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Records from the Aalenian–Bajocian of Patagonia (Argentina): an Overview

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Records from the Aalenian–Bajocian of Patagonia (Argentina): an Overview Geol. Mag.: page 1 of 11. c Cambridge University Press 2013 1 doi:10.1017/S0016756813000058 Ophthalmosaurian (Ichthyosauria) records from the Aalenian–Bajocian of Patagonia (Argentina): an overview ∗ MARTA S. FERNÁNDEZ † & MARIANELLA TALEVI‡ ∗ División Palaeontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina. CONICET ‡Instituto de Investigación en Paleobiología y Geología, Universidad Nacional de Río Negro, 8332 General Roca, Río Negro, Argentina. CONICET (Received 12 September 2012; accepted 11 January 2013) Abstract – The oldest ophthalmosaurian records worldwide have been recovered from the Aalenian– Bajocian boundary of the Neuquén Basin in Central-West Argentina (Mendoza and Neuquén provinces). Although scarce, they document a poorly known period in the evolutionary history of parvipelvian ichthyosaurs. In this contribution we present updated information on these fossils, including a phylogenetic analysis, and a redescription of ‘Stenopterygius grandis’ Cabrera, 1939. Patagonian ichthyosaur occurrences indicate that during the Bajocian the Neuquén Basin palaeogulf, on the southern margins of the Palaeopacific Ocean, was inhabited by at least three morphologically discrete taxa: the slender Stenopterygius cayi, robust ophthalmosaurian Mollesaurus periallus and another indeterminate ichthyosaurian. Rib bone tissue structure indicates that rib cages of Bajocian ichthyosaurs included forms with dense rib microstructure (Mollesaurus) and forms with an ‘osteoporotic-like’ pattern (Stenopterygius cayi). Keywords: Mollesaurus,‘Stenopterygius grandis’, Middle Jurassic, Neuquén Basin, Argentina. 1. Introduction all Callovian and post-Callovian ichthyosaurs, two different Early Jurassic taxa have been proposed as Ichthyosaurs were one of the main predators in the ophthalmosaurian sister taxa: Stenopterygius (Maisch oceans all over the world during most of the Mesozoic & Matzke, 2000; Sander, 2000; Druckenmiller & (Massare, 1987). Advanced parvipelvian ichthyosaurs Maxwell, 2010; Fischer et al. 2011b, 2012; Maxwell, were the first tetrapods to evolve a fish-shaped body Fernández & Schoch, 2012) or Ichthyosaurus (Motani, profile (Motani, 2005) and, based on the profound 1999; Thorne, Ruta & Benton, 2011). Internal topology morphological modifications of their body plan, are of the Ophthalmosauria clade is even more contentious, the paradigm of marine adaptation among sauropsids. and up to the present has been returned as weakly Parvipelvian evolutionary history spans the Late Trias- supported in all analyses. sic up to the Cenomanian–Turonian boundary, when Examination of parvipelvian topologies combined ichthyosaurs became extinct (Bardet 1994; Fischer with the stratigraphic occurrence of taxa depicts et al. 2011a). In recent years, knowledge of advanced a gap in the record from the Aalenian up to parvipelvians, termed ophthalmosaurians (Opthalmo- the Callovian, comprising most of the Middle sauria: Motani, 1999) has increased, triggered by new Jurassic (Fernández, 2003; McGowan & Motani, discoveries and the re-examination of historical collec- 2003). This gap is interrupted by three taxa, tions. Indeed, after publication of the comprehensive Stenopterygius aaleniensis Maxwell, Fernández & handbook of Ichthyopterygia (McGowan & Motani, Schoch (2012), S. cayi (Fernández, 1994) and the 2003), a number of ophthalmosaurian monotypic ophthalmosaurian Mollesaurus periallus Fernández, genera have been erected: Arthropterygius Maxwell, 1999. Stenopterygius aaleniensis was recovered 2010; Athabascasaurus Druckenmiller & Maxwell, from the Aalenian of southwestern Germany, while 2010; Sveltonectes Fischer et al. 2011b; Acamptonectes M. periallus and S. cayi were recovered from the Fischer et al. 2012; Cryopterygius Druckenmiller et al. lower Bajocian Los Molles Formation, which crops 2012; and Palvennia Druckenmiller et al. 2012. out in the Neuquén Basin (Argentina). Mollesaurus Although there is a consensus as to the basic periallus is of particular interest as it represents phylogenetic relationships among major groups, in- the oldest ophthalmosaurian material identifiable cluding the monophyly of derived ichthyosaurs (the to species level, and it has not been revised since parvipelvian clade of Motani 1999), the affinities its original description (Fernández, 1999). The of Ophthalmosauria are still controversial. While only other coeval ophthalmosaurian fossil is a consistently found as a well-supported clade including fragment of a forefin from the Aalenian–Bajocian boundary of Northwest Patagonia (Fernández, 2003), †Author for correspondence: [email protected] which represents the earliest ophthalmosaurian