Bothriolepis lohesti Leriche, 1931

Olive, Sébastien, 2015, Devonian antiarch placoderms from Belgium revisited, Acta Palaeontologica Polonica 60 (3), pp. 711-731 : 712-717

Bothriolepis lohesti Leriche, 1931

Fig. 3 .

1888 Bothriolepis or Pterychtis sp.; Lohest 1888: 60.

1889 Bothriolepis ; Lohest 1889: 58.

1895 Bothriolepis canadensis ; Lohest 1895: 39.

1931 Bothriolepis ; Leriche 1931: 15, pl. 3.

1932 Bothriolepis lohesti ; Gross 1932: 29.

1948 Bothriolepis lohesti ; Stensiö 1948: 513.

1978 Bothriolepis lohesti ; Denison 1978: 110.

Type material: No holotype was defined by Leriche (1931). Gross (1932) assigned as lectotype an AMD ( Leriche 1931: pl. 3: 4), bearing now the number PALULG-2011.12.14.14A.

Type locality: Chèvremont , Liège Province, Belgium

Type horizon: Montfort / Evieux Formation , Famennian, Upper Devonian .

Material.—Partial headshield: UCL-P.V.L.10.621 (cast); AMD: PALULG-2011.12.14.15, PALULG-2011.12.14.16, PALULG-2011.12.14.17A; ADL: PALULG-2011.12.14.17B;? ADL:IRSNB vert5179-001; PMD:PALULG-2011.12.14.19, PALULG-2011.12.14.18; MxL: UCL-P.V.L.10.533, UCLP.V.L.10.668; AVL: PALULG-2011.12.14.20; pectoral fin: PALULG-2011.12.14.14B, PALULG-2011.12.14.21, PALULG-2011.12.14.22; CV1: UCL-P.V.L. 10.656, PA- LULG-2011.12.14.23A, IRSNB vert 5179-002; ML2: PALULG-2011.12.15.9, PALULG-2011.12.15.10, PAL- ULG-2011.12.15.11, PALULG-2011.12.15.12, PALULG- -2011.12.15.13, PALULG-2011.12.14.23B. From Chèvremont and Vaux-sous-Chèvremont, both in Liège Province, Belgium, Montfort/Evieux Formation, Famennian, Upper Devonian. Chèvremont is the hill close to Vaux-sous-Chèvremont city. Old quarries and outcrops stretch between both localities. In old collections, there is often confusion between these two localities.

Fig. 2. Reconstructed dermal skeleton of the antiarch placoderm genera → found in Belgium. A. Bothriolepis canadensis , dorsal view, after Werdelin and Long (1986: fig. 2). B. Asterolepis maxima , dorsal view, after Traquair (1914). C. Grossilepis tuberculata , dorsal view, after Gross (1941).

D. Remigolepis walkeri , dorsal (D 1), lateral (D 2), and ventral (D 3) views, after Johanson (1997a). Abbreviations: CD1–4, plates of the dorsal central series; CV1, ventral central plate 1; DM1–4, plates of the dorso-medial marginal series; ML1–4, plates of the lateral marginal series; MM1–4, plates of the medial marginal series; VM1–3, plates of the ventro-medial marginal series.

Description

Headshield.—Partial headshield ( Fig.3A ):UCL-P.V.L.10.621 displays a partial headshield with the nuchal, paranuchal, premedian, and lateral plates in connection. It is badly preserved and the plate margins are difficult to observe. However it seems that the nuchal plate is excluded from the orbital fenestra. The postpineal notch forms a strong angle posteriorly, i.e., almost 90°. The central sensory line grooves and the principal section of infraorbital sensory line are well observed.

