Abstract
Three new Lower Devonian dalmanitid trilobite taxa are recognized from the Talacasto Formation in the Precordillera Basin, Argentina, which includes two monospecific genera (Ivanites leonorae n. gen., n. sp. and Aguaditaspis mediaspina n. gen., n. sp.), and one additional new genus and species left in open nomenclature. These taxa combine characters of the subfamilies Dalmanitinae Vogdes, 1890 and Synphoriinae Delo, 1935, challenging their taxonomic distinction. Forty percent of the dalmanitids from high paleolatitudinal basins from southwestern Gondwana (Malvinokaffric) exhibit non-homologous dorsal spinosity. Aguaditaspis mediaspina n. gen., n. sp. shares similar spines with Trypaulites calypso (Hall, 1861). The unnamed new genus resembles the spinosity of endemic Malvinokaffric dalmanitids, like Dalmanitoides Delo, 1935 and Fenestraspis Braniša & Vaněk, 1973. Considered defensive, the recorded spinosity along with putative sublethal-attack marks, suggest higher predation pressure than previously thought at high paleolatitudes. This evidence is in accordance with some hypothesis on rising predation pressure in the mid-Paleozoic marine ecosystems.
Highlights
The dalmanitid trilobites, abundant worldwide and diverse in the Silurian-Devonian, attracted comparatively little attention in basins of southwestern Gondwana
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
The dalmanitid trilobites in the context of the Malvinokaffric Realm Trilobite faunas from the Early Devonian of Southwestern Gondwana were characterized as noticeably endemic already since their first reports (Clarke 1913; Richter & Richter 1942), which allowed to recognize a major Early-Middle Devonian paleobiogeographical region of cold waters in high paleolatitudes (Dowding & Ebach 2018)
Summary
The dalmanitid trilobites, abundant worldwide and diverse in the Silurian-Devonian, attracted comparatively little attention in basins of southwestern Gondwana. The sampled unit is the Lower Devonian Talacasto Formation (Padula et al 1967), mainly exposed in the Central Precordillera in the San Juan Province, Argentina (Fig. 1B), isolated outcrops are known from the northernmost extension of the Precordillera, in the Sierra de las Minitas (Fig. 1A), La Rioja Province, Argentina (Rustán et al 2011a). This formation is composed of a marine succession of intensely bioturbated greenish-gray mudstones with intercalated sandstones. Specimens were collected in proximity of the GPS waypoints 28°42’50”S, 68°39’29”W and 28°42’43.8”S, 68°39’40.9”W
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