http://journals.cambridge.org Downloaded: 20 May 2013 IP address: 181.166.71.96 2 M. S. FERNÁNDEZ AND M. TALEVI recorded worldwide. Although not abundant, these (Fernández, 1999). However, in the same year, Motani remains are significant because they document the (1999) published an exhaustive phylogeny of Ich- earliest stage in the evolutionary history of the oph- thyopterygia and defined the clade Ophthalmosauria, thalmosaurian clade. The main aim of this contribution including all Callovian and stratigraphically later is, therefore, to present the first detailed description of forms, nested within a broader Thunnosauria. Since Mollesaurus, as well as update information on a second this contribution, most systematic revisions include poorly known taxon from the early Middle Jurassic Mollesaurus as a valid taxon, and all phylogenetic of Patagonia: ‘Stenopterygius grandis’ Cabrera, 1939. assessments return it within the Ophthalmosauria (Druckenmiller & Maxwell, 2010; Maxwell, 2010; Institutional abbreviations. MLP – Museo de La Fischer et al. 2011b, 2012). By contrast, Maisch & Plata, División Paleontología Vertebrados, La Plata, Matzke (2000) and Maisch (2010) considered the name Argentina; MOZ – Museo Prof. P. Olsacher, Zapala, Mollesaurus as a junior synonym of Ophthalmosaurus, Argentina. and Sander (2000) did not include it within his evaluation of ichthyosaurian diversity. Although Mollesaurus is consistently nested within 2. Historical overview the Ophthalmosauria, its phylogenetic position within The first Middle Jurassic ichthyosaur discovered in this clade is not clear. Maxwell (2010) described the Neuquén Basin (Central-West Argentina) corres- Mollesaurus as forming a basal polytomy within ponds to a fragment of rostrum, MLP 39-VII-2– ophthalmosaurians. This arrangement was resolved 2, found during a geological survey at Sierra de in an analysis by Druckenmiller & Maxwell (2010), Chacaico (Fernández, 1943). MLP 39-VII-2–2 was and Mollesaurus is described as more derived than recovered from sediments belonging to the Emileia the Aegirosaurus leptospondylus + Ophthalmosaurus giebeli ammonite zone in the Los Molles Formation, icenicus clade, and the sister taxon of Athabascasaurus an early Bajocian unit (Westermann & Riccardi, chrisorum + Brachypterygius extremus + Arthroptery- 1979) that crops out at Charahuilla Creek, Sierra de gius chrisorum + Caypullisaurus bonapartei + Platy- Chacaico, Neuquén province, Argentina. Based upon pterygius hercynicus + P.platydactylus + P.australis, this rostrum, Cabrera (1939) erected the new species and Maiaspondylus lindoe + P.americanus.However, Stenopterygius grandis. Since then, MLP 39-VII-2– node supports in these analyses were universally low. 2 has been illustrated and/or mentioned (Fernández, A phylogenetic analysis of the ophthalmosaurians 2007) but never specifically revised. In 1984, a by Fischer et al. (2011b) placed Mollesaurus in an geological commission of Yacimientos Petrolíferos even more derivative position than Druckenmiller Fiscales of Argentina recovered a fragment of an & Maxwell (2010), their strict consensus topology ichthyosaur forefin (MLP 92-III-2–1) from sediments depicting it as the sister taxon of Aegirosaurus + belonging to the Puchenquia malarguensis ammonite Sveltonectes, although node support was again weak zone (Aalenian–Bajocian boundary; Riccardi, 2008) (< 50 %). This topology was revised in a recent of the Los Molles Formation at Quebrada Remoredo, analysis (Fischer et al. 2012) that included the new Sierra de Reyes, Mendoza province, Argentina. This genus Acamptonectes, and resulted in a single most- forefin was described as Ophthalmosauria indet. by parsimonious tree with Mollesaurus recovered as the Fernández (2003). Spalleti, Gasparini & Fernández sister lineage to Ophthalmosaurus icenicus + O. natans (1994), when analysing the transition between the + Acamptonectes densus. Middle Jurassic Los Molles and Lajas formations in the southeast of the Neuquén Basin (Chacaico 3. Systematic palaeontology Sur), mentioned and illustrated a marine reptile assemblage recovered from sediments belonging to Order ICHTHYOSAURIA de Blainville, 1835 the Emileia giebeli ammonite zone at the top of the OPHTHALMOSAURIA sensu Motani, 1999 Los Molles Formation. These remains included the Genus Mollesaurus Fernández, 1999 Mollesaurus periallus Fernández, 1999 pliosaur Maresaurus coccai Gasparini, 1997, a trunk Figures 1–3 vertebra of a thalattosuchian crocodiliform, and two ichthyosaurs: MOZ 5803, initially described as a new 2000 Ophthalmosaurus periallus;Maisch&Matzke,p. genus and species Chacaicosaurus cayi Fernández, 91, nov. comb. 1994, later reassigned to the genus Stenopterygius 2010 Ophthalmosaurus periallus; Maisch, p. 153 (Fernández, 2007); and MOZ 2282, identified as aff. 2010 Mollesaurus periallus; Druckenmiller & Ophthalmosaurus
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