Sherbonaspis Young and Gorter, 1981

Plax, Dmitry P. & Lukševičs, Ervins, 2023, A new Early Devonian antiarch placoderm from Belarus, and the phylogeny of Asterolepidoidei, Acta Palaeontologica Polonica 68 (3), pp. 513-527 : 515-519

publication ID	https://doi.org/ 10.4202/app.01075.2023
persistent identifier	https://treatment.plazi.org/id/49453C70-573D-7725-275C-FAAFC4F3AD07
treatment provided by	Felipe
scientific name	Sherbonaspis Young and Gorter, 1981
status

Treatment

Genus Sherbonaspis Young and Gorter, 1981

Type species: Sherbonaspis hillsi Young and Gorter, 1981 , Goodradig- bee valley near Wee Jasper, New South Wales, Australia;?Emsian/ Eifelian, Lower/Middle Devonian.

Sherbonaspis talimaae sp. nov.

Figs. 3–6 View Fig View Fig View Fig View Fig .

2007 Antiarcha gen. nov.; Plaksa 2007: 7.

2008 a Antiarcha gen. nov.; Plaksa 2008 a: 84.

2008b Antiarcha gen. nov.; Plaksa 2008 b: 136.

2008 Antiarcha gen. nov. Plax; Plax 2008: 70, table 1, p. 76.

2012b Antiarcha gen. nov. Plax; Kruchek et al. 2012b: 95.

2012a Antiarcha gen. nov. Plax; Kruchek et al. 2012a: 100.

2012a Actinolepididae gen. indet.; Kruchek et al. 2012a: 100.

2015a Actinolepididae gen. indet.; Plax 2015a: 171, pl. 1: 5.

2015a Antiarcha gen. nov. Plax; Plax 2015a: 171, pl. 1: 6.

2015b Actinolepididae gen. indet.; Plax 2015b: 21, table (pars), pl. 1: 10. 2015b Antiarcha gen. nov. Plax; Plax 2015b: 21, table.

2018 Antiarcha gen. nov. Plax; Plax 2018: 409.

ZooBank LCID: urn:lsid:zoobank.org:actD 051501D-F1DA-4EE9-B 648-537AB4ED18B0

Etymology: In honour of Valentina Karatajūtė-Talimaa (Vilnius, Lithuania), who recently passed away (1930–2022), in appreciation of her works on antiarchs and thelodonts.

Holotype: BNTU 44 /2-2, left PVL plate.

Type locality: Borehole Osipovichi 6, 113.8 m deep, close to Osipovichi town , Mogilev Region, Belarus .

Type horizon: Lepel Beds, Vitebsk Regional Stage, probably upper Emsian, Lower Devonian .

Material.—Right Pn plate (BNTU 44/1-13a), left AVL plate

BNTU 121/20-1), fragmentary right AVL plate (BNTU 44/2-

), left PVL plates ( BNTU 44 /2-2, BNTU 158 /1-1). All from type horizon; for localities see Table 1 .

Diagnosis.—An asterolepidoid antiarch of small size, with length of the ventral wall of the trunk armour of about 75 mm, and with relatively high trunk armour. Paranuchal plate small, asymmetrical, slightly longer than it is wide. Ventral lateral ridge is well developed along the whole length of the trunk armour. The ventral wall of the trunk armour is flat. The contact face for mixilateral plate on the anterior ventral lateral plate is short. Lateral margin of the ventral lamina of posterior ventral lateral plate almost straight. Length/width index of lateral lamina of posterior ventral lateral plate is about 2.3. Ornamentation generally consists of small tubercles sometimes fused into tuberculate ridges; tubercles tend to form rows perpendicular or parallel to the margins of the plates. The anterior ventral lateral plates bear a fine-meshed network of the shallow pits in the anterior part of the ventral lamina, and small tubercles with sharp tips along the margins in the posterior portion of the plate.

