Fossil early vertebrates shed lights on the origin of the gnathostome face
2013 (English)In: Program and Abstracts of the 10th International Congress of Vertebrate Morphology, Barcelona, Spain, 2013, 245-245 p.Conference paper, Abstract (Refereed)
Jawless cyclostomes and jawed gnathostomes show very different face patterns. Cyclostomes have a single median nasohypophysial duct, an anterior hypophysis and a short telencephalon, while gnathostomes have a pair of nasal sacs opening externally, a more posterior separate hypophysis open in the palate and a longer telencephalon.
Embryonic processes differ as well. In cyclostomes, premandibular crest cells migrate forwards either side of the nasohypophysial placode to form the upper lip; in gnathostomes they migrate between the hypophysial and nasal placodes to form the trabecular region. Supraoptic neural crest remains posterior to the nasohypophysial duct in cyclostomes; it moves forward to create the nasal capsules in gnathostomes.
Some fossil forms illustrate a transition between these two patterns.
The jawless galeaspid Shuyu (-430 Ma) has a nasohypophysial duct, short telencephalon, and anteriorly oriented hypophysis, but the paired nasal sacs and hypophysis are separated by a rudimentary trabecula.
The jawed primitive placoderm Romundina (-415 Ma) shows a cranial cavity reminiscent of that of Shuyu (anteriorly directed hypophysis, very short telencephalon). The trabecular region is long and wide, the nasal capsule is small and located far behind the tip of the snout but just in front of the orbits. We interpret these features as uniquely primitive among gnathostomes. The premandibular crest of Romundina formed a trabecular region extending as anteriorly as the tip of the snout (like in extant cyclostome and the fossil Shuyu). The position of the nasal capsule suggests that the supraoptic crest had not migrated forwards.
We suggest that the evolutionary sequence for the creation of the extant gnathostome face from a cyclostome pattern involved 1) separation of the nasal and hypophysial placodes (galeaspids), 2) loss of the nasohypophysial duct (placoderms), and 3) lengthening of the telencephalon and the migration of the nasal capsules to the snout tip.
Place, publisher, year, edition, pages
Barcelona, Spain, 2013. 245-245 p.
IdentifiersURN: urn:nbn:se:uu:diva-213343OAI: oai:DiVA.org:uu-213343DiVA: diva2:681742
10th International Congress on Vertebrate Morphology, Barcelona, July 8–12, 2013
FunderEU, European Research Council, 233111
Anatomical Record, Volume 296, Special Feature 2013-12-202013-12-202015-07-28Bibliographically approved