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. 2016 Jun 16:4:e1850.
doi: 10.7717/peerj.1850. eCollection 2016.

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

Plamen Andreev  1 , Michael I Coates  2 , Valentina Karatajūtė-Talimaa  3 , Richard M Shelton  4 , Paul R Cooper  4 , Nian-Zhong Wang  5 , Ivan J Sansom  1
Affiliations

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

Plamen Andreev et al. PeerJ. .

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.

Keywords: Mongolepids; Morphogenesis; Odontocomplex; Ordovician; Scales; Solinalepis gen. nov..

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Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Principle morphological features of scales.
Line drawing of a Mongolepis scale (BU5296) from the Chargat Formation of north-western Mongolia in lateral view.
Figure 2
Figure 2. Character distribution within Mongolepidida.
Cladogram based on a yet-to-be-published scale-based phylogeny of early chondrichthyans by P Andreev, M Coates & I Samson (2014, unpublished data). Portion of a majority-rule consensus tree generated in TNT version 1.1 (Goloboff, Farris & Nixon, 2008) using a data matrix of 68 equally weighted scale-based characters (53 original and 15 revised/adopted) and 49 Palaeozoic jawed-gnathostome taxa.
Figure 3
Figure 3. Scale morphology of Mongolepididae.
(A–C) Mongolepis rozmanae scale BU5296 (Chargat Formation, north-western Mongolia) in (A) anterior (B) lateral, (C) and basal aspect and a M. rozmanae scale in (D) crown view (BU5351, Chargat Formation, north-western Mongolia); (E, G) Teslepis jucunda BU5322 (Chargat Formation, north-western Mongolia) in (E) crown and (G) basal view and a T. jucunda scale (BU5352, Chargat Formation, north-western Mongolia) in an (F) antero-lateral view; (H–J) Sodolepis lucens scales (Chargat Formation, north-western Mongolia) in (H) lateral (BU5305), crown (BU5304) and (J) basal (BU5355) views; (K–M) Rongolepis cosmetica scale BU5303 (Xiushan Formation, south China) in (K) crown, (L) lateral and (M) basal views;. Volume renderings, (A–C), (H) and (K–M); SEM micrographs, (D–G) and (I, J). Crown and base foramina indicated by arrows and arrowheads respectively. Anterior to the left in (B), (H), (L) and bottom in (A–G), (H–K), (M). Scale bar equals 500 µm in (D, I, J), 400 µm in (A–C), 300 µm in (H, K) and 200 µm in (E–G, L, M).
Figure 4
Figure 4. Scale morphology of Shiqianolepidae.
(A–C) Shiqianolepis hollandi scales (Xiushan Formation, south China) in (A) lateral (NIGP 130307), (B) crown (NIGP 130309) and (C) postero-basal (NIGP 130307) views; (D–F) Xinjiangichthys pluridentatus scale IVPP V X2 (Yimugantawu Formation, north-western China) in (D) anterior, (E) posterior and (F) antero-lateral views. All images volume renderings except (B). Crown foramina indicated by arrows. Anterior to the left in (A), to the right in (F) and bottom in (B). Scale bar equals 300 µm in (A, B) and 200 µm in (C–F).
Figure 5
Figure 5. SEM micrographs of Solinalepis levis gen. et sp. nov. scales from the Upper Ordovician Harding Sandstone of Colorado, USA.
(A–C) tessera-like head scales in (A, B) crown (BU5307, BU5308) and (C) lateral (BU5309) views; (D) bulbous head scale (BU5312) in lateral view; (E–I) polygonal trunk scales, (E) holotype (BU5310) in anterior view, (F) BU5345 in crown, (G) corono-lateral and (H) partial posterior views, (I) BU5313 in basal view; (J–L) lanceolate trunk scales in (J) anterior (BU5314), (K) lateral (BU5315) and (L) posterior (BU5311) views. Base foramina indicated by arrowheads. Anterior to the left in (G) and (K). Scale bar equals 300 µm in (A, B), 200 µm in (C), 100 µm in (D–G, I–L), and 50 µm in (H).
Figure 6
Figure 6. Scale histology of Mongolian and Chinese mongolepids.
