Bothriolepis obrutschewi Gross, 1942

Lukševičs, Erwin, 2001, Bothriolepid antiarchs (Vertebrata, Placodermi) from the Devonian of the north-western part of the East European Platform, Geodiversitas 23 (4), pp. 489-609 : 510-517

Bothriolepis obrutschewi Gross, 1942

( Figs 7-10 )

Bothriolepis obrutschewi Gross, 1942: 416-418 , abb. 7B.

HOLOTYPE. — MxL Riksmuseet, Stockholm.

MATERIAL EXAMINED. — LGI 5/2248, complete headshield with articulated anterior part of the trunk armour and proximal segments of the pectoral fin; 5/2249, articulated ventral wall of the trunk armour with proximal segments of the pectoral fin and headshield in visceral view; 5/2255, 2345, 2598, 2647, articulated head-shields; 5/2213-2235, 2238-2241, 2243, 2247, 2250-2252, 2255-2260, 2262, 2263, 2274-2284, 2288, 2320-2325, 2341-2345, 2350-2354, 2356-2382, 2416-2445, 2447-2450, 2452-2457, 2459-2482, 2484, 2485, 2487-2491, 2498, 2500-2531, 2533-2539, 2552, 2560-2582, 2584-2601, 2603-2615, 2622-2647, 2652- 2656, 2659-2661, 2664-2669, 2672, 2674-2683, 2685, 2687-2689, 2697, 2698, LDM 43/1004, articulated head shield; 43/585-671, 43/722: disarticulated plates of the armour from Pastamuiža locality. LGI 5/1099- 1101, AMD from Pelyša locality. LGI 5/1216-1222, disarticulated plates of the armour; Paroveja borehole. All this material collected by V. KaratajūteTalimaa and L. Lyarskaja. LDM 307/1-10, AMD, PMD, ADL, 2 MxL, Nu, 3 CD1 from Pērse locality, samples gathered and presented to LDM by amateur collector Jānis Blesse.

LOCALITIES AND HORIZON. — Pastamuiža locality on the right bank of Daugava River; outcrop on the Pērse River near Koknese close to the type locality, exposure on the left bank of Daugava River near Zvirgzdi, Latvia; Upper Devonian, lower Frasnian, upper part of the Amata Formation. Pelyša River; Paroveja borehole, 49.65-49.75 m deep, Lithuania; uppermost part of the Šventoji Formation. Piskovichi and YamTesovo, Russia; Podsnetnaya Gora Member of the Yam-Tesovo Formation. Out of the Main Devonian Field this species have been reported ( Ivanov & Lukševičs 1996) from the Middle Timan, Uste Chirka Formation and lowermost Uste Srednyaya Beds of the Uste Yarega Formation; and with slight doubt from the North Timan, Kumushka Formation. Probably, the same species occurs in lower part of the Matusevich Formation of Severnaya Zemlya (Lukševičs 1999).

DIAGNOSIS. — Bothriolepis of moderate size with a median dorsal armour length reaching 80-85 mm. B/L index of the head-shield 127. Preorbital recess of simple type. Weakly convex rostral margin slightly short- er than the posterior margin. Orbital margin of Prm is straight, without nasal notches. Rostral margin of Prm weakly convex. Anterior and posterior margins of orbital fenestra straight, lateral division of Pn narrow, and Pmg elongated. Dorsal wall of trunkarmour low and relatively narrow (B/L index 86), tergal angle weakly defined, and situated in anterior part of middle third of AMD. Median dorsal ridge on AMD present only in very small individuals, and more clearly defined in PMD. AMD B/L index about 94. Anterior division of the lateral margin is more than twice as long as the posterior division. Overlap area for MxL often of Remigolepis - type. PMD B/L index about 91. Dorsal and lateral laminae of ADL enclosing an angle of about 121°. Angle between the dorsal and lateral walls on MxL about 114°. Lateral and ventral walls enclosing an angle about 119° on PVL. Lateral lamina of PVL is relatively high with well-defined dorsal corner. MV is small. Proximal segment of the pectoral appendage of moderate length, 3.8-4.5 times as long as it is broad. Distal segment is relatively long and narrow, 5-5.4 times as long as it is broad. Mesial spines on the proximal segment, as well as dorsal and ventral spines on the distal segment are well-defined, separate and relatively long. Lateral spines on the proximal segment are long and fused in their base forming well-defined ridge. Ornamentation is always reticulate on the ventral wall. Ornamentation of the head-shield and dorsal wall of the trunk-armour, as well as dorsal surface of the proximal segment is tuberculate, which is not always well-defined.

