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 : 28-30

Aguaditaspis mediaspina ?

MATERIAL. — Two incomplete pygidia from Quebrada de las Aguaditas ( CEGH-UNC 27421; Fig. 6A, B, D, E, I, J ; and CEGH-UNC 27420; Fig. 6C, F-H).

OCCURRENCE. — Lochkovian-Pragian, Talacasto Formation, Quebrada de las Aguaditas, San Juan, Precordillera Argentina, Argentina.

DESCRIPTION

Because of their tentative assignment, description of these specimens is herein focused on characters differing from others A. mediaspina n. gen., n. sp.

Axial furrows broad tr., moderately deep, with a weak divergence from sagittal line (7°). Axis with at least 24 axial rings. Axial terminus poorly defined. Anterior 13 rings strongly defined, lateral sides convex in dorsal view. Anterior side of first 13 rings convex and posterior side slightly concave. First four axial rings bearing dorsal posteromedial nodes. Pleural field with 15 pleurae, bearing pleural furrows, gradually less impressed backward and more posteriorly directed. Anteriormost pleural furrows (particularly the first six) slightly convex adaxially, interpleural furrows nearly straight. Interpleural furrows slightly impressed in first six pleurae, then even less impressed, and symmetrical in cross-section. Posterior pleural bands narrower exsag. and more vaulted than anterior pleural bands. Pygidial caudal border with a little sag. and relatively broad tr. spine with a rounded base ventrally directed. Regular granular ornamentation in posterior region of axial rings and anterior pleural bands, also present in surface of terminal piece but less densely distributed.

REMARKS

A number of pygidia assigned herein with doubt to A.mediaspina n. gen., n. sp., co-occur in the same layers of the Quebrada de las Aguaditas type locality, and are very similar to those of A. mediaspina n. gen., n. sp. They share some diagnostic characters: a steeply inclined anterior slope of pleural furrows; ornamentation posteromedially located on some axial rings, similar sinuosity of pleural bands and furrows and an elongated and stout post-axial region. Differences include, in these specimens, wider pleural bands exsag., interpleural furrows better impressed, higher number of axial rings, an extremely subdued post-axial ridge, and ornamentation of axial rings compound of medial nodes or subtle inflations instead of spines. Furthermore, the number of axial rings is remarkably high for most dalmanitids (more than 20 is extremely rare) and the anterior slope of pleural furrows is typically much steeper than the posterior slope. However, the general pygidial morphology is consistent, and no similar taxa are known outside from the fossiliferous interval of the Quebrada de las Aguaditas. Hence, we interpret that differences among specimens might be due to intraspecific variability and refer putatively the specimens to A. mediaspina n. gen., n. sp. A caudal spine, relatively short, stout, subtriangular in cross-section and ventrally directed distally is a unique character within dalmanitids. However, this character cannot be appreciated in A. mediaspina n. gen., n. sp., which caudal part is unknown in type specimens. Thus, the downturned caudal spine could not be used to solve the taxonomic proposal, nor was included in the diagnosis.

New genus and species of Dalmanitidae

( Fig. 7)

MATERIAL. — One incomplete pygidium from Quebrada de las Aguaditas CEGH-UNC 27422; Fig. 7.

OCCURRENCE. — Lochkovian-Pragian, Talacasto Formation, Quebrada de las Aguaditas, San Juan Province, Argentina.

