Nostolepis scale remains (stem Chondrichthyes) from the Lower Devonian of Qujing, Yunnan, China

Systematic paleontology

Referred Chinese Material: 69 trunk scales (IVPP V26830.1–V26830.68, V26831).

Description:

Morphotype 1 (Figs. 1A–1D):

This type of scales has a broad triangular, flat, or slightly inclined crown. Crowns are about 0.21–0.56 mm long and 0.28–0.51 mm wide. Three to four short, stout, longitudinal and converging ridges bend inward from the anterior face of the crown to a quarter of the crown’s length. The distal pointed end of the crown overhangs the base slightly. Most of the scales have two smooth lateral slopes rising from the anterior crown-neck boundary and joining at the pointed posterior apex. The neck is smooth and indistinctive. The rhomboid base is strongly or moderately convex and its outline matches that of the crown except for the crown’s posterior end.

Morphotype 2 (Figs. 1E–1H):

One triangular scale form has an inclined crown surface with an angle of nearly 30°. The crowns are slightly wider than long (0.5 mm wide and 0.3 mm long). Two strong ridges are running back from the anterior crown margin to one-third of the crown length with 1–3 subparallel ridges between them. The ridges slope down almost to the base rim anteriorly, thus distinctly lowering the scale’s anterior neck. The lateral slopes of the crown are sharp and well developed. The neck is short, smooth anteriorly, deeper and more concave posteriorly. The base is rhomboid and moderately convex.

Histology (Figs. 2A–2B):

The scales of the first and second varieties have the same histology. The crown is composed of 3–4 odontodes, which are thick on top and sides. All the odontodes are filled with a network of dentine tubules. The canal system is visible on the crown’s posterior part, but Stranggewebe only present in the primordial odontode. The base is pyramid-shaped and is composed of cellular bone. Long Sharpey’s fibers penetrate the base radially.

Remarks:

Nostolepis striata has been frequently redefined since it was erected by Pander (1856) for scales from Estonia. Gross (1947) reassigned some acanthodian species, erected by Brotzen (1934), to Nostolepis striata. Vergoossen (2002b) agreed with this assignment, but thought the morphological variations of N. striata were still too large. After careful observation, Vergoossen (2002b) defined seven different morphotypes according to the Nostolepis scales from Klinta, southern Sweden. We follow his classification and recent descriptions by Burrow et al. (2013) and Turner & Burrow (2018). We suggest two kinds of morphotypes here based on the new materials from the Xitun Formation. We combine morphotypes 1, 2 and 6, defined in Vergoossen (2002b), within one group, and morphotypes 3 and 7 within another group. This second kind of scales comprises pinnal scales or tesserae as in Turner & Burrow (2018). Both morphotypes have the typical Nostolepis structure with mesodentine and Stranggewebe.

Referred Chinese Material: 43 trunk scales (IVPP V7216.4, IVPP V26832.1–V26832.41, IVPP V26833).

Description:

Morphotype 1 (Figs. 1I–1N):

The scale crown’s length and width range from 0.32–0.66 mm and 0.31–0.65 mm, respectively. Morphotype 1 is elongated with rhombic to ellipsoidal in outline. The crown surface is side to side and inclined, with one-fourth of the crown’s length overhanging the base posteriorly. Two longest prominent ridges from an elevated median area of the crown, are sculptured by 2–6 rounded, short, subparallel anterior ridges. Most scales have an unsculptured lateral slope on each side of the median area, but some have several ridgelets (Figs. 1M–1N), which are finer and lower than those on the median area. The neck is well defined and constricted. A row of 3–5 regularly spaced small openings are located on each side of the neck, and sometimes they are also visible anteriorly below the ridges (Figs. 1I–1L). The convex and rectangular base is equal or slightly larger than the crown.

Morphotype 2 (Figs. 1O–1P):

These scales have an inclined median crown surface with two strong main ridges and two inconspicuous lateral slopes on each side of the median area. Crown length varies from 0.60–0.75 mm and width from 0.50–0.65 mm. A shallow and wide longitudinal groove between the two central ridges is another characteristic, forming an incised anterior crown margin. The neck has a row of regularly spaced small openings locating on the side or below the ridge. The scale base of both morphotypes is shaped from isometric to asymmetric rhomboid, sometimes wider than long. The base protrudes anteriorly beyond the crown. It is moderately deep to deep, with the deepest point locating centrally or anteriorly.

Histology (Figs. 2C–2D):

The structures of tissues are similar in the two types. The crown is composed of mesodentine but the stranglacunae are not visible due to the recrystallization.The winding and short dentine tubules orient upwards. Ascending and vaulted vascular canals can be seen in the posterior part of the crown. Stranggewebe is short, narrow, and medium dense, developing only in the first and second lamellae on the crown’s posterior part. The base consists of cellular bone with osteocytes.

