Machaeracanthus goujeti, Botella & Martínez-Pérez & Soler-Gijón, 2012

Botella, Héctor, Martínez-Pérez, Carlos & Soler-Gijón, Rodrigo, 2012, Machaeracanthus goujeti n. sp. (Acanthodii) from the Lower Devonian of Spain and northwest France, with special reference to spine histology, Geodiversitas 34 (4), pp. 761-783 : 765-774

publication ID	https://doi.org/ 10.5252/g2012n4a3
persistent identifier	https://treatment.plazi.org/id/145E87DC-4567-F10F-FCA6-DD9784698B95
treatment provided by	Marcus
scientific name	Machaeracanthus goujeti
status	sp. nov.

Treatment

Machaeracanthus goujeti n. sp. ( Figs 3-10 View FIG )

Machaeracanthus sp. – Goujet 1976: 313, pl. 61, figs 3-17; pl. 63, fig. 1a, b; text-fig. 54A-E. — Wang 1993: 143, pl. 14, fig. 13. — Fernández-Herrero et al. 2009: 180, figs 2, 3. — Burrow et al. 2010b: 60.

Machaeracanthus sp. A – Mader 1986: 30, pl. 3, figs 3-5, 7-11; text-figs 10c, b; 11a, b. — Wang 1993: 141, pl. 15, figs 1-8; text-fig. 21E-H. — Botella & Valenzuela-Ríos 2005: 126, fig. 11.

Machaeracanthus ? sp. – Mader 1986: 31, pl. 1, fig. 8.

Machaeracanthus sp. B – Wang 1993: 143, pl. 15, figs 9, 10.

Machaeracanthus stonehousensis – VALENZUELA- RÍOS & BOTELLA 2000: 150, text-figs 3, 6.

HOLOTYPE. — MPZ 2010 View Materials /948 ( Fig. 4A); a complete spine, c. 53 mm long, slightly damaged at distal end, preserved in a marly sandstone slab from the late Lochkovian (Nogueras Fm.) of the Locality Poyales-East.

PARATYPES. — A complete spine, 50 mm long, from the late Lochkovian of the Nogueras Fm. ( MPZ 2009 View Materials /28, Fig. 4B) from the Mariplo Locality ( ADRC) ; and scales including MGUV-15.075 to MGUV-15.082 and MPZ 2010 View Materials /949 ( Fig. 9 View FIG ) from the Localities Barranco Sur Santo Domingo and Poyales-East ( ADRC) and Nigüella ( NI), Nogueras Fm .

ETYMOLOGY. — In honour of Dr. Daniel Goujet for his important contributions to the knowledge of the early vertebrates. He described for the first time, scales now assigned to M. goujeti n. sp. from the Lower Devonian of Saint-Céneré ( France).

TYPE LOCALITY AND HORIZON. — Poyales-East, Axial Depression of the Río Cámara ( ADRC), Iberian Chains ( Spain). Lower Devonian, Late Lochkovian (unit d2aβ5).

GEOGRAPHICAL AND STRATIGRAPHICALDISTRIBUTION. — In Celtiberia the stratigraphical distribution of Machaeracanthus goujeti n. sp. ranges from the Upper Middle Lochkovian to the Lower Pragian, submembers d2aα to d2cβ (Nogueras Fm.). Machaeracanthus goujeti n. sp. has also been recovered from the Lochkovian and Pragian of Saint-Céneré Fm. (northwest France), from the Upper Lochkovian and Lower Pragian of Lebanza Fm. in Palencia and La Vid Fm. in León (Cantabrian Mountains, North Spain).

MATERIAL EXAMINED. — More than 1000 scales, more than 50 spines and 5 scapulocoracoids from the localities Barranco Sur Santo Domingo, Poyales-East, Maripló and Viñas sections from ADRC (see Carls 1988; Dojen 2005) and sections Ni-2 and Ni-4 from NI (Valenzuela-Ríos 1989; Valenzuela-Ríos & Botella 2000).

DIAGNOSIS. — Machaeracanthus species with relatively slender spines with a maximum width to length ratio c. 1:7; two morphotypes can be distinguished, both showing narrow lateral expansions (keel and wing) of similar width (upper surface view); morphotype 1 is represented by spines with longitudinal striation mainly on the proximal third, and showing, in cross section, a triangular to sub-triangular longitudinal ridge on the upper surface and a more rounded longitudinal ridge on the lower surface; morphotype 2 is represented by densely striated spines which present, in cross section, a subsquare-shaped longitudinal ridge on the upper surface and a rounded and broader longitudinal ridge on the lower surface. Scales with eight to twelve ridges which converge from the anterior part to the centre of the crown and diverge posteriorly; the ridges extending behind the upper part of the neck, never reach the base; neck is pronounced, presenting a slight narrowing; base moderately convex.Mesodentine forms most of the crown, and the base is formed by cellular bone with bone cell lacunae arranged parallel to growth lines.

