Kenichthys is a genus of Sarcopterygii fish from the Devonian period, and a member of the clade Tetrapodomorpha. The only known species of the genus is Kenichthys campbelli (named for the palaeontologist Ken Campbell), the first remains of which were found in China in 1993.[Chang, M. and Zhu, M. (1993) A new Middle Devonian osteolepidid from Qujing, Yunnan. Mem. Assoc. Australas. Palaeontol. 15 183-198] The genus is important to the study of the evolution of due to the unique nature of its , which provide vital evidence regarding the evolutionary transition of fish-like nostrils to the tetrapod choanae.[Zhu, M. and Ahlberg, P. (2004) The origin of the internal nostril of tetrapods. Nature 432 94-97 ]
Description
Kenichthys was a small tetrapodomorph, with a
skull about long.
While only areas of the front of the body are known, it seems likely that
Kenichthys would have been similar in general body form to other basal sarcopterygians, with two
Fish anatomy, paired
Fish anatomy and
Fish anatomy and an
Fish anatomy.
An important way in which Kenichthys differed from other tetropodomorphs however is in the positioning of its posterior nostril. Whilst in other tetrapodomorphs this nostril is positioned in the roof of the mouth (the palate), in Kenichthys it is found at the jaw margin, between the premaxilla and maxilla.
Fossils
The original
fossil material of
Kenichthys was first described in 1993, and consists of various parts of the
skull roof,
Mandible and
pectoral girdle. They were found in Southwestern China, in the Chuandong Formation in
Yunnan, and are now housed in the Institute of Vertebrate Paleontology and Paleoanthropology,
Beijing.
The fossils date from the early Devonian, specifically from the
Emsian period, about 395 million years ago.
Further material from the skull was later found, and described in 2004. It was this material that established the presence of a transitional external nostril in the species' skull.
Evolutionary significance
Kenichthys is important to the study of the evolution of tetrapods due to the nature of its nostrils. Most non-tetrapod vertebrates (e.g.
Actinopterygii) possess two sets of nostrils, one set at either end of the
nasal cavity, and both sets of which are external. These nostrils play no part in respiration, instead serving an
Olfaction role. However, in all
crown group tetrapods and many
stem group tetrapods one set of nostrils is found exteriorly, and another interiorly, in the roof of the palate. This arrangement means that the nasal passage leads from the outside of the body into the
mouth. The 'inner set' of nostrils are known as
choanae, and allow tetrapods to breathe through their
nose.
Prior to the description of Kenichthys' nasal passages, exactly how the transition between these two forms had taken place was a source of debate. Various suggestions had been put forward including that the choana was homologous to either the posterior or anterior nostril of non-tetrapods, that it had 'budded off' from one of these passages, or that it was an entirely novel form, unrelated to either of the other nostrils. The debate was further complicated by the fact that , another group of sarcopterygian fish, also possess a choana with a different form to that of tetrapodomorphs.[Panchen, A. L. The nostrils of choanate fishes and early tetrapods. (1967) Biol. Rev. 42 374–420 ]
The character state of the nostrils of Kenichthys demonstrates that the vertebrate choana did in fact evolve by migration of the posterior external nostril around the jaw and up onto the roof of the mouth. Kenichthys suggests that this migration took place on the route between the premaxilla and the maxilla.
This evolutionary transition appears to have left its mark on tetrapod Ontogeny. A cleft palate may form in (and other tetrapods) where the tissues that will become the premaxilla and the maxilla fail to join during development. This condition is similar to the situation found in Kenichthys.
Systematics
Kenichthys has consistently been seen as one of the most basal tetrapodomorphs since its discovery.
Below is a recent
cladogram modified from Swartz, 2012:
See also
