Aguaditaspis mediaspina, Randolfe & Rustán & Bignon, 2022

Randolfe, Enrique A., Rustán, Juan José & Bignon, Arnaud, 2022, New dalmanitid trilobite taxa from the Lower Devonian of Argentina: comments on spinosity in endemic taxa from southern high paleolatitudes, Comptes Rendus Palevol 21 (2), pp. 17-38 : 25-28

Aguaditaspis mediaspina n. gen., n. sp.

( Figs 5; 6)

urn:lsid:zoobank.org:act:53FB3DCF-9ECD-4B37-9664-F24CA0CF9FDA

TYPE MATERIAL. — Holotype. 1 enrolled specimen CEGH-UNC 27418 ; Fig. 5 A-D, G, from Quebrada de las Aguaditas (Lochkovian-Pragian).

Paratypes. 4 pygidia: CEGH-UNC 27419, from Quebrada de las Aguaditas (Lochkovian-Pragian); Fig. 5E, F, H, I, K, L; CEGH-UNC 12735, from Quebrada de las Aguaditas (Lochkovian-Pragian); Fig. 5N, Q; PULR-I 006, from Sierra de las Minitas (Lochkovian-Pragian); Fig. 5J, M, P; PULR 132, from Sierra de las Minitas (Lochkovian-Pragian); Fig. 5O.

TYPE LOCALITY. — Quebrada de Talacasto, San Juan Province, Argentina.

ETYMOLOGY. — From Latin. Refers the presence of one dorsal median spine in some pygidial axial rings.

OCCURRENCE. — As for the genus.

DIAGNOSIS. — As for the genus (monospecific).

DESCRIPTION

Cephalon approximately subparabolic in dorsal view. Shallow epiborder furrow running close to the margin.

Glabella expanding tr. rather evenly forward in dorsal view, maximum width tr. located in frontal lobe, equivalent to half of maximum cephalic width tr. Posterior part of glabella sloping slightly forward up to S3, in lateral view. Glabellar frontal lobe incompletely known but with a barely depressed or gently convex medial dorsal area in the posterior part, in lateral view.

Glabellar axial furrows, nearly straight and weakly divergent (approx. 16°) forward up to junction with S3, where describe a local outward convexity, and then slightly more divergent (approx. 25°), shallowest adjacent to the anterior part of the eye and deepest adjacent to S1. SO slightly convex anteriorly in medial part where is shallowest, and concave forward laterally becoming apodemal. S1 nearly posterolaterally directed, broader exsag. abaxially, shallowest at junction with axial furrows, more adaxially with a short tr. apodemal pit with a little convex intrusion of the L1. S2 nearly straight and barely posterolaterally directed, shallowest at junction with axial furrows, apodemal adaxially with a little convex intrusion of the L2. S3 anterolaterally directed, effaced sagitally, narrow adaxially then broadening and deepening anterolaterally nearly at junction with axial furrows. LO not well-preserved but long sag.; exsag., longer sagitally, slightly convex anteromedially, lateral sides nearly concave forward in dorsal view. L1 short exsag., subrectangular to elongated, laterally acuminated, scarcely inflated, higher than L 2 in lateral view. L2 approximately twice longer (sag.; exsag) than L1, subrectangular, inflated. L3 subtriangular, expanded anterolaterally, barely longer exsag. than L1 abaxially, inflated, protruding slightly anterolaterally.

Eye kidney-shaped in dorsal view, with posterior edge opposite to posterior part of L2 and anterior edge opposite to contact between axial furrows and S3. Eye maximum width tr. about 10% of cephalon total width tr., ocular surface taller in anterior part, gently convex and steeply inclined in general, more vertical in posterior part. Complete lens formula unknown. Lenses regularly ordered in approximately 48 vertical rows with not more than 4-18 lenses each.

Palpebral furrow shallow and slightly convex laterally, broader tr. anteriorly.

Anterior branch of facial suture running subparallel to and near external sides of axial furrows from the anterior part of eye, cuts subocular ridge ( Fig. 5A), then circumscribing exteriorly the anterolateral corner of the glabellar frontal lobe. Posterior branch of facial running anterolaterally from behind the eye along the posterior part of the eye socle furrow, then strongly and evenly convex forward across the lateral border furrow opposite to S2, and turning backwards to the margin across the border opposite to L2. Subocular ridge robust and extended anteriorly almost reaching axial furrows. Eye socle furrow shallow, broadest tr. anteriorly. Palpebral area elevated, slightly convex dorsally in posterior view. Postocular area moderately inclined towards lateral border, gently convex in lateral view. Posterior border furrow wide exsag. and deep, shallowest and narrower exsag. adaxially. Librigenal very narrow tr., steeply sloped downward to lateral border furrow.

