A review of the brachiopod subfamily Septosyringothyridinae (Spiriferinida) from the Carboniferous of Laurussia and Gondwana

MOTTEQUIN, B.; LEFÈVRE, U.; CISTERNA, G.A.

Milán

Congreso; 8th International Brachiopod Congress; 2018

Università degli studi di Milano

The Syringothyridoidea is one of the most important lineages of spire-bearer brachiopods that survived the global Hangenberg Crisis and flourished in many parts of the world during the Carboniferous-Permian interval (Carter & Gourvennec, 2006; Mottequin et al., 2014). Only three genera of this superfamily are distinguished by the presence of a syrinx, namely the Syringothyridinae Syringothyris Winchell, 1863 and the Septosyringothyridinae Septosyringothyris Vandercammen, 1955 and Histosyrinx Termier & Termier (in Massa et al., 1974), which are both characterized by a syrinx supported by a median septum.Septosyringothyris is defined as a syringothyridid with the syrinx suspended between the delthyrial plate and the floor of the valve by a median septum (Carter, 2006). The poorly preserved type material of its type species (S. demaneti Vandercammen, 1955) from the lower part of the Tournaisian Yvoir Formation (southern Belgium) has been herein studied using for the first timecomputed tomography (CT scan), allowing us to discriminate specimenswithout a median septum that were initially considered in the definition of the genus by Vandercammen (1955). Although these specimens are externally similar to S. demaneti, they are markedly different internally as their syrinx is attached to the ventral valve floor by a septal pillow, resulting from the thickening of the internal faces of the dental plates. This feature has been described inSyringothyris species and is sometimes mistakenly interpreted as a median septum (e.g., Abramov & Grigorjewa, 1986). They are thus assigned to a new species of Winchell's (1863) genus.Unlike Septosyringothyris, there is no delthyrial plate in the genus Histosyrinx and the syrinx is inserted at the junction of two partly joined plates. Dental adminicula thickened by callus deposits converge to simulate a delthyrial plate and the syrinx is supported by a stout median septum (Legrand-Blain, 1974; Carter,2006). The type species of Histosyrinx (H. vautrini Termier & Termier in Massa et al., 1974) is from the Tournaisian (Ivorian) Marar Formation (Serdeles area in the Murzuq Basin, Western Libya) and the type material consists of internal moulds and worn limonitic moulds, as is the case of the holotype. The ventral interior of H. vautrini is strongly variable (syrinx size, development of the septum) as suggested by Legrand-Blain (1974) and our recent observations of the material collected by Delépine (Legrand-Blain, 1974) and Massa (Termier & Termier in Massa et al., 1974; Mergl & Massa, 1992).The genus Septosyringothyris has a particular palaeogeographic pattern of distribution with occurrences in tropical latitudes of Laurussia (Belgium), and in higher latitudes of Gondwana, in western basins of South America (Argentina and Chile). In Argentina, Septosyringothyris shows a temporal diversification that appears to increase from the Carboniferous glacial sequences (late Palaeozoic ice age) to those associated with the climatic amelioration intervalthat took place during the latest Carboniferous?earliest Permian. Argentinean species of this latter interval have been referred to the subgenus Septosyringothyris (Precosyringothyris) (Cisterna, 2011), to characterize species with a large syrinx partially enclosed by the delthyrial plate, with a median septum that becomes more conspicuous in mature specimens. Occurrences of Carboniferous Septosyringothyris species in South America, as is the case withother Carboniferous genera (Cisterna & Simanauskas, 1999; Taboada & Shi, 2009), could be reflecting a faunal interchange with the Northern Hemisphere regions. However, many of Argentinean species must be reviewed in the light of the new results of the study of the Septosyringothyris type material.