External gills

Abstract: Yunnanozoans are a distinctive clade of Lower Cambrian metazoans. Although widely accepted as deuterostomes, their exact placement within this superphylum is controversial. Here we describe a new species of Haikouella (H. jianshanensis) from the Chengjiang Lagerstatte (Yunnan, China) with exceptional preservation of a number of features. These include external gills, which suggest that the origin of the pharyngeal clefts was independent of the gills. The diagnostic branchial arches of chordates may, therefore, be composite structures. No evidence was found for the chordate-like structures that have been described in other yunnanozoans. We propose that yunnanozoans are stem-group deuterostomes, allied to the vetulicolians.

Abstract: The embryonic and larval development of the Polypteriformes, the presumed sister group of all other living actinopterygians, is poorly known The main reason is the scarcity of successful breedings in captivity and therefore the lack of developmental series of any one polypterid species A series of five successful breedings of P senegalusnow makes it possible to define developmental stages of this species based on numerous closely timed specimens The staging given here focuses on external embryonic and larval features: epidermal surface structures documented by SEM, colour pattern, development of fins and squamation, larval feeding and locomotory behaviour The development of P senegalusis characterized by a long free embryonic phase Suction feeding is performed from the beginning of larval life (apterolarval phase) The pectoral fins start to become employed for slow locomotion and as supportive structures at around the same time Olfactorily guided prey capture, however, is observed later in the pterolarval phase Quantitative kinematic data also demonstrate a change in the mode of undulatory locomotion during this phase Sustained axial undulation becomes confined to the posterior abdominal and caudal region of the body At about the same time the paraxial high frequency undulation of the pectoral fin fold is replaced by the characteristic propeller-like movement of much greater amplitude and wavelength Surfacing for aerial breathing is not seen before a marked change in colouration has taken place at the beginning of the juvenile period The external gills slowly become reduced during this period The definitions of larval and juvenile stages given here may advance understanding of developmental processes in the ontogeny of these primitive actinopterygians, and may serve as a tool for comparison with the ontogeny of Tetrapoda and Dipnoi, as well as to that of some “primitive” groups of Actinopterygii Judging from its distribution among extant taxa, embryonic and larval ciliation is a character that most probably belongs to the grundplan‡ of Osteognathostomata Phylogenetic evaluation is not so clear for the two other prominent embryonic and larval specializations found in Polypterus: upper labial attachment glands and opercular external gills © 1997 The Royal Swedish Academy of Sciences Published by Elsevier Science Ltd

Abstract: Schoch, R.R. and Witzmann, F. 2011. Bystrow’s Paradox – gills, fossils, and the fish-to-tetrapod transition. —Acta Zoologica (Stockholm) 92: 251–265. The issue of which breathing mechanism was used by the earliest tetrapods is still unsolved. Recent discoveries of stem tetrapods suggest the presence of internal gills and fish-like underwater breathing. The same osteological features were used by Bystrow to infer a salamander-like breathing through external gills in temnospondyl amphibians. This apparent contradiction – here called Bystrow’s Paradox – is resolved by reviewing the primary fossil evidence and the anatomy of the two gill types in extant taxa. Rather unexpectedly, we find that internal gills were present in a range of early crown tetrapods (temnospondyls), based on the anatomy of gill lamellae and location of branchial arteries on the ventral side of gill arch elements (ceratobranchials). Although it remains to be clarified which components are homologous in external and internal gills, both gill types are likely to have been present in Palaeozoic tetrapods – internal gills in aquatic adults of some taxa, and external gills in the larvae of these taxa and in larvae of numerous forms with terrestrial adults, which resorbed the external gills after the larval phase. Future developmental studies will hopefully clarify which mechanistic pathways are involved in gill formation and how these might have evolved.