Abstract
The Mongolepidida is an Order of putative early chondrichthyan fish, originally erected to unite taxa from the Lower Silurian of Mongolia. The present study reassesses mongolepid systematics through the examination of the developmental, histological and morphological characteristics of scale-based specimens from the Upper Ordovician Harding Sandstone (Colorado, USA) and the Upper Llandovery–Lower Wenlock Yimugantawu (Tarim Basin, China), Xiushan (Guizhou Province, China) and Chargat (north-western Mongolia) Formations. The inclusion of the Mongolepidida within the Class Chondrichthyes is supported on the basis of a suite of scale attributes (areal odontode deposition, linear odontocomplex structure and lack of enamel, cancellous bone and hard-tissue resorption) shared with traditionally recognized chondrichthyans (euchondrichthyans, e.g., ctenacanthiforms). The mongolepid dermal skeleton exhibits a rare type of atubular dentine (lamellin) that is regarded as one of the diagnostic features of the Order within crown gnathostomes. The previously erected Mongolepididae and Shiqianolepidae families are revised, differentiated by scale-base histology and expanded to include the genera Rongolepisand Xinjiangichthys, respectively. A newly described mongolepid species (Solinalepis levis gen. et sp. nov.) from the Ordovician of North America is treated as family incertae sedis, as it possesses a type of basal bone tissue (acellular and vascular) that has yet to be documented in other mongolepids. This study extends the stratigraphic and palaeogeographic range of Mongolepidida and adds further evidence for an early diversification of the Chondrichthyes in the Ordovician Period, 50 million years prior to the first recorded appearance of euchondrichthyan teeth in the Lower Devonian.
Highlights
Middle Ordovician to upper Silurian strata have yielded a number of isolated scale remains that have been assigned to the chondrichthyans with varying degrees of confidence
These, largely microscopic, remains include the elegestolepids (Karatajute-Talimaa, 1973; Žigaite & Karatajute-Talimaa, 2008; Andreev et al, in press), sinacanthids (Zhu, 1998; Sansom, Wang & Smith, 2005; Zeng, 1988), taxa such as Tezakia and Canyonlepis from the Ordovician of North America (Sansom, Smith & Smith, 1996; Andreev et al, 2015), Tantalepis (Sansom et al, 2012), Kannathalepis (Märss & Gagnier, 2001) and Pilolepis (Thorsteinsson, 1973), and, perhaps the most widely distributed and diverse collection of what Ørvig and Bendix-Almgreen, quoted in Karatajute-Talimaa (1995), referred to as ‘praechondrichthyes,’ the mongolepids (Karatajute-Talimaa et al, 1990; Karatajute-Talimaa & Predtechenskyj, 1995; Sansom, Aldridge & Smith, 2000). It is the latter which this work concentrates on, re-assessing and re-defining previously described members of the Mongolepidida, and describing a new taxon that extends the range of the Order into the Ordovician, adding further evidence for a diversification of early chondrichthyans as part of the Great Ordovician Biodiversification Event that encompasses a wide variety of taxa, both invertebrate (e.g., Webby, Paris & Droser, 2004; Servais et al, 2010) and vertebrate (Sansom, Smith & Smith, 2001; Turner, Blieck & Nowlan, 2004)
Remarks The current study has determined scale crown growth to be a characteristic shared by all mongolepid taxa, contrary to previous interpretations of synchronomorial development of scale odontodes in Mongolian mongolepid species (Karatajute-Talimaa et al, 1990; Karatajute-Talimaa & Novitskaya, 1992; Karatajute-Talimaa & Novitskaya, 1997)
Summary
Middle Ordovician to upper Silurian strata have yielded a number of isolated scale remains that have been assigned to the chondrichthyans with varying degrees of confidence. These, largely microscopic, remains include the elegestolepids (Karatajute-Talimaa, 1973; Žigaite & Karatajute-Talimaa, 2008; Andreev et al, in press), sinacanthids (Zhu, 1998; Sansom, Wang & Smith, 2005; Zeng, 1988), taxa such as Tezakia and Canyonlepis from the Ordovician of North America (Sansom, Smith & Smith, 1996; Andreev et al, 2015), Tantalepis (Sansom et al, 2012), Kannathalepis (Märss & Gagnier, 2001) and Pilolepis (Thorsteinsson, 1973), and, perhaps the most widely distributed and diverse collection of what Ørvig and Bendix-Almgreen, quoted in Karatajute-Talimaa (1995), referred to as ‘praechondrichthyes,’ the mongolepids (Karatajute-Talimaa et al, 1990; Karatajute-Talimaa & Predtechenskyj, 1995; Sansom, Aldridge & Smith, 2000) It is the latter which this work concentrates on, re-assessing and re-defining previously described members of the Mongolepidida, and describing a new taxon that extends the range of the Order into the Ordovician, adding further evidence for a diversification of early chondrichthyans as part of the Great Ordovician Biodiversification Event that encompasses a wide variety of taxa, both invertebrate (e.g., Webby, Paris & Droser, 2004; Servais et al, 2010) and vertebrate (Sansom, Smith & Smith, 2001; Turner, Blieck & Nowlan, 2004)
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