Mimetaster florestaensis, Aris & Corronca & Quinteros & Pardo, 2017

Aris, María J., Corronca, Jose A., Quinteros, Sebastián & Pardo, Paolo L., 2017, A new marrellomorph euarthropod from the Early Ordovician of Argentina, Acta Palaeontologica Polonica 62 (1), pp. 1-8 : 3-5

Mimetaster florestaensis sp. nov.

Fig. 2 .

Etymology: In reference to the locality from where the specimen was collected.

Holotype: CNS-I 133/1-1, cephalic shield (mold). Type locality: Mojotoro Mountains (24°48’27”S, 65°22’03”W), Villa Floresta, Salta. Argentina.

Type horizon: Santa Victoria Group, Floresta Formation, lower Tremadocian ( Lower Ordovician ) .

Material.—Cephalic shield (CNS-I 133/1-1, part; CNS-I 133/1-1’, counterpart).

Diagnosis.—Cephalic field slightly inflated; short, anterolateral spines directed forward and outward ( Fig. 2A–C ). Anterolateral spines with small and strong secondary spines in the proximal inner and outer borders ( Fig. 2B–D ); distal portion smooth.

Description.—Cephalic shield has a central field with three pairs of primary spines ( Fig. 2A 1, C); each with a different length and orientation. Central cephalic field subtrapezoidal, slightly convex. In the central cephalic field the imprint of the posterior ventral margin is present ( Fig. 2B, C ). In addition, a delicate line is well marked in the posterior section of the field and seems to complete a circular route approaching the anterior edge. This structure could be interpreted as a suture line ( Fig. 2B, C ), but since only a single specimen is available, this cannot be confirmed unequivocally. The anterior margin curves forward slightly, elongating into a pair of short anterolateral spines (the shortest of the three pairs), which are directed forward and outward. The bases of secondary spines are present in the proximal section of the anterolateral spines on both the inner and outer margins. These secondary spines disappear distally, providing a smooth look to the remainder of the anterolateral spines. In the counterpart three secondary spines that are diminutive and well-defined (ca. 1 mm) are preserved ( Fig. 2D ).

Behind these spines, the cephalic field extends into a pair of mediolateral spines (the longest of the three pairs) that arch inward and backward. These are three times larger than the anterolateral spines, and extend considerably beyond the posterior margin of the cephalic field ( Fig. 2A 1, C). The widest point of the cephalic shield is measured between the most curved sections of the mediolateral spines. The base of these spines is relatively narrow ( Fig. 2A, C ), and extends from the base of the anterolateral spines to nearly the center of the central field of the cephalic shield.

A third pair of spines (the posterior pair) extends from the posterior border of the central field of the cephalic shield ( Fig. 2A 1, C), following a path similar to that of the mediolateral spines, though the length of these spines is uncertain because they are truncated. The internal borders are in contact medially, forming the posterior margin of the central part of the shield, which has a U-shape. The bases of the mediolateral and posterolateral spines do not touch, and both have triangular base of insertion of secondary spines. Some complete secondary spines are observed on the inner edge of the middle and posterolateral spines ( Fig. 2A 2 , B, C). The secondary spines in the posterolateral pair are less numerous and have a wider base than those of the mediolateral secondary spines.

Remarks.—The presence of the secondary spines on the mediolateral and posterolateral spines is a shared character in Furca bohemica , Mimetaster hexagonalis , the Moroccan unnamed marrellid and Mimetaster florestaensis . sp. nov., but its position and number differs in the four species. The secondary spines on the anterolateral pair in the new species differs significantly not only in shape and length, but also in their unique position, occurring only in the proximal part of the spines. On the other hand, based on the phylogenetic analysis, Mimetaster , the Moroccan unnamed marrellid and the new taxon described here are more closely related to each other than to Furca bohemica , supporting the suggestion that this new taxon represents a new species of the genus Mimetaster .

Stratigraphic and geographic range. — Type locality and horizon only.

Figures

→ Fig. 2. Marellomorph arthropod Mimetaster florestaensis sp. nov. from Tremadocian of Mojotoro Mountains, Salta, Argentina. A–C. CNS-I 133/1-1, part. A. Cephalic shield and spines. Detail of the secondary spines on mediolateral spine (A2). B. View of the imprint of the ventral posterior margin of the cephalic shield. C. Explanatory drawing revealing the most important morphological characters. D. CNS-I 133/1-1´, counterpart showing detail of strong secondary spines on anterolateral spine. Arrows indicate the secondary spines.