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quinta-feira, 1 de outubro de 2015

[PaleoIchthyology • 2015] 

A Streamlined Jawless Fish (Galeapida) from the Lower Devonian of Yunnan, China and Its Taxonomic and Paleoecological Implications

Rhegmaspis xiphoidea  Gai, Zhu, Jia & Zhao, 2015 
BRhegmaspis xiphoidea with streamlined body, which is more suggestive of a superbenthic habitat
Fig. 2 Photograph of a complete head-shield of Rhegmaspis xiphoidea gen. et sp. nov. (IVPP V 19354.1A, holotype)
Fig. 5 Restoration of Rhegmaspis xiphoidea gen. et sp. nov.
Abstract   
A new genus and species of the Huananaspiformes (Galeaspida), Rhegmaspis xiphoidea, is described from the Posongchong Formation (Pragian, Early Devonian), Zhaotong, Yunnan Province, China. The new taxon is characterized by having a torpedo-shaped head-shield, a long rostral process, ventrolaterally set orbits, and ventrally curved branchial fossae, but no corners, inner corners, and ventral rim of head-shield. It is assigned to the family Gantarostrataspidae that include Gantarostrataspis and Wumengshanaspis. According to the new material and new observation, the Gantarostrataspidae is emended and a fresh look is prososed for Gantarostrataspis and Wumengshanaspis. As a streamlined jawless fish, Rhegmaspis displays an adaptation for a suprabenthic lifestyle with more active feeding behavior among galeaspids. The new form not only enriches the diversity of the Huananaspiformes, but also provides evidence for the last adaptive radiation of galeaspids by occupying an unexploited ecological niche during the Pragian of the Early Devonian.

Key words   Zhaotong, Yunnan; Lower Devonian; Galeaspida, Huananaspiformes; taxonomy; paleoecology

Rhegmaspis xiphoidea  Gai, Zhu, Jia & Zhao, 2015
B. Rhegmaspis xiphoidea with streamlined body, which is more suggestive of a superbenthic habitat;
B1. a streamlined body can minimize water pressure drag largely when swimming;
B2. a cross section of the head-shield, showing the ventrally curved branchial fossae and absence of ventral rim.
Abbreviations: bra.c. brain cavity; br.f. branchial fossae; obr.c. oralobrachial chamber; vr. ventral rim.
Fig. 2 Photograph of a complete head-shield of Rhegmaspis xiphoidea gen. et sp. nov. (IVPP V 19354.1A, holotype) 
Fig. 5 Restoration of Rhegmaspis xiphoidea gen. et sp. nov.
A. dorsal view; B. ventral view; C. lateral view. 

Abbreviations: br.f. branchial fossa; md.o. median dorsal opening; obr.f. oralobrachial fenestra; orb. orbital opening; pi. pineal fossa; ro. rostral process; soc2. posterior supraorbital canal; m. mouth.
Systematic paleontology
Subclass: Galeaspida Tarlo, 1967
Supraorder: Polybranchiaspidida Janvier, 1996
Order: Huananaspiformes Janvier, 1975
Family: Gantarostrataspidae Wang & Wang, 1992
Genus: Rhegmaspis gen. nov.
Type species: Rhegmaspis xiphoidea sp. nov.
Etymology: Rhegma, Greek, rupture, breach; aspis Greek, shield.

Rhegmaspis xiphoidea sp. nov.
(Figs. 2−6, 9E, 10B, cover image)
Etymology: Xiphoideus (swordlike).
Holotype: A complete head-shield (IVPP V 19354.1A, B).
Paratype: A nearly complete head-shield (IVPP V 19354.2), an incomplete head-shield (V 19354.4), an incomplete endocast of skull (V 19354.3).
Horizon and type locality: Early Devonian, Pragian, Posongchong Formation,
Zhaotong, Yunnan, China.
Fig. 10 The lifestyle of galeaspids in benthic (A) and superbenthic (B) habitats
A. ‘Dongfangaspisqujingensis with a flattened head-shield, which is suggestive of a benthic habitat; A1. a flattened head-shield bears much water pressure drag when swimming, but permit remaining on the bottom without a significant energetic cost; A2. a cross section of the head-shield, showing the transversely elongated branchial fossae and ventral rim;
B. Rhegmaspis xiphoidea with streamlined body, which is more suggestive of a superbenthic habitat; B1. a streamlined body can minimize water pressure drag largely when swimming; B2. a cross section of the head-shield, showing the ventrally curved branchial fossae and absence of ventral rim.
Abbreviations: bra.c. brain cavity; br.f. branchial fossae; obr.c. oralobrachial chamber; vr. ventral rim.
GAI Zhi-Kun, ZHU Min, JIA Lian-Tao and ZHAO Wen-Jin. 2015. A Streamlined Jawless Fish (Galeapida) from the Lower Devonian of Yunnan, China and Its Taxonomic and Paleoecological Implications. VERTEBRATA PALASIATICA. 53(2); 93-109. 
New jawless fish found from the Lower Devonian of Yunnan, China
http://phy.so/360919467 via @physorg_com

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