Trunkshield.— Anterior median dorsal plate, AMD ( Fig. 3B, C ): This plate is as long as wide. Externally, it is almost flat, with only a slight bulge on the dorsal median ridge. The latter is only defined by a longitudinal row of fused tubercles on PALULG-2011.12.14.16. The tergal angle is well marked and situated between the anterior and middle thirds of the plate. It is roughly of 45°. The posterior oblique dorsal sensory line grooves are usually well defined. On PALULG-2011.12.14.14, the right posterior oblique dorsal sensory line groove is shortened or interrupted. The posterior oblique dorsal sensory line grooves run from the tergal angle and cut the posterior division of the lateral margin at the junction between its anterior and middle thirds. The anterior margin is fairly straight, except on PALULG-2011.12.14.14 where it is slightly concave and not convex as Stensiö 1948: 514) noticed. It is twice the length of the posteri- or margin. The antero-lateral and lateral corners are well defined. The posterior division of the lateral margin is as long as the anterior division except on the biggest AMD PALULG-2011.12.14.14), where the posterior division is one third smaller than the anterior division. Only the right area overlapped by the mixilateral plate is observable (PAL- ULG-2011.12.14.14). It is quite small. The posterior lateral margin displays a sigmoid shape, as observed in a specimen of B. canadensis ( Stensiö 1948: fig. 182) and suggesting that the AMD both overlaps and is overlapped by the MxL. Internally, the AMD shows a triangular-shaped levator fossa stretching on all the anterior third length. It is limited laterally by fairly conspicuous postlevator thickenings, observable directly on PALULG-2011.12.14.16 and indirectly on impressions of the visceral surface (PALULG-2011.12.14.15 and PALULG-2011.12.14.17A). The postlevator cristae are quite sharp and high. Their confluence is located at the posterior edge of the anterior ventral pit. The median ventral ridge and ventral median groove are well marked.

Posterior median dorsal plate, PMD ( Fig. 3D ): The anteri- or margin is convex, with an obtuse median corner.The lateral process is well developed and extended. In fact, the margin between the lateral corner and postero-lateral corner is quite long compared to what is observed in Bothriolepis canadensis Stensiö 1948 : text-fig. 138A–R). The lateral margin situated in front of the lateral corner and running to the antero-lateral corner is straight. The posterior margin is convex, with an obtuse median angle. The plate is slightly arched with a low dorsal median ridge. The dorsal median ridge extends almost straight from the anterior angle to the posterior angle area.

Anterior dorso-lateral plate, ADL ( Fig. 3E ): Only the anterior part of a left ADL, in internal view, is preserved in the material. Both lateral and dorsal anterior laminae are present. The anteriormost part of the plate is hidden by a displaced AMD. The articulation area with the headshield is thus not observable. The ventral margin is convex on the preserved anterior part. The notch, for the external postlevator process of the AMD, is smooth and the postnuchal corner strongly developed. The dorso-medial margin is concave along all its preserved length. The anterior part of the lateral lamina is twice less wide than the median part. The dorso-lateral ridge is well pronounced, and the lateral lamina forms an angle of almost 150° with the dorsal lamina, but that value may be incorrect as a result of compression. The main lateral line groove is not directly observed, but suggested by a tiny edge running along the length of the lateral lamina that tends to join the dorso-lateral ridge anteriorly and to remain parallel to the dorso-lateral ridge posteriorly.

Mixilateral plate, MxL ( Fig. 3F ): As Stensiö (1948) noticed for Bothriolepis canadensis , the lateral lamina of MxL is often missing, crushed, compressed or damaged. It is the case in the available material of Bothriolepis lohesti , where the lateral lamina is crushed and damaged. Because of its bad preservation, it is difficult to evaluate the angle between the lateral and dorsal laminae. However, the specimen UCLP.V.L.10.668 shows quite a well preserved area of transition between these two laminae and suggests that the angle is approximately 150°, as noticed for the ADL, with a sharp prominent dorso-lateral ridge. The crushed lateral lamina, in that specimen, presents a broad area overlapped by the PVL. The area overlapped by the posterior median dorsal plate is long and narrow. The posterior oblique dorsal sensory line groove crosses the anterior part of the dorso-medial margin and ends in the posterior third of the dorsal lamina.

Anterior ventro-lateral plate, AVL ( Fig. 3G ): This plate is badly preserved, with the anterior part and the lateral lamina damaged. The length of this plate is at least twice the width. The angle between the ventral and lateral laminae is roughly of 160°, and the ventro-lateral ridge is smooth. The subcephalic division is not well preserved. The posterior margin of the ventral lamina is straight and the posterior ventro-lateral corner well defined. The different portions of the medial margin seem straight but this is not obvious, because they are badly preserved. The brachial process is almost entirely damaged but the axillary foramen is observable and quite small. Internally, the crista transversalis interna anterior is well developed, as well as the transverse thickening on the visceral surface and the depression between the two cristae.