DESCRIPTION

The head-shield in small individuals ( Figs 7B ; 8A ) has a large orbital fenestra and short anteri- or division (Karatajūte-Talimaa 1966). In larger individuals ( Figs 7A, C, D ; 8B ), the head-shield is moderately broad with B/L index about varying from 123 to 131. In general, the head-shield of B. obrutschewi is narrower than that of B. cellulosa and B. prima (Karatajūte-Talimaa 1966) . She shown the rostral margin of the head-shield sometimes bearing the rostral angle, almost straight posterior margin, gently defined anterolateral corners (alc) and prelateral notch (nprl). Karatajūte-Talimaa (1966) noted that the obtected nuchal area (nm) is relatively broad, well-defined only on the Nu, but I found that in some cases it extends also into Pn. The shape of the preorbital recess was not clearly shown by Karatajūte-Talimaa (1966); it is well seen in a newly prepared specimen LDM 43/1004 being of simple type.

The Prm is moderately broad, the B/L index changing from about 134 in smallest individual with the Prm 6 mm long to 94-95 in well-grown individuals with 16-18 mm long Prm. The rostral angle (ac) is present only on the two large Prm. The orbital margin is 1.4-1.8 time shorter than the rostral margin.

The La is moderately broad with the L/B index 130-173, 148 on the average. The rostral margin is almost straight, the antero-median and anterolateral corners are well-defined ( KaratajūteTalimaa 1966). The infraorbital sensory line groove crosses the plate in its anterior part not far from the rostral and lateral margins. The central sensory line groove (csl) usually finishes at the level of the orbital margin of the orbital fenestra. The visceral plate surface shows the antero-lateral corner of otico-occipital depression (pr.po) extending forward slightly over the middle of the orbital fenestra. The transverse lateral groove (tlg) is broad and clearly defined. The lateral pit (p) is broad, shallow and situated equally spaced from an orbital edge of the La and prelateral notch.

The Nu is relatively broad with a L/B index 57- 74, 66 on the average. The plate is usually broadest across the lateral corners. The anterior division of the lateral margin usually is straight and a little shorter than the strongly concave posterior division. The posterior margin is usually almost straight, but in LGI 5/2654 it is strongly concave. It always bears the median posterior process (mppr). The central sensory line groove is clearly distinct.

The Pn is of moderate breadth, L/B index about 74-97, 84 on the average. As it was stressed by Karatajūte-Talimaa (1966), the lateral division of the Pn in B. obrutschewi is comparatively narrower than that in B. prima , B. panderi and especially B. cellulosa and comprises 42-55% (48% on the average) of the general breadth of a plate. The median division of the plate is broad. The Pmg is moderately broad with the lateral margin slightly longer than the median margin (Karatajūte-Talimaa 1966).

The Sm (extralateral) plate is known from one specimen, LGI 5/2230 (Karatajūte-Talimaa 1966: pl. IX, fig. 12). It is relatively long with a L/B index of about 250. The dorsal margin has a prominent antero-dorsal process and narrow posterior attachment area for the skull. The posterior margin is strongly convex, the ventral margin is weakly convex. The well-marked lateral notch is rather deep separating the plate into a short thick anterior division and more thin posterior division. There is a groove running along the dorsal margin of the ornamented part of the plate, which is similar to that of B. macphersoni Young, 1988 . It has the character of a sensory groove.

