Lower Devonian aulacopleurids trilobites from Argentina

RUSTÁN, JUAN JOSÉ

Toledo, España

Conferencia; Fouth International Trilobite Conference (Trilo08); 2008

Aulacopleurids are widespread, frequent and diverse elements with long stratigraphic ranges (Ordovician-Devonian) out of the Malvinokaffric Realm. Nevertheless, their Malvinokaffric record almost exclusively restricted to the Devonian, is scattered and of low diversity. Due to this particularity and, probably, to the fragmentary nature of the type material, and the lack of proper illustrations and detailed studies, the only two genera recognized (Malimanaspis Baldis and Longobucco, 1977 and Maurotarion Alberti, 1969) (Adrain & Edgecombe, 1996), have received secondary attention, and no specific investigations have been carried out after the excellent review performed by Adrain and Edgecombe (1996). The principal controversies about these faunas include their origin, their role in evolutionary events, and their paleobiogeographic significance (Elredge & Ormiston, 1979; Elredge & Braniša, 1980; Lieberman, 1993). Lower and Middle Devonian information about Maurotarion from Bolivia is of great significance, accounting for a monophyletic small-scale radiation hypothesis that involves five formally nominated species, separated in two informal groups (Adrain & Edgecombe, 1996). The origin of this clade has been explained by means of two independent migration events of cosmopolitan groups from low latitudes, one for Lochkovian-Pragian and the other during the Emsian, that were be interpreted to represent cladogenetic pulses, according to Adrain and Edgecombe (1996). Nevertheless, new evidences from eastern Bolivia confirm the presence of Maurotarion in South America already for the Silurian, questioning the nature and age of this evolutionary event (Edgecombe and Fortey, 2000). A sixth Emsian species from the Bokkeveld group of South Africa, of discussed affinities to the proposed informal groups, completes the list of known Malvinokaffric Maurotarion species up to the present (Cooper, 1982; Adrain and Edgecombe, 1996) In this scenario, the knowledge of Aulacopleurids from Argentina is very relevant. However, Malimanaspis sarudianskii (Baldis and Longobucco, 1977), Middle-Upper Devonian in age, from the Chigua Formation in western Precordillera is the only one Aulacopleurid described from this country. Unfortunately, this species is known from a single specimen poorly illustrated, and thus several diagnostic features are obscured. In this contribution the record of Lower Devonian Aulacopleurid trilobites from the Talacasto Formation in the Argentine Precordillera, is documented. A detailed taxonomic study based upon abundant and well preserved materials as well as related evolutionary implications is now in progress. Hence, systematic aspects and evolutionary significance of these new faunas are preliminary analyzed herein. MATERIALS The Talacasto Formation is a classical, widespread lower Devonian unit from Argentine Precordillera, and has been dated as Early Lochkovian-Late Emsian, based on its rich brachiopod fauna (Herrera,1991,1993). It consists of a dominantly shaly succession at the base and a mainly psamitic succession toward the top, and was interpreted as deposited in a shallow muddy shelf under the fluvial influence (Astini,1991). The Aulacopleurid material comes from several sections of the Talacasto Formation in San Juan Province: Río Talacasto, La Chilca, and Loma de Los Piojos (Fig. 1), and it mostly consist on molds, although occasionally shell remains are preserved. So far, six species assigned to Maurotarion are recognized. They are closely related with congenerics from Bolivia and South Africa. Two species recovered from the Río Talacasto section, Early Lochkovian in age, probably are the most ancient Devonian Aulacopleurids for the Malvinokaffric Realm. The remainder taxa come principally from the Loma de Los Piojos section, and belong to the brachiopod assemblage zone B (Herrera, 1991) of Pragian age (Fig. 2). Together with the latter, other trilobites resembling assemblages recognized in the Scaphiocoelia zone from Bolivia (Vaccari et al.1994, Racheboeuf et al., 1993, 1998) occur. In this stage of research, it is still not possible to confirm the presence of Aulacopleurids in Emsian levels. All these species share the single prominent apomorphy indicated for Malvinokaffric Maurotarion: the extreme posterior expansion of the posterior and lateral librigenal borders, and the base of the genal spine (Adrain and Edgecombe, 1996). They also exhibit a reduction of the ornamentation, the cephalic border usually flat and expanded forward, wide glabellae with often angular outline, effacement of the lateral border furrow, long wide and flat genal spines, absence of thoracic spines in the sixth segment, a constant number of 14 thoracic segments, and small pygidia. DISCUSSION According to Adrain and Edgecombe (1996) expanded genal spines bases and wide posterior and lateral librigenal borders relate Bolivian Maurotarion with M. periergum (Haas, 1969) as a sister group of this subclade. At first glance, these apomorphies are also present in Argentine Maurotarion species, nevertheless, greater precisions based upon detailed morphologic studies now in progress would possible alter this proposed ancestry relation. The Bolivian monophyletic radiation recorded in Altiplano and Tarija has been dated as post-Upper Lochkovian (Adrain & Edgecombe, 1996); nevertheless, the presence of Maurotarion in the late Silurian (Pridolian?) of eastern Bolivia (Edgecombe & Fortey, 2000), suggest an earlier age for this event. The early Lochkovian age of the oldest Argentine Maurotarion species, is in agreement with eastern Bolivia information. The diversity of Argentine forms, their stratigraphic constrained data, and the morphological evidences, suggest that a diversification event within this subgroup of Maurotarion took place. Their relationship with Bolivian records, probably implies a more extended South American event. These macroevolutionary events might be related with similar ones documented among other broadly coeval trilobite groups in these basins; for example, the paradigmatic event of the family Calmoniidae (Elredge & Ormiston, 1979; Elredge & Braniša, 1980; Lieberman, 1993; Vaccari et al.1994). Several important aspects such as the cladogenetic structure of the event, phylogenetic relationships among proposed informal groups, the nature and age of possible migratory or exchange events and other significant aspects will be investigated and discussed in further contributions. CONCLUSIONS Our new records suggest that the diversity of Lower Devonian Argentine Aulacopleurids (with at least 6 taxa of specific level for Maurotarion) would to be comparable to that of the remainder of the Malvinokaffric Realm. This one among other evidences would indicate a more ancient and complex South American Aulacopleurid diversification event (Pridolian? – Emsian) than previously thought, principally registered in Argentina and Bolivia. The unravelling of evolutionary and phylogenetic aspects of these new faunas, would result of great importance in the discussion of similar events proposed for other trilobite groups in broadly similar times.