morphology have to be gathered.