Description.—Only the Pn plate is known from the skull roof of Sherbonaspis talimaae sp. nov. This plate has not been described either in Sherbonaspis hillsi ( Young and Gorter 1981) or in Sherbonaspis andreannae ( Panteleyev 1993) . The right Pn plate BNTU 44/1-13a ( Fig. 3C View Fig ) is subsquare in shape, with the maximum width in its posterior part. It is longer than broad, like in Pterichthyodes milleri ( Miller, 1841) ( Hemmings 1978: 16) , with the B/L index of 0.88, in a sharp contrast to species of Asterolepis (e.g., Lyarskaya 1981) or Walterilepis speciosa ( Gross, 1933) ( Lukševičs 2021) with the B/L index of 1.12–1.34. The posterior margin of the bone is straight, with a narrow (short) and weakly developed smooth obtected nuchal area. The lateral segment of the bone is narrow, only 0.16 times as wide as the medial segment, in contrast to Walterilepis speciosa with the index of 0.35–0.49. The mpg is well seen and long almost reaching the mesial margin ( Fig. 4B View Fig ) thus differing well from Byssacanthus dilatatus ( Eichwald, 1844) ( Karatajūte-Talimaa 1960) and Walterilepis speciosa . The infraorbital sensory line canal on the head shield (ifc1) is weakly defined, in contrast to the cephalic division of the main lateral line (ptc) that is well discernible in the posterior third of the lateral margin. The lateral segment is narrowest posteriorly slightly increasing in width anteriorly; this shape significantly differs from that in species of Byssacanthus , Pterichthyodes , and Walterilepis where the lateral segment is narrowest anteriorly (e.g., Lukševičs 2021: fig. 3G 1). The internal surface shows a well-defined paramarginal crest (cr.pm) dividing the surface into two regions approximately equal in width. The posterolateral corner of the shallow oticooccipital depression is rounded. The external ornamentation consists of low small tubercles in places fused into radially arranged tuberculate ridges radiating from the posterior lateral angle of the plate.

Thorax: The left AVL plate BNTU 121/20-1 is partially preserved bone showing only the ventral lamina; almost all of the lateral lamina and processus brachialis are missing ( Fig. 3A View Fig ); this specimen is very thin, extremely fragile, and relatively small (only 41 mm in length). The right AVL plate BNTU 44/2-1 is even less complete, comprising approximately half of the posterior part of both ventral and lateral laminae ( Fig. 3B View Fig ). Length of the ventral lamina in BNTU 44/2-1 is estimated at about 20 mm. Almost all edges of the ventral lamina of the AVL plate in BNTU 121/20-1 are well preserved, except the slightly damaged middle portion of the mesial edge. In contrast, most edges of the lateral lamina of the AVL in BNTU 121/20-1 are damaged, except the ventral lateral ridge and a narrow strip of bone along the ridge. The angle between the ventral and lateral laminae cannot be measured because the lateral lamina is almost absent in BNTU 121/20-1, and because both ventral and lateral laminae in BNTU 44/2-1 form too obtuse angle due to deformation of the specimen. In its squarish outline the ventral lamina resembles that of Byssacanthus dilatatus ( Eichwald, 1844) , Sherbonaspis hillsi Young and Gorter, 1981 , Sherbonaspis andreannae Panteleyev, 1993 , or Stegolepis jugata Malinovskaya, 1973 ( Karatajūte-Talimaa 1960; Malinovskaya 1973; Young and Gorter 1981; Panteleyev 1993), and differs from species of Asterolepis (e.g., Lyarskaya 1981). It is moderately elongated with a B/L index of about 0.5, resembling that in Sherbonaspis hillsi , being larger than the index in Stegolepis jugata or Walterilepis speciosa ( Malinovskaya 1973; Young and Gorter 1981; Lukševičs 2021). The ventral lamina is almost flat, as in Byssacanthus dilatatus , Stegolepis jugata , Sherbonaspis hillsi , and Walterilepis speciosa ( Karatajūte-Talimaa 1960; Malinovskaya 1973; Young and Gorter 1981; Lukševičs 2021), and in contrast to Gerdalepis rhenana ( Beyrich, 1877) and Gerdalepis jesseni Friman, 1982 ( Gross 1941; Friman 1982) or Wurungulepis Young, 1990 ( Young 1990), where it is convex. The subcephalic division is relatively short and comprises about 21% of the length of the ventral lamina similarly as in Sherbonaspis hillsi ( Young and Gorter 1981) ; the subcephalic division is rather short also in Stegolepis jugata ( Malinovskaya 1973: 75) . This division is only slightly shorter than the subcephalic division in Walterilepis speciosa ( Lukševičs 2021) and significantly shorter than that in Pterichthyodes milleri ( Miller, 1841 ex Agassiz MS) ( Hemmings 1978: 30) . The anterior margin is well preserved in BNTU 121/20-1, which shows a distinct anterolateral corner (c.al), as is the notch for the semilunar plate ( Fig. 4A View Fig 2 View Fig : cf.Sm). The anterior margin of the AVL is rounded; it is formed of two almost equal parts: the anterior lateral part is almost straight forming about 135° wide angle with a rounded anterior mesial part of this margin. The right AVL overlies the left one similarly as in Pterichthyodes milleri , Sherbonaspis hillsi , and Stegolepis jugata , but in contrast to Walterilepis ( Malinovskaya 1973; Hemmings 1978; Young and Gorter 1981; Lukševičs 2021). The lateral margin is almost straight, it is only slightly convex in the anterior part. It is easy to discern that the mesial margin is subdivided into three portions for contact with the semilunar plate anteriorly (cf.Sm), with the opposite AVL plate in the middle (oa.AVL), and with the median ventral plate posteriorly (cf.MV). The shape of the notch for the semilunar plate (cf.Sm; Fig. 4A View Fig 1 View Fig ) suggests a narrow trapezoidal shape for this plate, and not a quadrilateral shape as in Stegolepis and Pterichthyodes (e.g., Malinovskaya 1973; Hemmings 1978) or in at least one AVL of Sherbonaspis hillsi ( Young and Gorter 1981: fig. 18C); the semilunar plate is about as long as it is wide. However, the semilunar plate remains unknown, hence it is unclear whether it is paired as in Pterichthyodes milleri or not. The contact area with the opposite AVL plate is rather strongly convex, it is about three times longer that the notch for the semilunar plate (cf. Sm), and about 1.8 times longer than the overlapping area of the MV plate (cf.MV). The area overlapping the median ventral plate is moderately wide. The area overlapping the PVL plate (cf. PVL) is rather long; the ventrolateral corner is rounded, but well discernible particularly on the visceral surface. The ventral division of the crista transversalis interna anterior (cit1) is rather low laterally, and decreases in height mesially. It is not subdivided mesially, similarly as in Sherbonaspis hillsi ( Young and Gorter 1981: fig. 18), but in contrast to species of Asterolepis , Pterichthyodes , and Walterilepis (e.g., Hemmings 1978: 23; Lyarskaya 1981; Lukševičs 2021). A low ridge stretches from the lateral end of the crista transversalis interna anterior to the anterolateral corner, similarly as in Sherbonaspis hillsi ( Young and Gorter 1981: fig. 18A, C).