Anterior cephalic doublure ventrally convex ( Fig. 5B).

Hypostome big (of about 28 mm sagittal length), with a shield-like shape resembling an inverted bell, slightly wider than longer (length/width index, 0.9), convex ventrally; with anterior margin evenly convex forward; conspicuous anterior wings, lateral side concave anteriorly and evenly convex posteriorly, posterior margin incompletely known but convex with at least one posterolateral denticle. Separation between anterior border and middle body without any furrow. Middle body more elevated medially than anterior border, convex in lateral view. Middle furrow and macula only slightly expressed. Lateral border furrow shallow and narrow. Posterior border furrow broader sag. and deeper than lateral border furrow. Posterior body not completely known, subtriangular in ventral view and lower than middle body in lateral view ( Fig. 5C).

Thorax known from a few incomplete segments ( Fig. 5A, D-F, L). Axis about 25-30% of thoracic width tr. Axial furrows well-impressed. Axial rings subrectangular in dorsal view, strongly convex medially in transversal view; with a discrete dorsal inflation in the anterolateral corner ( Fig. 5D). Pleural tips unknown, pleural furrows deep, broad exsag., and asymmetrical in cross section, the anterior slope more steeply inclined, shallowest and narrowest at junction with axial furrows. Anterior pleural band narrower exsag. than posterior pleural band. Short and stout dorsal spine posteromedially located in at least some posterior thoracic segments ( Fig. 5E, F, L).

Subtriangular pygidium, with length/width index of nearly 1.0, based on an estimated total length sag. Axial furrows nearly straight, narrow tr. and shallow, with a low divergence from the sagittal line (about 6°). Pygidial axis approximately 30% maximum pygidial width tr., measured at the first axial ring. As much as 18-20 well-defined pygidial axial rings plus a terminal piece. First six pygidial axial rings slightly convex forward anteriorly and concave backward posteriorly, adaxially, in dorsal view; with lateral sides somewhat broader and barely inflated; posteriormost ones more subrectangular in dorsal view. Each axial ring gently inclined forward dorsally and more convex and higher posteriorly in lateral view, with a short posteromedial spine or inflation variably developed typically in anteriormost 12 axial rings. Inter-ring furrows narrow (sag., exsag) but well-impressed, becoming apodemal abaxially. Apodemal pits without contacting axial furrows. Pleural field with at least 13 pleurae gently inclined forward in lateral view, bearing broad exsag. and deep pleural furrows. Anteriormost pleurae (particularly the first five) somewhat sinuous (slightly convex backward near axial furrow, then strongly bending backward at fulcrum). Posteriormost pleurae progressively becoming nearly straight and backwardly oriented, trending to be parallel to the axis, straighter and more posterolaterally directed. Interpleural furrows shallow, sinuous as pleural bands, barely impressed in internal molds. Anterior pleural bands broader exsag. and extended more distally than posterior band. Anterior pleural bands broader exsag. near pygidial margin. Pleural furrows asymmetrical with anterior slope steeper than posterior one. Pleural bands and furrows become progressively faint distally to efface next to pygidial margin, leaving an insinuated pygidial border. Short pygidial doublure, slightly convex ventrally ( Fig. 5J, M, P). Postaxial region wide tr. and elongated, with a gently dorsal postaxial keel-like ridge ( Fig. 5N), but unknown in the very rear part from the available material, in appearance, tapering in widebased and stout mucro or spine.

REMARKS

Pygidial interpleural furrows of Aguaditaspis mediaspina n. gen., n. sp. are extremely shallow in external molds ( Fig. 5E), and barely impressed in internal molds ( Fig. 5D, F). Thus, the apparent absence of interpleural furrows is a doubtful character to use when external information of the carapace is not available.

The distribution pattern of dorsal median spinosity on axial rings of the pygidium, and the expression of the spines, varies among specimens. We interpret it as intraspecific variability, as was recognized in other dalmanitids ( Rustán & Vaccari 2012) such as Dalmanitoides drevermanni ( Delo, 1935) .

External morphology of the thoracic axial rings of A. mediaspina n. gen., n. sp. is known from a single segment derived from the specimen CEGH-UNC 27419 ( Fig. 5E, F, L), which exhibits a dorsal postero-sagittal spine. However, as spines expression seems to be variable, we did not include spines of thoracic axial rings as a diagnostic character.