The trunk-armour ( Figs 9 ; 10 ) is moderately broad as it was reconstructed by KaratajūteTalimaa (1966), weakly arched anteriorly and more arched posteriorly with B/L index 86. The length of the dorsal wall, probably, is more than 110 mm. The ventral wall is not quite flat, but slightly arched in rostrocaudal direction. The subcephalic division of the ventral wall is relatively narrow and short. The subanal division of moderate length. The median dorsal ridge is weakly defined, in well-grown individuals it is developed only on the PMD. The dorso-lateral and ventro-lateral ridges are well-marked.

The AMD ( Fig. 7A ) is moderately broad, B/L index about 89-98, 94 on the average. This plate was well described by Karatajūte-Talimaa (1966). The anterior part of the plate is weakly arched, the posterior part is flattened. The anterior margin may be weakly convex or less often fairly straight. It is relatively narrow. The antero-lateral and lateral corners, as well as the postlevator processes are well-defined. The posterior division of the lateral margin is 1.5- 2.3 times shorter than the relatively long anterior division. The median dorsal ridge is weakly defined as a longitudinal row of fused tubercles. Overlap areas for the MxL are normally developed as usually in Bothriolepis , but in LGI 5/2215, 5/2639, 5/2697, the AMD overlaps the MxL by its anterior part of the posterior division of the lateral margin similar as in Remigolepis . There are some cases the ADL overlaps the AMD in the posterior portion of the common suture: LGI 5/2639, LDM 43/656. The anterior (dlg1) oblique dorsal sensory line grooves are well-defined only on the plates of individuals of small size: LGI 5/2251 and 5/2252 with the length of the AMD 13 and 18.4 mm respectively. The posterior (dlg2) oblique dorsal sensory line grooves usually are well-defined. In some cases the posterior oblique dorsal sensory line grooves are shortened or interrupted ( LGI 5/2639), in one specimen ( LDM 307/1) there are two parallel branches of this groove on the left side of the plate (pers. observation).

The visceral surface of the AMD shows a slightly lengthened triangular-shaped levator fossa, which is limited by the low postlevator thickenings and in a posterior part also by the postlevator cristae. The anterior ventral pit and the median ventral ridge are well-defined.

The PMD ( Fig. 7E ) is moderately broad, B/L index about 82-101, 91 on the average. The posterior margin is usually strongly convex, with pronounced posterior corner, but there are specimens with rounded posterior corner noticed. The width of the anterior margin varies between 48-57% of total breadth, it is relatively broader than that of B. prima , but narrower than that of B. cellulosa (Karatajūte-Talimaa 1966) . The lateral corners are well-defined, the postero-lateral corners are often rounded. The median dorsal ridge is present only in the posterior half of the plate in well-grown individuals (Karatajūte-Talimaa 1966). The median ventral ridge and median ventral groove with short posterior ventral pit (pt2) are well-defined on the visceral surface of the plate. The crista transversalis interna posterior (cr.tp) is normally developed, the postmarginal area (pma) is rather broad.

The ADL is relatively broad, the dorsal lamina is 1.9-2.1 times (2.4-2.5 by Karatajūte-Talimaa 1966) as long as it is broad and its breadth 1.2- 1.5 time exceeds height of the lateral lamina. The dorso-lateral ridge (dlr) is well-defined. The postnuchal ornamented corner (pnoa) is sharp, moderately long and narrow. Specimen LGI 5/2379 shows the dorsal overlap area partly overlapping the AMD plate. The processus obstans is strongly developed ( KaratajūteTalimaa 1966).

The MxL is moderately broad. The dorsal lamina of the plate is 1.4-1.6 time as long as it is broad. The dorso-lateral ridge is well-defined. The posterior oblique sensory line groove (dlg2) terminates close to the lateral margin, in some distance from the dorso-ventral pit-line groove (dxp) crossing the dorso-lateral ridge. Specimen LDM 307/6 has no posterior oblique sensory line groove. The overlap area for the AMD is often restricted to half the length of the anterodorsal margin (Karatajūte-Talimaa 1966) ( LGI 5/2485, 5/2490, 5/2491), as in Remigolepis ( Stensiö 1931) .