Remarks:

This species was erected by Valiukevičius (2003c) for scales from Pridoli and Ludlow of Lithuania. Valiukevičius (2003c) indicated that N. amplifica was similar to the specimens of N. striata described by Vergoossen (2002b) (pl. 14, figs. 85–87) obtained from Öved-Ramsåsa, the Sandstone Formation, Ludlow to Pridoli and (Vergoossen, 2000) pl. 1, figs 4–5 and 7–8), obtained from Welsh Borderland, Silurian to Devonian. We consider that the specimens described by Vergoossen (2002b) (pl. 14, figs 85–87) should be assigned to N. striata as originally suggested. Morphologically, N. amplifica is similar to N. striata, and the former usually has a shallow and longitudinal groove between the two central ridges. Another difference is that the neck has a row of regularly spaced small openings for N. amplifica, but it is rarely reported for N. striata. Only two exceptional specimens with neck openings were reported in N. striata, which differ from the trunk scales described by Gross (1947), pl. 7, fig. 13 and Gross (1971), pl. 5, fig 3a. Histologically, Valiukevičius (2003c) also figured that both species have similar mesodentine, Stranggewebe, vascular canals, and large number of osteocytes, but the canal system is more advanced and Stranggewebe is shorter and denser in N. amplifica. N. amplifica was first found by Wang (1984) (figs. 3D–3F) and described as Nostolepis sp. indet. from the Xitun Formation. In terms of morphology, some specimens described by Gagnier, Jahnke & Shi (1989) (pl. 1, figs 3–4) from the Yulungssu Formation, Hongmiao (Shiyanpo), Qujing, which differ from the holotype of N. sinica (pl. 1, figs 1–2), probably belong to N. amplifica.

Emended diagnosis: The crowns are flat, never inclined, and sculptured with 6–12 short and parallel ridges. Low anterior ridges fade out at one-third of the crown’s length. The posterior part of the crown is composed of Stranggewebe and widened ascending vascular canals. The Stranggewebe is dense in each odontode.

Referred Chinese Material: around 25 trunk scales (IVPP V7216.7, IVPP V26834.1–V26834.23, V26835).

Description (Figs. 1Q–1X):

The scale crown is flat, never inclined, and has a nearly triangular or ovoid shape. Crown length varies from 0.37–0.70 mm and width from 0.41–0.55 mm, respectively. Most of the scale crowns have 3–12 subparallel and short ridges extending one–third of the crown’s length, converging towards the posterior end. Lateral slopes are not well developed in most of the scales. The anterior edge of the crown is convex, curving down to the base. The neck is smooth and constricted for most of the scales. A rhombic base is slightly convex, anteriorly vaulted, so that the base protrudes in front of the crown.

Histology (Figs. 2E–2F):

The crown has a relatively big primordial scale and a few growth lamellae thin on top but thick on anterior and posterior parts. The odontodes have dentine tubules, which are dense anteriorly. The crown’s posterior part is composed of Stranggewebe and widened ascending and vaulted vascular canals (over the base). Stranggewebe is short and densely presented in each odontode. The base is composed of compact, thin-lamellar bone, containing a large number of osteocytes.

Remarks:

For trunk scales, Valiukevičius (2003c) identified two morphotypes of N. consueta. The first morphotype has 8–12 ridges, and the second morphotype has fewer than 6 ridges. In our collection, most of the referred scales have 6–10 ridges. N. consueta is so similar to N. striata that some N. consueta probably have been misassigned to the latter. N. consueta has a flat crown, never inclined, and the anterior corner of the crown is markedly angular. More differences are present in the histological features. N. consueta has dense Stranggewebe in each odontode; N. consueta has a low number of osteocytes and no obvious Sharpey’s fibers in the base compared with N. striata. The scales assigned to Nostolepis sp. indet. (Wang, 1984), figs. 3A–3C) should be Nostolepis striata. And the specimen (V11528.1) described as Nostolepis striata (Wang et al., 1998, pl. 1, fig. A) from the lower Lochkovian Xiaputonggou Formation, West Qinling, China, probably belong to N. consueta.

Deviation of name: After Qujing city, the fossil site.

Holotype: IVPP V26838.1.

Type locality and horizon: Xitun, Xishan subdistrict, Qujing, Yunnan, China; Xitun Formation, Lochkovian, Lower Devonian.