DESCRIPTION

Pygidium subtriangular in dorsal view, with length/width index approximately 1.1. Axial furrows nearly straight, narrow tr., moderately deep, with a weak divergence from sagittal line (about 7°) along the first 15 axial rings, and then nearly parallel to the sagittal line, after a subdued constriction of the axis. Axis with 24 axial rings plus a terminal piece, slightly concave in lateral view. First axial ring approximately 30% of the maximum pygidial width tr. Axial rings narrow sag., exsag. almost rectangular with slightly convex lateral sides in dorsal view. First 14 axial rings bearing typically two strong adaxial dorsal small spines or spine-like tubercles, variably developed and arranged in two longitudinal rows (each on different sides of the sagittal plane), and usually two or more additional much smaller spines either laterally (in rings 1 to 5) or between them (in rings 1-2?; 5-11). Inter-ring furrows well-impressed, apodemal abaxially. Apodemal pits without contacting axial furrows. Pleural field with 16 pleurae, bearing broad exsag. and deep pleural furrows, gradually less impressed backward and more posterolaterally directed. Anteriormost pleurae (particularly the first nine) nearly straight to slightly concave forward in the pleural field adaxially from fulcrum, slightly convex backward toward the fulcrum, then bending strongly backward abaxially at fulcrum, distinguishable up to near the margin. Posteriormost pleurae progressively becoming nearly straight and backwardly oriented, trending to be parallel to the axis. Interpleural furrows well-impressed but incised and shallow. Anterior pleural bands convex dorsally in cross-section, higher, slightly broader exsag., and extending more distally than the posterior bands, becoming broadest exsag. near fulcrum. Anterior band of first six pleurae bearing typically 3-4 spine-like small tubercles irregularly and asymmetrically distributed. Posterior pleural band of the first two pleurae with little spines abaxially near fulcrum. Pleural bands and furrows becoming progressively faint distally to efface next to the pygidial margin, leaving a barely defined pygidial border. Caudal region elongated backward, with no ornamentation. Pygidial terminus continued in a gently upturned caudal spine of unknown total length. Regular granular ornamentation.

REMARKS

The new genus and species of Dalmanitidae combines diagnostic characters of Dalmanitinae, such as asymmetrical width exsag. of pygidial pleural bands, with characters of Synphoriinae as a poorly defined pygidial border and apodemal pits of pygidial axis without contacting axial furrows.

Ornamentation appears to be similar to that of Roncellia Lespérance & Bourque, 1971 , from the Lower Devonian of Canada; Fenestraspis , from the Lower Devonian of Bolivia; and especially Dalmanitoide s. These genera share a very narrow tr. pygidial border, and spines/tubercles on several axial rings, usually paired and trending to define longitudinal rows. However, specimens of Dalmanitoides and Roncellia differ from our specimen by spines developed on the posterior pleural bands, instead of being located on the anterior ones. In turn, the broad bases of spines in Dalmanitoides protrude into the following anterior pleural band, which contrasts with the delicate spine bases of the new genus and species of Dalmanitidae . Our specimen could be differentiated from Dalmanitoides and Roncellia from its narrower tr. pygidium, a higher number of axial rings and pleurae, a straighter lateral margin, and absence of dorsal ornamentation on the last axial rings. Fenestraspis , in contrast, is clearly different by its unique intersegmental fenestrae and nearly straight pleurae. Hence, similarities in the ornamentation pattern among these genera are most probably due to evolutionary convergence, although this must be tested in a phylogenetic analysis.

The combination of the spines distribution pattern, the high number of axial rings and the sharp subtriangular shape with weakly convex lateral margins, is unique for a dalmanitid and suggest that this specimen corresponds to a new genus. We leave it in open nomenclature due to the scarcity of material.

It is worth mentioning an irregularity in the last three pleurae on the left side specimen ( Fig. 7I). These pleurae appear to be fused adaxially, suggesting a malformation or a post-damage regeneration. It is closely similar to interpretations of post predation-marks ( Šnajdr 1981). These abnormalities are explained as evidence of regeneration after failed predation, in which the margin regenerates first while successive moults change the pre-damage distribution of pleural furrows and bands. The typical result of this process would be a pygidial margin without any damage but pleural furrows and bands irregularly distributed, as in our case. However, it is very difficult to objectively distinguish post-damage regeneration from some teratological conditions ( Owen 1985). Bicknell et al. (2019) proposed to reject a teratological origin for distinctive shapes of marginal embayments, marks of cicatrization and/or single spine injuries. Nevertheless, none of these characteristics have been seen in our specimen. In a study about abnormalities and spinosity in Cambrian trilobites, a correlation was found between development of spines and abundance of abnormalities ( Pates & Bicknell 2019). The conclusion of that publication was that in such cases abnormalities would correspond to post-predation marks. In either case, this is the first abnormality of this kind registered in post-Ordovician dalmanitids.

On the other hand, this specimen exhibits two little circular marks also located in the left pleural field, which look like sessile epibiont attachment structures ( Fig. 7H). We interpret that these structures were probably post mortem and not related to predation.