Pectoral fin ( Fig. 3H ).—PALULG-2011.12.14.22 and PA- LULG-2011.12.14.21 present only a part of the pectoral fin proximal segment, in ventral view. It is a long segment with the estimated width being approximately one quarter the length. It is quite incomplete because only the ventral central plate 1 and lateral marginal plate 2 are clearly preserved and connected. The margins of these plates are not well defined and the presence or absence of the ventral central plate 2 cannot be checked. The mesial marginal plates 1 and 2 are not observable. The material also comprises isolated plates referred to CV1 and ML2. The ventral central plate 1 is more than twice longer than broad. The external ventral articular area is well developed. The ventro-medial margin is prominent on the largest specimen ( UCL-P.V.L. 10.656). Lateral spines are numerous and closely set. These spines are not very elongated but strong and quite sharp. Basally all the spines are fused to a lamina, forming a tiny lateral crest.

Ornament.— On the dorsal elements, the ornament is coarse, with well-marked tubercles. “Short pieces of ridges with very few anastomoses” can also be observed on the material. “The pieces of ridges are clearly nodose and somewhat vermiculating” ( Stensiö 1948: 515). The ornament is the same on all the plates. However, the ornament of the pectoral fin proximal segment, for specimens of the same size, can be either reticulate or nodose (PALULG-2011.12.14.21 and PALULG-2011.12.14.22), a retention of immature character already noticed by Stensiö (1948) for B.canadensis . The uniformity of the ornamentation and the equivalent size of all the plates suggest that all the material belongs to the same species.

Remarks.— On the AMD, the strongly developed ventral median ridge, the slightly marked dorsal median ridge and the lack of anterior oblique dorsal sensory line, indicate that these plates belong to an adult specimens ( Stensiö 1948). The relative breadth of the proximal segment of the pectoral fin and numerous few developed lateral spines argue for the same conclusion ( Stensiö 1948).

The remains of this taxon were first noticed by Lohest 1888: 60), who referred them to the genera “ Bothriolepis or Pterychtis ”, and then to the genus Bothriolepis ( Lohest 1889: 58) . Later, the same author ( Lohest 1895: 39) assigned these remains to Bothriolepis canadensis . They consisted of une tête complète, des organes natatoires et de nombreuses plaques dorsales et ventrales” ( Lohest 1895: 39), i.e., a complete head, swimming appendages and numerous dorsal and ventral plates. Finally, Leriche (1931) studied this material and found only several anterior median dorsal plates, dorso-lateral plates and pectoral fins. In the present study, I have considered the historical material examined by Lohest (1888, 1889, 1895), Leriche (1931), and new material in the Université de Liège and in the Université Catholique de Louvain-la-Neuve collections. There is no trace of the complete head, never figured, and of the ventral plates studied by Lohest 1895). Leriche (1931) did not assign the historical material to B. canadensis because of (i) the more anastomosed ornamentation of B. canadensis , (ii) the different internal side of the anterior median dorsal plate, and (iii) the less developed lateral spines of the lateral marginal plate 2 of B. canadensis . He therefore assigned this material to a new species, Bothriolepis lohesti , and concluded that B. lohesti is a relative of B. canadensis and Bothriolepis hydrophila . According to Stensiö (1948), B. lohesti resembles B. canadensis by the development of the ornament, whereas it recalls B. hydrophila by the general shape of the AMD.

In the adult stage, B. canadensis could reach a total armour length of 18–19 cm ( Stensiö 1948: 224), which corresponds approximately to a trunk armour length of 12–13 cm. This is different from B. lohesti , which reached a maximum trunk armour length of 8 cm. The shape of the anterior median dorsal plate of B. lohesti is quite similar to that of B. canadensis except that the dorsal median ridge is more pronounced in B. canadensis . Internally, and as noticed by Leriche (1931: 17), the postlevator cristae of B. lohesti are rather sharper than those of B. canadensis . Concerning the posterior median dorsal plate, both plates of both species are quite similar, except that the lateral process of B. lohesti is longer and more extended. On the anterior dorso-lateral plate of B. canadensis , the notch for the external postlevator process of AMD is pronounced. This is not the case for B. lohesti , where this notch is smooth. Leriche (1931: 18) argues that the spines of the pectoral fin lateral margin are more developed in B. lohesti than in B. canadensis . According to the variability shown by B. canadensis in this respect, it seems that several specimens of this species present spines that are similar to those observed in B. lohesti (i.e., Stensiö 1948: 355, fig. B). This criterion can consequently not discriminate the two species. Concerning the ornament, Stensiö (1948: 515) argued that the development of the ornament of B. lohesti “reminds mostly of large individuals of B. canadensis ”. In fact, in the adult stage, both can present a coarse ornament with tubercles and short pieces of ridges with very few anastomoses on the dorsal elements. Even if the adult ornament of B. canadensis can be more complicated, with frequent stellate tubercles at their bases, in general the ornament in both species is quite similar. Concerning the ornament on the proximal segment of the pectoral fin, B. canadensis often retains immature, reticular characters ( Stensiö 1948). It is also observed for B. lohesti , with proximal segments showing a reticulate or nodose ornament (PALULG-5011 and PALULG-5012).