Referred Material: 19 trunk scales (IVPP V26838.1–V26838.16, IVPP V26839-V26839.3)

Diagnosis:

Trunk scales are of moderate size, ornamented with longitudinal ridges in the crown. Crown length is 0.3–0.5 mm, width 0.3–0.5 mm. The distal pointed end of the crown always overhangs the base at one-third of its length. The angle between crown and base can reach 45°. The neck is absent anteriorly, but deeper and concave posteriorly. The base is rhomboid, thin, and concave. Dentine tubules rarely present in the anterior part of the crown. Scale base is composed of bone with few osteocytes.

Description (Figs. 3A–3H):

The scale crowns are triangular in shape or irregularly shaped (Figs. 3E–3H). They incline antero-posteriorly at an angle of 30°–45°. Crown length varies from 0.33–0.50 mm and width from 0.36–0.51 mm, respectively. The crown is sculptured with slightly sinuous ridges extending the whole surface (Figs. 3A–3D) or ornamented with radial ridges and deep grooves (Figs. 3E–3H). The lateral slope usually develops in the former morphotype (Figs. 3A–3D). The neck is absent anteriorly, but deeper and concave posteriorly. The base is rhomboid and thin, never protrudes beyond the crown on all sides.

Histology (Figs. 4A–4D):

The crown’s odontodes are very thin, and the dentine tubules are rarely present in each odontode. Wide vascular canals rise from the neck and penetrate the posterior part of the crown. Scattered osteocytes form a low and flat base.

Remarks:

Nostolepis qujingensis is similar to N. lineleyensis in the crown sculptured with ridges and grooves at the anterior margin, sloping down at an angle of 30°–45°. But N. lineleyensis has a much thicker crown and more ridges than N. qujingensis. More differences are found in histology. N. qujingensis and N. lineleyensis have a similar system of vascular canals, but the latter has a typical mesodentine in the crown and cellular bone with numerous large and elongate osteocytes (Miller & Märss, 1999).

Nostolepis digitus sp. nov.

Derivation of name: From the Latin digitus, referring to the appearance of the crown ornamentation.

Holotype: IVPP V26840.1

Type locality and horizon: Xitun, Xishan subdistrict, Qujing, Yunnan, China; Xitun Formation, Lochkovian, Lower Devonian.

Referred Material: 28 trunk scales (IVPP V26840.1–V26840.25, IVPP V26841-IVPP V26841.3).

Diagnosis:

Large-sized scale having a sub-rhomboid crown with rounded and widened anterior edge and a slightly tapered posterior end. Short and parallel ridges on the crown’s anterior and lateral margins, curving down to the base. The neck is constricted with small pores. Scale bases are strongly convex; their maximum depth is slightly in front of the center. Dentine tubules are dense in each odontode of the crown, and the Stranggewebe is oriented parallel to the growth zone lines and distributed in the posterior part of the odontodes. Ascending vascular canals are well developed. The base is filled with numerous osteocytes.

Description (Figs. 3I–3P):

Scale crowns are usually flat, or slightly inclined. Crown length varies from 0.45–0.76 mm and width from 0.41–0.70 mm. The rhombic crown has a round and wide anterior edge and a blunt posterior cusp. The posterior parts of crowns tilt to one side. The crown is ornamented with short and parallel ridges that do not extend to the middle of the crown posteriorly, but bend down anteriorly and laterally. The crown has a smooth lateral slope flanking the posterior part. The neck is short, lowered anteriorly, but deeper and concave posteriorly. Only small pores are visible on the neck. The base is strongly convex, vaulted anteriorly and protruding in front of the crown.

Histology (Figs. 4E–4H):

The crown is composed of 3–4 odontodes, and the young one envelops the older one completely. Dentine tubules are dense and discernable in each odontode. The growth lamella contains long ascending vascular canals placed posteriorly in the neck over the scale base. The Stranggewebe is oriented parallel to the growth zone lines and only presented in the posterior part of the outermost odontode. The upper surface of the base forms a high pyramid, and the base is filled with many osteocyte cavities.

Remarks:

The referred specimens are similar to Nostolepoides mingyinensis in morphology and histology, which was described from another Devonian site in Yunnan Province (Wang, 2003) (p. 8, fig. 3). Morphologically, the crowns of both species are rhomboid, sculptured with short and parallel ridges, not extending half of the crown length, and the lateral slopes are less developed. N. minyingensis has very sharp ridges and anterior crown margin, and N. digitus has short rounded ridges and crown margin. Histologically, their base tissues are similar in having dense osteocytes. The Stranggewebe is not visible in Nostolepoides mingyinensis, and the crown is composed of more odontodes. Morphologically, N. digitus is also similar to Nostolepis sp. aff. N. multicostata reported by Burrow, Lelièvre & Janjou (2006) (figs. 3, 11–12 and 15–16) from the Lower Devonian Jawf Formation of northwestern Saudi Arabia. But the Arabian form is much larger than N. digitus, and its crown has more ridges.