The systematics of the Mongolepidida (Chondrichthyes) and the Ordovician origins of the clade

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.

Early Silurian chondrichthyans from the Tarim Basin (Xinjiang, China)

“…Class Chondrichthyes Huxley, 1880 [44] Order Mongolepidida Karatajūtē-Talimaa, Novitskaya, Rozman & Sodov, 1990 [18] sensu [7] Remarks. The present study follows the most recent revision of the Order in recognising linear odontocomplexes formed by an acellular and atubular dentine (lamellin [18,45]) as diagnostic for the Mongolepidida [7].…”

“…Class Chondrichthyes Huxley, 1880 [44] Order Mongolepidida Karatajūtē-Talimaa, Novitskaya, Rozman & Sodov, 1990 [18] sensu [7] Remarks. The present study follows the most recent revision of the Order in recognising linear odontocomplexes formed by an acellular and atubular dentine (lamellin [18,45]) as diagnostic for the Mongolepidida [7]. Although not harbouring the tubular system of traditionally-defined dentines [46,47], lamellin topology and deposition via infill of a pulp-like canal are evidence for its odontogenic origin [45].…”

“…Family Mongolepididae Karatajūtė-Talimaa et al, 1990 [18] sensu [7] Genus Rongolepis Sansom, Aldridge and Smith, 2000 [5] sensu [7] Rongolepis cosmetica Sansom…”

“…Lower Silurian vertebrate assemblages of the Tarim and South China tectonic blocks are dominated by galeaspid agnathans and putative chondrichthyans and are united together within the recently erected Zhangjiajie Vertebrate Fauna [1,2]. The chondrichthyans are represented by two enigmatic groups, the Sinacanthida ordo nov. and the Mongolepidida [3][4][5][6][7]. Sampling in South China has revealed sinacanthids and mongolepids in Telychian strata of the Xiushan Formation in Guizhou Province (southeast [5,8]) and similar faunas from the Ymogantau Formation (sensu [9]) on the northwestern margin of the Tarim Basin (Xinjiang Uygur Autonomous Region, northwest [3,5,8,10]).…”

“…The taxa identified here are placed within the total-group Chondrichthyes and add to the burgeoning diversity of Lower Palaeozoic scales that are shark-like in their overall appearance, growth and histology. These include, in approximate stratigraphic order from the Darriwilian (Middle Ordovician) through to the Lower Devonian, Tantalepis [16], Tezakia and Canyonlepis [7,17], mongolepids [5,[18][19][20], elegestolepids [20,21] and Tuvalepis [22]. The relationship of these taxa to conventionally defined chondrichthyans (sensu [23]) remains contentious despite recent progress in integrating scale-based trees into the phylogenetic framework of early jawed gnathostomes [24].…”