DESCRIPTION

Spine morphology ( Figs 3 View FIG ; 4 A-K; 5)

More than 50 spines, from nearly complete to very fragmentary specimens, were studied.The preserved material indicates a wide range of sizes suggesting the assemblage of remains belonging to individuals of different ontogenetic stages (see below). The smallest complete specimen ( Fig. 3 View FIG ;MPZ 2010/950) is a spine 1 cm long and 2.6 mm in maximum width (maximum width to length ratio c. 1:4, characteristic of juvenile individuals; see discussion below). The holotype and paratype MPZ 2010/948 and MPZ 2009/28 ( Fig.4A, B), represented by nearly complete specimens, are very similar in size, about 53 mm long and 7.5 mm in maximum width (maximum width to length ratio 1:7, a ratio considered as characteristic of adult individuals and included in the diagnosis of the new species).However, several large fragmentary spines indicate the presence in the locality of bigger specimens. For example, specimen MPZ 2010/951 and MPZ 2009/26 (see Fernández-Herrero et al. 2009: fig. 2B) are about 1.5 cm in maximum width pointing to complete spines more than 10 cm long. All the spines are asymmetrical, curved posteriorly and have a characteristic saber shape with a thick central axis (the body of the spine) and two narrow lateral expansions, an anterior keel and a posterior wing.As shown in specimen MPZ 2010/950 ( Fig.3 View FIG ), the lateral expansions start close to the distal termination of the spine, gradually increase their width in proximal direction to reach the maximum and then decrease in width to the proximal termination of the spine. Upper surfaces of keel and wing exhibit a similar width with the exception of the most proximal part of the spine where the keel is narrower than the wing. Concerning the lower side, the keel is narrower than the wing along the whole spine.Two different morphotypes can be recognised according to the morphology in transverse section of the upper and lower parts of the central axis (upper and lower longitudinal ridges following the terminology of Burrow et al. 2010b) and the development of longitudinal striation. The morphotype 1, including the holotype MPZ 2010/948 ( Figs 3 View FIG ; 4A, F-I), is characterised by spines with, in cross section, a triangular to sub-triangular upper longitudinal ridge and a more rounded lower longitudinal ridge on the lower surface, with some spines having a slightly asymmetrical triangular upper ridge ( Figs 4K; 5A, B View FIG ). The longitudinal striation on the upper side occurs in the proximal third of the spine whereas in the lower side it extends for one third or more of the spine ( Figs 3 View FIG ; 4 F-I; 5A-C).Morphotype 2, including paratype MPZ 2009/28 ( Fig. 4B), is characterised by having a narrowly subsquare-shaped upper longitudinal ridge and a rounded and broader lower longitudinal ridge ( Figs 4J; 5D View FIG ). A dense longitudinal striation occurs along the upper and lower sides of the complete spine except along the most distal part ( Fig. 4 C-E). The upper surfaces of the spines show a longitudinal groove at the junction of the upper longitudinal ridge and the lateral expansions that in some elements appears as a secondary expansion or double edge (more marked on the keel joining) in cross section.

Spine histology ( Figs 5-8 View FIG View FIG View FIG View FIG )

The histological structure of the spines changes from distal to proximal ends ( Figs 5-8 View FIG View FIG View FIG View FIG ). A pulp cavity extends along the central axis of the spine ( Fig. 5 View FIG ); the cavity is very narrow in the distal part of the spine, becomes wider proximally and opens at the most proximal end of the spine ( Fig. 3A, C View FIG ). At the proximal part of the spine, the pulp cavity is higher than wide (transverse section view) in morphotype 1 and nearly rounded to wider than high in the morphotype 2 ( Fig. 5 View FIG ).

The wall of the central axis (body of the spine) and lateral expansions (keel and wing) consist of trabecular and lamellar dentine.

Centripetally growing trabecular dentine surrounds the pulp cavity in the distal and middle parts of the spine ( Figs 5A, B, D View FIG ; 6A View FIG ; 7A View FIG ). Centrifugally growing trabecular dentine surrounds the pulp cavity proximally ( Figs 5C View FIG ; 8A View FIG ), covers the centripetally growing trabecular dentine in the mid- and distal parts of the spine and extends laterally to form the internal and main part of the keel and wing ( Figs 5 View FIG ; 6A View FIG ; 7A View FIG ; 8 View FIG ). There is no evidence of sharp structural discontinuity between centripetal and centrifugal trabecular dentines, but both hard tissues can be easily distinguished by the difference in the dimension of the intertrabecular spaces (see below). In addition, growth marks in the trabecular dentine are also less apparent than in the lamellar dentine.

Centrifugal lamellar dentine with clear growth marks covers the centrifugal trabecular dentine at least in the distal half of the spines (morphotypes 1 and 2) ( Figs 6 View FIG ; 7A, B View FIG ).

Vascular pattern of the centripetally growing trabecular dentine is extremely regular as observed in transverse section ( Figs 5A, B, D View FIG ; 6A View FIG ; 7A View FIG ). Rows of round cavities and fairly straight canals radiate from the pulp cavity and connect with the centrifugal trabecular dentine. The round cavities represent the transverse sections of longitudinal canals. In general view, the intertrabecular spaces are wider in the centripetal dentine than in the centrifugal one.