Bothriolepis hydrophila was certainly described on the basis of juvenile material ( Stensiö 1948; Miles 1968). The comparison with other Bothriolepis species is therefore difficult. The trunk armour length of that species is maximally about 9 cm ( Stensiö 1948). This corresponds to the trunk armour length of B. lohesti , which is of 8 cm. Contrary to the quite flat dorsal wall of B. lohesti , the dorsal wall of B. hydrophila is somewhat elevated, with a strongly developed dorsal median ridge. In both species, the anterior median dorsal plate is approximately as long as broad and the tergal angle is situated between the anterior and middle thirds of this plate. In both species, the postnuchal notch and external postlevator process are slightly developed or absent. The anterior median dorsal plate of B. lohesti overlaps the mixilateral plate but is also overlapped by that plate. This pattern is also observed in certain specimens of B. hydrophila ( Stensiö 1948: 508, fig. 262B; Miles 1968: fig. 30). The posterior median dorsal plate of B. lohesti possesses stronger and more extended lateral processes than in B. hydrophila . The lateral spines of the pectoral fin proximal segment of both species are quite similar, i.e., broad and rather sharp. They are more numerous in B. lohesti than in B. hydrophila . Concerning the ornament, it is of a reticulate type and of uniform distribution in B. hydrophila . In one specimen ( Miles 1968: pl. 19: 5) the ornament is both reticulate and tubercular. The ornament of B. lohesti is tuberculate but corresponds to the ornament of adult specimens, contrary to that of B. hydrophila .

Lukševičs (2001) noticed that B. lohesti and Bothriolepis jani Lukševičs, 1986 differ in that the AMD is less arched and the ornament consists of tubercles and nodose ridges in B. lohesti . He also noticed that the two species are similar in the shape and proportions of the AMD and in the sutural connections between AMD and MxL and concluded that they may be phylogenetically very close. The new material of B. lohesti , studied in this article, brings new insights in the comparison of the two species. B. lohesti differs also from B. jani by (i) the postpineal notch of the nuchal plate, which forms a strong angle posteriorly (rounded in B. jani ), (ii) the well developed lateral processes of the PMD, (iii) the less arched dorsal median ridge (on the AMD and on the PMD), and (iv) the well developed postnuchal corner of the ADL. Therefore, the new material studied here slightly modifies Lukševičs’ (2001) conclusion: both species seem phylogenetically less close than previously thought.

The material characterising B. lohesti has been complet- ed with new specimens in collections. Even if this material is poor, and B. lohesti is only known by some elements of the trunk armour and pectoral fin, this species appears quite different from the other species of Bothriolepis , e.g., B. canadensis , B. jani , and B. hydrophila , and its status is not reconsidered. B. hydrophila is defined on juvenile material and, as noticed by Stensiö (1948) and Werdelin and Long (1986) for B. canadensis , ontogenetic changes are numerous from juvenile to adult. Therefore it is injudicious to use such material for morphological comparisons. However, B. hydrophila continues to be widely used for comparisons with other Bothriolepis species defined on adult characters and is included in phylogenetic analyses ( Young 1988; Lukševičs 2001), whereas the data included in the matrix might reflect the growth stage. More data on adult material is necessary for comparisons with B. lohesti .

Stratigraphic and geographic range.—Famennian, Upper Devonian of Belgium.

Figures

Fig. 3. Dermal plates of antiarch placoderm Bothriolepis lohesti Leriche, 1931, from Vaux-sous-Chèvremont and Chèvremont (Liège Province), Famennian, Devonian. A. UCL-P.V.L.10.621, partial headshield, external view. Photograph, whitened with ammonium chloride (A 1), explanatory drawing (A 2). B. PALULG-2011.12.14.14A, AMD, external view. C. PALULG-2011.12.14.17A,AMD, internal view. D. PALULG-2011.12.14.18, PMD, external view. E. PALULG-2011.12.14.17B, ADL, internal view. F. UCL-P.V.L.10.668, MxL, external view. G. PALULG-2011.12.14.20, AVL, external and internal views. H. PALULG-2011.12.14.22, pectoral fin, ventral view. Scale bars 10 mm.