First Diagnostic Marine Reptile Remains from the Aalenian (Middle Jurassic): A New Ichthyosaur from Southwestern Germany

Schoch RR (2012) First Diagnostic Marine Reptile Remains from the Aalenian (Middle Jurassic): A New Ichthyosaur from Southwestern Germany. PLoS ONE 7(8): e41692. doi:10.1371/journal.pone.0041692 First Diagnostic Marine Reptile Remains from the Aalenian (Middle Jurassic): A New Ichthyosaur from Southwestern Germany Erin E. Maxwell 0 Marta S. Ferna ndez 0 Rainer R. Schoch 0 Andrew A. Farke, Raymond M. Alf Museum of Paleontology, United States of America 0 1 Staatliches Museum fu r Naturkunde , Stuttgart, Germany, 2 Museum f u r Naturkunde , Leibniz-Institut f u r Evolutions- und Biodiversita tsforschung , Berlin, Germany , 3 Departamento Paleontolog a de Vertebrados, Museo de La Plata , La Plata , Argentina Background: The Middle Jurassic was a critical time in the evolutionary history of ichthyosaurs. During this time interval, the diverse, well-studied faunas of the Lower Jurassic were entirely replaced by ophthalmosaurids, a new group that arose sometime prior to the Aalenian-Bajocian boundary and by the latest middle Jurassic comprised the only surviving group of ichthyosaurs. Thus, the Middle Jurassic Aalenian-Bathonian interval (176-165 million years ago) comprises the time frame during which ophthalmosaurids not only originated but also achieved taxonomic dominance. However, diagnostic ichthyosaur remains have been described previously from only a single locality from this interval, from the Bajocian of Argentina. Methodology/Principal Findings: In this paper, we describe a new species of ichthyosaur based on a partial articulated specimen from the Middle Jurassic of southwestern Germany. This specimen was recovered from the Opalinuston Formation (early Aalenian) and is referable to Stenopterygius aaleniensis sp. nov. reflecting features of the skull and forefin. The genus Stenopterygius is diverse and abundant in the Lower Jurassic of Europe, but its presence has not previously been confirmed in younger (Middle Jurassic) rocks from the northern hemisphere. Conclusions/Significance: This specimen represents the only diagnostic ichthyosaur remains reported from the Aalenian. It bears numerous similarities in size and in morphology to the Lower Jurassic species of the genus Stenopterygius and provides additional evidence that the major ecological changes hypothesized to have occurred at the end of the Toarcian took place sometime after this point and most likely did not occur suddenly. There is currently no evidence for the presence of ophthalmosaurids in the northern hemisphere during the Aalenian-Bathonian interval. - Funding: This project was funded through Natural Sciences and Engineering Research Council of Canada and Alexander von Humboldt postdoctoral fellowships to EM, and PIP 0426, Consejo Nacional de Investigaciones Cientficas y Tecnicas, Argentina, to MF. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. The Middle Jurassic was a critical time in the evolutionary history of ichthyosaurs: during this time interval, the diverse, wellstudied faunas of the Lower Jurassic were entirely replaced by ophthalmosaurids, a new group that arose sometime prior to the Aalenian-Bajocian boundary [1], and by the latest middle Jurassic (Callovian) comprised the only surviving ichthyosaur clade [2]. Thus, the Middle Jurassic Aalenian-Bathonian interval (176165 million years ago) comprises the time frame during which ophthalmosaurids not only originated but also achieved taxonomic dominance. However, the Middle Jurassic is extremely poorly documented from the standpoint of marine reptile paleontology [3,4]. Excluding the Callovian-aged Oxford Clay Formation, which has produced a diverse marine reptile fauna including ichthyosaurs [5,6], diagnostic ichthyosaur remains have been described from only a single Middle Jurassic locality the Bajocian of Chacaico Sur, Argentina. This site has produced two ichthyosaur taxa, each known from a single incomplete specimen [7,8]: the basal ophthalmosaurine (sensu [9]) Mollesaurus periallus Fernandez, 1999, and Chacaicosaurus cayi Fernandez, 1994. Chacaicosaurus cayi is considered to be referable to the Lower Jurassic genus Stenopterygius (see [10]), but disagreement exists on this point: some authors consider S. cayi to be intermediate between Stenopterygius and Ophthalmosauridae [9,11]. Fragmentary ichthyosaurian material is not abundant in the Aalenian-Bathonian interval, but isolated rostral fragments, teeth, and vertebrae have been reported from the early Aalenian to late Bajocian of Germany [1214], earliest Aalenian and Bathonian of France [14,15], early Bajocian of western Argentina [16], early Bathonian of Russia [17], and Bathonian of the UK [18]. McGowan [19] concluded that none of these Middle Jurassic finds were diagnostic. However, anecdotal reports indicate more complete ichthyosaur material from the lower Aalenian of southwestern Germany [8,11]. The affinities of these specimens are of great interest, because no diagnostic ichthyosaur material is Materials and Methods In 1976, an articulated ichthyosaur was discovered in a hard limestone concretion of the basalmost Opalinuston Formation, 700 m SE of Zell near Go ppingen (Baden-Wu rttemberg) (Fig. 1). It was prepared by Michael Maus in 200507. The specimen (SMNS 90699), an ichthyosaur prepared in right lateral view, is preserved in three dimensions (Fig. 2). Some general measurements are presented in Table 1. The skull is oriented at approximately a 90u angle to the rest of the body, and the rostrum is broken anterior to the external nares. Both the right and left sides of the skull have been prepared. The vertebral column and ribs are also three-dimensionally preserved. The cervical region is disrupted, and slight disruption also occurs in the anterior thoracic region. Disarticulation is significant in the anterior caudal region, and the column is truncated prior to the apical region. Few ribs are preserved in their entirety, and a large gap is present in the mid torso. Only a few gastralia are present; these have been displaced and are located around the pectoral girdle. The torso, measured from the posterior skull to the pelvic girdle, is approximately 102 cm in length. The coracoids, interclavicles, right clavicle and scapulae are exposed in ventral view. Only the proximal-most elements are in articulation, and most of the distal paddle is missing. The left paddle is visible in ventral view, and the right in dorsal view. The hind limb and pelvic girdle are only represented by the proximal-most femur and the end of a second element. The specimen is currently on display in the public gallery. A second, less well-preserved specimen (SMG uncatalogued) was discovered in the early 1970s in the commercial brick quarry Mohring at Heiningen, again in the basalmost Opalinuston [20] (Fig. 1). This is on displa (...truncated)