Redescription of the Sanqiaspidae (Galeaspida) from the Lower Devonian of South China and its biostratigraphic significance

Gai, Zhikun; Jiang, W.-S.; Zhao, Wei; Li, Qiang; Shi, Xiaodong; Zhu, Min

doi:10.1007/s12549-021-00486-z

Cited by 3 publications

(1 citation statement)

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“…This indicates that a strong muscle probably for the velum was attached in this fossa. In addition, the whole oralobranchial chamber of galeaspids is closed posteriorly by a complete postbranchial wall (Wang, 1991;Janvier et al, 2009;Gai et al, 2022), which precludes tidal ventilation by the hypobranchial muscle. Therefore, a velum must have been present in galeaspids to provide ventilation from the oronasal cavity to the gill pouches as in osteostracans (Janvier, 1996b).…”

Section: Resultsmentioning

confidence: 99%

The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods

et al. 2022

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The spiracular region, comprising the hyomandibular pouch together with the mandibular and hyoid arches, has a complex evolutionary history. In living vertebrates, the embryonic hyomandibular pouch may disappear in the adult, develop into a small opening between the palatoquadrate and hyomandibula containing a single gill-like pseudobranch, or create a middle ear cavity, but it never develops into a fully formed gill with two hemibranchs. The belief that a complete spiracular gill must be the ancestral condition led some 20th century researchers to search for such a gill between the mandibular and hyoid arches in early jawed vertebrates. This hypothesized ancestral state was named the aphetohyoidean condition, but so far it has not been verified in any fossil; supposed examples, such as in the acanthodian Acanthodes and symmoriid chondrichthyans, have been reinterpreted and discounted. Here we present the first confirmed example of a complete spiracular gill in any vertebrate, in the galeaspid (jawless stem gnathostome) Shuyu. Comparisons with two other groups of jawless stem gnathostomes, osteostracans and heterostracans, indicate that they also probably possessed full-sized spiracular gills and that this condition may thus be primitive for the gnathostome stem group. This contrasts with the living jawless cyclostomes, in which the mandibular and hyoid arches are strongly modified and the hyomandibular pouch is lost in the adult. While no truly aphetohyoidean spiracular gill has been found in any jawed vertebrate, the recently reported presence in acanthodians of two pseudobranchs suggests a two-step evolutionary process whereby initial miniaturization of the spiracular gill was followed, independently in chondrichthyans and osteichthyans, by the loss of the anterior pseudobranch. On the basis of these findings we present an overview of spiracular evolution among vertebrates.

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Section: Resultsmentioning

confidence: 99%

The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods

et al. 2022

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New data on the cranial anatomy ofPterogonaspis(Tridensaspidae, Galeaspida) from the lower Devonian of Yunnan, China and its evolutionary implications

Meng

Zhu

et al. 2022

The Anatomical Record

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New material of the tridensaspid Pterogonaspis yuhaii is described from the Xujiachong Formation in Qujing City, Yunnan Province, revealing the morphology of the ventral side of the headshield for the first time. Most significantly, the cranial anatomy of Pterogonaspis provides the first fossil evidence for the position of the esophagus in galeaspids. The esophagus and dorsal aortae display a central placement in the postbranchial region and do not share a common canal as in osteostracans. Moreover, the paired and symmetrically extending dorsal aortae in galeaspids strikingly resemble those of jawed vertebrates, probably representing a plesiomorphic condition for vertebrates because they are also present in amphioxus and hagfishes. In contrast, the unpaired and asymmetrically extending dorsal aorta in osteostracans is similar to that of lampreys, probably resulting from convergent evolution. In addition, our new data also exhibit some characters that galeaspids uniquely share with osteostracans, such as the complete endoskeletal postbranchial wall and the extensive lateral expansion of the headshield with the marginal arteries and veins.

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