The lateral lamina is reconstructed at about 2.4–2.8 times as long as high, resembling proportions of the lateral lamina in Sherbonaspis hillsi and Stegolepis jugata ( Malinovskaya 1973; Young and Gorter 1981), being proportionately higher than in Byssacanthus dilatatus ( Karatajūte-Talimaa1960:pl. 3: 2), but significantly lower than in Wurungulepis denisoni and Walterilepis speciosa ( Young 1990: fig. 12; Lukševičs 2021: fig. 6). Judging from the outline of the dorsal margin in BNTU 44/2-1, it could be highest at the posterodorsal corner (pdc), similarly to Pterichthyodes milleri ( Hemmings 1978: figs. 13, 14), Stegolepis jugata ( Malinovskaya 1973: 77) , and Sherbonaspis hillsi ( Young and Gorter 1981: 106, fig. 18) but in contrast to Walterilepis speciosa ( Lukševičs 2021) , where it is highest at the anterodorsal corner. The contact face for the MxL is shorter than in Byssacanthus dilatatus , Pterichthyodes milleri , and Sherbonaspis hillsi . The shape of the foramen axillare (f.ax) is not known since it is not preserved in both available specimens.

The ornamentation is not well seen on the photographs of the specimen BNTU 121/20-1 due to glue covering the surface; however, it is well seen on the surface of the specimen. AVL plates are ornamented with a fine-meshed network of the shallow pits in the anterior part of the ventral lamina, and small tubercles with sharp tips along the margins in the posterior portion of the plate, which are well seen in BNTU 44/2-1 ( Fig. 3 View Fig ), as well as in BNTU 121/20-1 under oblique lighting. In places these sharp small tubercles are fused by their bases in rows forming narrow ridges distributed almost perpendicularly to the margins of the plate. The lateral lamina in specimen BNTU 44/2-1 shows similar ornamentation consisting of small tubercles.

