Abstract
A fossil of a colonial organism with pyritized soft tissues of elongated fusiform zooids, found in the middle part of the early Floian Fenxiang Formation in Hubei Province of China, probably represents the oldest reliable record of a hydroid cnidarian. The preservation of the fossil is sufficiently different from that of associated carbonized skeletons of graptolites to exclude affinities with this group. The fossil is unlikely to be a bryozoan because of the mode of budding from proximal, not distal, parts of parent zooids, which is typical rather of hydroids. Although no thecae are preserved, the fossil, named Sino- bryon elongatum gen. et sp. nov., is suggested to be a thecate hydroid, possibly related to the Haleciidae. The apparent presence of an advanced member of the thec- aphoran Macrocolonia clade in strata 470 Ma old means that much of the hydroid (and cnidarian) diversification preceded the Middle Ordovician.
Highlights
It is popular wisdom that hydras and medusae are among the most primitive of living animals
It was once customary to classify any fossil of a colonial organism with organic skeleton among the hydrozoans (e.g., Kozłowski 1959)
In this paper we critically evaluate a new finding of a possible ancient hydrozoan in the Ordovician of the Three Gorge area of southern China
Summary
Kurzfassung Ein fossilisierter kolonialer Organismus mit pyritisierten Weichteilen aus dem mittleren Teil der Fenxiang Formation (fruhes Floium, Hubei Formation, China) stellt wahrscheinlich den altesten Vertreter der hydroiden Cnidaria dar. Die Erhaltung dieses mutmaßlichen Vertreters der Hydrozoa ist hinreichend verschieden von der kohligen Erhaltung dort ebenfalls vorkommender Graptolithen, um eine Zugehorigkeit zu dieser Gruppe auszuschließen. Das Fossil zeigt einen typisch hydroiden Knospungsmodus, der vom proximalen und nicht vom distalen Teil des Zooids erfolgt. Obwohl Thecae nicht erhalten sind, wird vorgeschlagen, das Fossil mit dem Namen S. elongatum gen. Es ist moglicherweise mit den Haleciidae verwandt. Die offenkundige Prasenz eines abgeleiteten Vertreters der Klade Macrocolonia (Thecata) vor 470 Mio a legt nahe, dass die wesentlichen stammesgeschichtlichen Aufspaltungen der Hydrozoa (und Cnidaria) bereits vor dem mittleren Ordovizium stattgefunden haben. Dzik Instytut Zoologii Uniwersytetu Warszawskiego, Banacha 2, 02-097 Warszawa, Poland
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