The ventral lamina of the AVL is 1.6-1.8 time as long as it is broad. The subcephalic division is short and comprises about 20% of total length of the ventral lamina. The antero-lateral corner is situated slightly medially the axis of the ventro-lateral ridge. The ventral lamina is 2.9- 3 times as broad as the low lateral lamina high. The right AVL overlaps the left AVL. The axillary foramen (f.ax) is relatively large and rounded or slightly elongated in shape (Karatajūte-Talimaa 1966). The visceral surface of the AVL shows the high transverse anterior crista running almost mesially subparallel to the gently defined low and broad transverse thickening (Karatajūte-Talimaa 1966), but nevertheless a sharp angle between cit1 and cit2, unlike as in B. ciecere or B. karawaka Young, 1988 is clearly seen.

The PVL has similar proportions to the AVL, as in most Bothriolepis species. It is of moderate breadth, the ventral lamina is 2.1-2.6 times as long as it is broad. The subanal division is relatively broad (Karatajūte-Talimaa 1966), it occupies about one forth (23-28%) of the total PVL plate length. The lateral lamina is moderately high, it is relatively higher than that in B. prima (Karatajūte-Talimaa 1966) ; the ventral lamina is 1.2-1.3 time as broad as the lateral lamina high. The ventro-lateral ridge (vlr) is well-defined.

The pectoral fin is represented by many disarticulated bones, and five specimens showing articulated plates of the proximal segment. There are 14 examples of the distal segment or it fragments. The CV1 contacts the MM2. The CD1 is of moderate size with L/B index varying from 2.7 to 3.1 (2.8 on the average). The CV1 is slightly longer than the CD1 and have the L/B index about 3.1-3.4 (3.2 on the average). The ML2 is 4.2-4.4 times as long as it is broad. The distal segment shows only CD3. Both segments bear prominent lateral and mesial spines. On the proximal segment the isolated mesial spines are large and high.

The ornamentation is generally tubercular with coarse tubercles and short vermiculated ridges of fused tubercles on the head-shield in wellgrown individuals (Karatajūte-Talimaa 1966). The short radially arranged ridges are developed along the margins of the La and Nu. The network of anastomosing ridges, which are broken into short ridges could be sometimes seen on the posterior part of the Prm. The ornament on the Sm consists of the network with weak elevations in the points of anastomoses on the central part of the plate and is almost smooth on the posterior part. The ornament is typically tubercular in general on the dorsal and lateral walls of the trunk-armour and reticular on the ventral wall (Karatajūte-Talimaa 1966). The tubercles are usually relatively low, often fused forming the short, sometimes vermiculated ridges. The tubercles and ridges are arranged into rows, parallel to the mesial margin of the ADL and anterior margin of the MxL. The ornamentation is fine-tuberculated on the lateral wall of the trunk-armour. The ornamentation of the pectoral appendage also is variable. It is reticulate in general, on the CD1 the ornament consists of radially arranged ridges of fused or isolated tubercles. The network of ridges bears weak elevations in the points of anastomoses on the ML2 and CV1. The distal segment bears the longitudinal well-defined ridges on the dorsal wall. The ventral wall of the distal segment is almost smooth with gently defined ridges in its proximal part.

REMARKS

The above account is the first full treatment in English, which supplements KaratajūteTalimaa’s description (in Russian) of Bothriolepis obrutschewi (1966) , using specimens from the collection of the Latvian Museum of Natural History and adding some details such as structure of the visceral surface of the head shield.

DISCUSSION

Bothriolepis obrutschewi resembles B. prima , but may be distinguished by 1) its narrower headshield; 2) relatively broader dorsal and ventral walls of the trunk-armour; 3) relatively longer subanal division of the PVL; 4) shape of the postlevator cristae on the visceral surface of the AMD; 5) proportions of the ADL; 6) more strongly defined tubercular ornament; 7) development of the central sensory line groove (csl) and the anterior oblique dorsal sensory line groove (dlg1) in smallest individuals. B. obrutschewi differs from B. evaldi in its 1) much broader dorsal wall of the trunk-shield; 2) shape and proportions of the AMD; 3) shape of the postnuchal ornamented corner of the ADL; 4) narrower ventral wall of the trunk-shield; 5) shorter proximal segment of the pectoral fin.