Left PVL plate BNTU 44/2-2 is well preserved despite being flattened so that the angle between the ventral and lateral laminae appears very obtuse ( Fig. 3D View Fig ). Another left PVL plate BNTU 158/1-1 is less complete, particularly the ventral lamina is fairly damaged; however, the lateral lamina is well preserved ( Fig. 3E View Fig ). The PVL plate is rather small, reaching only 18.7 mm in length in the specimen BNTU 44/2-2 and about 30 mm in BNTU 158/1-1. Almost all edges of the ventral lamina of the PVL plate in BNTU 44/2-2 are well preserved, except the middle and posterior portion of the mesial edge, which are represented by the impression on the surface of the rock, and the anterior margin coinciding with the area overlapped by the AVL plate. In contrast, most edges of the ventral lamina in BNTU 158/1-1 are damaged. Almost all edges of the lateral lamina of the PVL are well preserved in both specimens, except small portion along the anterior dorsal margin in BNTU 158/1-1. The angle between the ventral and lateral laminae cannot be measured with confidence due to deformation of both specimens of the PVL plate; the measured angle is about 145° wide, which is too large to be undeformed. The ventral lamina is moderately elongated with a B/L index of about 0.44, resembling that as it is restored in Sherbonaspis hillsi , with the B/L index of 0.47 ( Young and Gorter 1981: fig. 15), and being relatively longer than that in Sherbonaspis andreannae , with the B/L index of 0.4 ( Panteleyev 1993: 66). The index is larger than the B/L index in Stegolepis jugata ( Malinovskaya 1973: pl. 9: 6; B/L index of 0.4), and slightly smaller than that in Pterichthyodes milleri ( Hemmings 1978: text-fig. 15; B/L index 0.47–0.52, 0.5 on the average). The subanal division is relatively short and comprises about 21% of the total length of the ventral lamina similarly as in Sherbonaspis hillsi ( Young and Gorter 1981) , which is less than the corresponding proportion in Sherbonaspis andreannae ( Panteleyev 1993) , Stegolepis jugata , and Pterichthyodes milleri . The lateral margin is almost straight, being gently convex in the anterior part; it bears small closely set prominent tubercles along the ventrolateral ridge and the whole lateral edge. The area overlapping the MV plate is rather long ( Fig. 5B View Fig ) indicating a relatively large MV plate ( Fig. 6 View Fig ), as in Stegolepis , Byssacanthus , and Sherbonaspis , and in contrast to Pterichthyodes and Gerdalepis ( Gross 1941; Karatajūte-Talimaa 1960; Malinovskaya 1973; Hemmings 1978; Young and Gorter 1981). The left PVL overlies the right one similarly as in species of Pterichthyodes , Sherbonaspis , and Stegolepis . The ventral lamina is about 1.5 times broad as the lateral lamina is high. The length/width index for the lateral lamina is 2.3 in BNTU 44/2-2, similarly as in Byssacanthus dilatatus , proportionally higher than in Stegolepis jugata or species of Asterolepis , but proportionally lower than in Sherbonaspis hillsi , Wurungulepis denisoni and Gerdalepis jesseni ( Karatajūte-Talimaa 1960: pl. 3: 4; Malinovskaya 1973: pl. 9: 6; Young and Gorter 1981: 107; Friman 1987: fig. 5; Young 1990: fig. 12). The dorsal margin of the lateral lamina is almost straight anteriorly from the prominent dorsal corner (dc). The posterior margin of the lateral lamina is concave; overall shape of the lateral lamina resembles that in Sherbonaspis hillsi and Sherbonaspis andreannae ( Young and Gorter 1981: fig. 15D; Panteleyev 1993: fig. 4a), but differs from that in Pterichthyodes milleri , Byssacanthus dilatatus , Wurungulepis denisoni , or Stegolepis jugata . The external surface of the posterior portion of the PVL plate BNTU 44/2-2 has been lost, but the slightly elevated area in the impression of the visceral surface indicates a broad transverse thickening on the ventral lamina. The PVL plates are ornamented with small tubercles sometimes fused into tuberculate ridges; tubercles tend to form rows usually perpendicular or in places parallel to the margins of the plate.

Remarks.— Sherbonaspis talimaae sp. nov. differs well from such asterolepidoids as Wurungulepis denisoni , Byssacanthus dilatatus , asterolepidids, Pterichthyodes milleri , Gerdalepis rhenana and Gerdalepis jesseni , and Walterilepis speciosa by many features. Sherbonaspis talimaae sp. nov. resembles Sherbonaspis hillsi in (i) small size; (ii) B/L index of ventral lamina of AVL; (iii) short subcephalic division; (iv) semilunar plate which is as long as it is broad; (v) semilunar plate with a narrow trapezoidal shape; (vi) crista transversalis interna anterior (cit1) which is not subdivided mesially; (vii) proportions and shape of the lateral lamina of AVL; (viii) shape and proportions of the ventral lamina of PVL; (ix) short subanal division; (x) tubercular ornamentation. Sherbonaspis talimaae sp. nov. differs from Sherbonaspis hillsi in (i) that the lateral margin of the ventral lamina of PVL is almost straight; (ii) L/B index of lateral lamina of PVL. Sherbonaspis andreannae is a somewhat larger fish; however, it resembles S herbonaspis talimaae sp. nov. in various features, but differs in: (i) B/L index of the ventral lamina of PVL; (ii) shorter subanal division; (iii) L/B index of lateral lamina of PVL; (iv) more ridged ornamentation. Sherbonaspis talimaae sp. nov. resembles Stegolepis jugata in (i) the short subcephalic division; (ii) proportions of the lateral lamina of AVL; (iii) tuberculate ornamentation. However, it differs from Stegolepis jugata in (i) B/L index of ventral lamina of AVL; (ii) shape and (iii) proportions of the semilunar plate; (iv) L/B index of lateral lamina of PVL; (v) less ridged ornamentation.