The Santa Clara Abajo Formation in the north of Mendoza Province, Argentina, is part of the Triassic Cuyana rift Basin infill and consists of fluvial, deltaic and lacustrine units. The combination of progradational to aggradational stacking pattern plus the identification of a fluctuating profundal facies association suggests the Santa Clara Abajo palaeolake was a balanced-fill lake system. Lake-centre sediments (finely laminated mudrock facies) preserve pseudobeaconiid actinopterygian fish represented primarily by bones and scales, generally undeformed and concordant with bedding planes. We propose that variation of articulation degree of these fish combined with sediment features provides insights into palaeolimnological changes in this single lake system over time, primarily water temperature. The fish fossils are sorted into three taphonomic modes: Mode A, isolated and dispersed scales; Mode B, associated but dispersed scales, loosely to well sorted, and low to moderate density; and Mode C, scale patches and articulated fishes. We conclude these modes represent, respectively, warm, fluctuating warm-cold and cold water conditions. Stratigraphical analysis of the taphonomic modes for the balanced-fill lake model revealed an up-section pattern of fluctuating water temperatures (cold and warm), to a stable-cold pattern, to a stable-warm pattern and to fluctuating thermal conditions (cold and warm) again. This pattern is linked to lake-level fluctuations resulting in lacustrine intervals that reflect hydrological closure to opening to closure again. □ Actinopterygii, lake-basin-type model, Mendoza, Santa Clara sub-basin, Triassic.
Abstract. Scales of a new species of Teleosteomorpha from the continental Aptian of the south of South America are studied. These neopterygians are from the La Cantera Formation in central Argentina, and were previously identified as Pholidophoriformes. They present ganoid scales; most of them are rhombic with well-developed peg-and-socket articulations and possessing a smooth surface. They have a straight posterior margin, but occasionally, some scales of the flank have a sinuous posterior margin with one or two serrations. The shape of the scales varies along the body from large, rectangular and deeper than long scales behind the head to the preanal region to smaller and rhomboidal scales in the caudal region. There are a few horizontal rows along the flank and about 32 lateral line scales. Thick, round ganoid scales are present in the prepelvic region close to the ventral margin. The round and rhombic scales present growth lines, which form concentric ridges on the external side. A characteristic row of deep scales forms the dorsal margin on each side of the body; a row of median ridge scales is not present. This is a unique feature of the studied fishes. Scutes covered with unornamented ganoine precede the pelvic, dorsal, and anal fins, as well as the dorsal and ventral margins of the caudal fin. The posterior margin of the dorsal lobe of the caudal fin is formed by a single line of scales, which continues and covers the base of the first principal caudal ray. Histological studies reveal a lepisosteoid-scale type with multiple ganoine layers, lack of dentine, and the presence of canaliculi of Williamson. The macro-and micromorphology of the scales shows features that are found in other teleosteomorphs, but also in other neopterygians.
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