B. obrutschewi resembles B. panderi by the ornamentation, but differs from it in 1) shape and proportions of the head shield; 2) proportions of the lateral division of the Pn, which is narrower in B. obrutschewi ; 3) broader AMD; 4) relatively narrower anterior margin of the AMD; 5) relatively shorter pectoral fin.

B. obrutschewi can be distinguished from B. cellulosa ( Gross 1941) , other lower Frasnian Bothriolepis representative from the Main Devonian Field in 1) its smaller size; 2) tubercular ornamentation; 3) narrower head-shield; 4) shape of the Prm possessing more long orbiral margin; 5) course of the infraorbital sensory line groove on the Prm and La; 6) shape of the Nu; 7) narrower lateral division of the Pn; 8) longer Sm; 9) features of the visceral surface of the trunk armour; 10) CV1, which is making contact with the MM2.

Figures

FIG. 7. — Bothriolepis obrutschewi Gross, 1942; A, head-shield and AMD, probably belonging to the same individual LGI 5-2345; B, head-shield of small individual LGI 5-2255; C, D, head-shield LDM 43/1004, in visceral and dorsal views; E, PMD LDM 43/635. Daugava River near Pastamuiža, Latvia. Amata Formation. Abbreviations: AMD, anterior median dorsal plate; PMD, posterior median dorsal plate. Scale bars: 10 mm.

FIG. 8. — The head-shields of Bothriolepis obrutschewi Gross, 1942; A, head-shield of small individual LGI 5-2255; B, headshield of well-grown individual LGI 5-2345. From Karatajūte- Talimaa 1966: figs 6, 1, 2. Daugava River near Pastamuiža, Latvia.Amata Formation. Abbreviations: csl, central sensory line groove; csl1, additional branchs of central sensory line groove; ifc1, principal section of infraorbital sensory line; ifc2, branch of infraorbital sensory line diverging on La; La, lateral plate; mp, middle pit-line groove; soc, anterior section of the supraorbital sensory line. Scale bars: A, 5 mm; B, 10 mm.

FIG. 9. — Bothriolepis obrutschewi Gross, 1942, trunk armour, based on the AMD LGI 5-2632, PMD LGI 5-2561, right ADL LGI 5- 2377, left ADL LGI 5-2363, left MxL LGI 5-2498, right AVL LGI 5-2504, left AVL LGI 5-2511, left PVL LGI 5-2252 in lateral, ventral and dorsal views (plasticine reconstruction produced by V. Talimaa). Daugava River near Pastamuiža, Latvia. Amata Formation. Abbreviations: ADL, anterior dorso-lateral plate; AMD, anterior median dorsal plate; AVL, anterior ventro-lateral plate; MxL, mixilateral plate; PMD, posterior median dorsal plate; PVL, posterior ventro-lateral plate. Scale bar: 10 mm.

FIG. 10. — The trunk armour of Bothriolepis obrutschewi Gross, 1942 reconstructed by Karatajūte-Talimaa (1966: fig. 10); A, dorsal view; B, lateral view; C, ventral view. Daugava River near Pastamuiža, Latvia. Amata Formation. Abbreviations: ADL, anterior dorsolateral plate; AMD, anterior median dorsal plate; AVL, anterior ventro-lateral plate; MV, median ventral plate; MxL, mixilateral plate; PMD, posterior median dorsal plate; PVL, posterior ventro-lateral plate; SM, semilunar plate; dlg2, posterior oblique dorsal sensory line groove; dlr, dorso-lateral ridge of trunk armour; dmr, dorsal median ridge of trunk armour; dxp, dorso-ventral pit-line groove; f.ax, axillary foramen; lcg, main lateral line groove; p.br, processus brachialis; vlr, ventro-lateral ridge of trunk armour.Scale bar: 10 mm.