Denteons around the vascular canals and interdenteonal areas can be distinguished in sections with less taphonomic alteration ( Fig. 7 View FIG ). Dentinal tubules radiating from the vacular canals form a dense network in the borders of denteons and in the interdenteonal areas ( Fig. 7B, C View FIG ). Numerous interglobular spaces appear in the interdenteonal areas ( Figs 7C View FIG ; 8B View FIG ). The spaces are usually filled by opaque authigenic minerals and present evidence of severe alteration postmortem in some regions of the spines ( Fig. 8B View FIG ; see discussion below).

The most superficial layer of dentine, centrifugally deposited, is pierced by dentinal tubules that exhibit their finer distal branches in centripetal direction ( Fig. 6B, C View FIG ). Consequently, there is no evidence of hypermineralized enameloid.

Scale morphology ( Fig. 9 View FIG )

Scales are large, 1 to 2 mm long and wide, and 0.6 to 1.5 mm height. Largest specimens are up to 2.4 mm long. The crown is flat, nearly parallel to the interface between the base and neck and extends posteriorly beyond the base ( Fig. 9B, G, H View FIG ). The crown is ornamented with a variable number of ridges (8-12) which rise from the upper part of the neck, more or less parallel along the rostral margin and then converge to the center of the crown. On

the posterior crown, when it is preserved, ridges diverge as radial ridges towards the caudal edge of the crown ( Fig. 9A, C, E, G, L View FIG ), which is divided into eight to twelve long parallel denticulations, with each of these denticulations corresponding to the end of a ridge ( Fig. 9H, J, L View FIG ). However, as the posterior part of the crown is thin and delicate, denticulation at the caudal margin is broken or damaged in most of the specimens ( Fig. 9A, C, E View FIG ). The anterior margin of the crown is rounded ( Fig. 9A, C, E, G View FIG ). The neck is pronounced and presents a slight narrowing between the crown and the base ( Figs. 9D, F, J View FIG ). The base varies from low to moderately convex and protrudes rostrally in front of the anterior edge of the crown ( Fig. 9A, C, E, L View FIG ). The shape of the base is rhomboidal and smaller than the crown ( Fig. 9B, H View FIG ). Small vascular canal openings can be observed in the neck.

Some scales have a long and narrow crown, with a long neck; in these scales the ridges are strongly

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cfld dt dt igsp vc ad marked and do not follow the pattern of convergence to the center, but run subparallel along the crown ( Fig. 9I View FIG ). These forms are similar to some figured by Goujet (1976: pl. 61, figs 15-17). Others have an asymmetrical rounded crown ( Fig. 9K View FIG ), where the ridges are smooth and short and are arranged throughout the crown. These morphologies are not present in the material figured by Goujet (1976) and are very similar to the forms “verwachsene” and “schamale” figured by Wang (1993: pl. 15, figs 5, 6).

Scale histology ( Fig. 10)

The crown is formed by apposed growth layers of mesodentine in the posterior part of the crown, and superposed growth layers in the anterior ( Fig. 10A, H). A dense network of dentine tubules and lacunae occupies the internal zones of growth layers, although in zones of the outermost part of every growth layer (especially in areas that correspond with ridges) the network of dentine tubules is less dense, with large sinuous branched tubules, and no lacunae, so that the tissue resembles orthodentine ( Fig. 10G, H). No vascular canals can be identified either in the base or in the crown, although it may be due to fossilisation problems. The base shows numerous concentric growth lines consisting of successive alternating of dark and light layers ( Fig. 10A, C, F). It is formed by cellular bone with some bone cell lacunae aligned with the different growth lines ( Fig. 10F) and is pierced by numerous Sharpey’s fibers, thick and arranged radially and obliquely ( Fig. 10F). The apex of the base, immediately above the center of the crown, is often crystallized and occupied by calcite that hides the inner structure ( Fig. 10C).

Scapulocoracoid ( Fig. 4 L-O)

Three right and two left perichondrally ossified scapulocoracoids of typical Machaeracanthus morphology (see Burrow et al. 2010b) occur associated with scales and spines of M. goujeti n. sp. All the scapulocoracoids found are of small size, the preserved height (dorso-ventrally) of the largest specimen MPZ 2010/952 ( Fig. 4 L-O) is 13 mm (estimated not more than 18 mm if it was complete). Unfortunately, none of the elements is entirely preserved, missing the dorsal end of the scapular shaft and, to a greater or lesser extent, the ventral areas of the scapulocoracoid blades. The preserved specimens show an elongate constricted scapular shaft, which is subcircular in cross section (about 0.25 cm in dimension anteroposterior), and a triangular scapulocoracoid portion ( Fig. 4L, M). The transition between the scapular shaft and the triangular scapulocoracoidal areas is not abrupt but gradual, although the scapular shaft slightly narrows and bends anteriorly. The better preserved scapulocoracoid (MPZ 2010/952, Fig. 4 L-O, broken during photography) shows a flat medial face on the blade while the preserved part of the lateral face slightly flares out in its most ventral part, consistent with the presence of a ventrolateral expansion of the scapulocoracoid.