(A) medial longitudinal section of a Mongolepis rozmanae scale (BU5297; Chargat Formation, north-western Mongolia); (B) detail of (A) depicting primary and secondary odontodes at the anterior crown margin; (C) primary odontode lamellin microstructure in a longitudinally sectioned Mongolepis rozmanae scale (BU5298; Chargat Formation, north-western Mongolia), etched for 10 min in 0.5% orthophosphoric acid; (D) basal bone microstructure of a longitudinally sectioned Mongolepis rozmanae scale (BU5354; Chargat Formation, north-western Mongolia) etched for 10 min in 0.5% orthophosphoric acid; (E) detail of BU5354 depicting the bone tissue of the anterior basal platform; (F) medial longitudinal section of a Teslepis jucunda scale (BU5324; Chargat Formation, north-western Mongolia); (G) lamellin architecture of two odontodes in a longitudinally sectioned Sodolepis lucens scale (BU5306; Chargat Formation, north-western Mongolia) etched for 10 min in 0.5% orthophosphoric acid; (H) basal bone microstructure in BU5306 at the anterior projection of the base; (I), sagittal longitudinal section of a Sodolepis lucens scale (BU5344; Chargat Formation, north-western Mongolia); (J) anterior third of BU5306 showing the contact between the globular crown dentine and the underlying basal bone; (K) sagittal longitudinal section of a Rongolepis cosmetica scale (NIGP 130328; Xiushan Formation, south China); (L) detail of NIGP 130328 showing the mid third of the scale crown; (M) Xinjiangichthys pluridentatus scale (IVPP V X1; Yimugantawu Formation, north-western China) in longitudinal section; (N) sagittal longitudinal section of a Shiqianolepis hollandi trunk scale (NIGP 130312; Xiushan Formation, south China). Nomarski differential interference contrast optics micrographs, (A), (B), (D), (F), (G), (I) and (K–N); SEM micrographs, (C), (E), (H) and (J). Anterior towards the left in (A–J, L) and towards the right in (K), (M) and (N). Abbreviations: gb, globular dentine; lb, lamellar bone; red dotted lines, contact surfaces between primary and secondary odontodes; white dotted lines, border between globular dentine and basal bone; white dashed line, contact surfaces between primary odontodes in Rongolepis. Asterisks mark bone layers with fibre orientation parallel to the section axis. Scale bar equals 400 µm in (A), 100 µm in (B, G, H, M), 20 µm in (C), 200 µm in (D, F, K, N), 50 µm in (E, J, L), and 300 µm in (I).
Figure 7
Figure 7. Histology of Solinalepis levis gen. et sp. nov. scales.
(A) thin-sectioned head scale (BU5317) from the Harding Sandstone, Colorado, USA; (B) transverse section of a Solinalepis levis gen. et sp. nov. trunk scale (BU5316) from the Harding Sandstone, Colorado, USA. Scale bar equals 200 µm in (A) and 100 µm in (B).
Figure 8
Figure 8. Canal system of mongolepid scales.
Volume renderings. (A–C) canals (red) inside a translucent Mongolepis rozmanae scale (BU5296) in (A) lateral view, in (B) posterior view sliced along the plane 1 and in (C, C1) crown view sliced along plane 2; (D, D1) canals in a transversely sliced Teslepis jucunda scale (BU5325) shown in posterior view; (E) pulp cavities (red) in a transversely sliced Sodolepis lucens scale (BU5305) shown in postero-lateral view; (F) longitudinally sliced Shiqianolepis hollandi scale (NIGP 130307) in baso-lateral view; (G, H) longitudinally sliced Xinjiangichthys pluridentatus scale IVPP V X2 in (G) posterior and (H) lateral views; (I, J) canals system (red) inside a transversely sliced Solinalepis levis gen. et sp. nov. scale (BU5318) shown in posterior view, (J) detail of (I). Horizontal canals depicted in purple in c1 and d1. Yellow arrowheads point at canal openings on the sub-crown surface. Red dotted line, contact surfaces between primary and secondary odontodes; grey dotted line, crown/base border. Scale bar equals 400 µm in (A–C), 100 µm in (D, H, I), 200 µm in (E), 300 µm (F, G) and 50 µm in (J).
Figure 9
Figure 9. Odontocomplex organization of mongolepid scale crowns.
(A) Teslepis jucunda (BU5323) scale, medial portion of the crown; (B) Shiqianolepis hollandi (NIGP 130309) scale, medial portion of the crown; (C) Solinalepis levis gen. et sp. nov. trunk scale (BU5314), lateral portion of the crown. Primary odontocomplex structure in Mongolepidida demonstrated by line drawings of longitudinally sectioned (D) Mongolepis rozmanae (BU5297) and (E) Shiqianolepis hollandi (NIGP 130312) scales. In (A–C) some of the odontocomplexes are highlighted in red and green. Dark green and dark red, odd numbered odontodes; light green and light red, even numbered odontodes. In (D, E)—light grey, primary odontodes; light yellow, secondary odontodes. Anterior towards the bottom in (A–C) and towards the left in (D, E). Scale bar equals 100 µm in (A), 200 µm in (B) and 50 µm in (C).
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