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Fröbisch et al. 10.1073/pnas.1216750110 SI Methods convergence. The analysis was run twice, once with and once Geologic Age of Ichthyosaurian Apex Predators. The holo- without gamma shape distribution. type and thus far only specimen of Thalattoarchon saurophagis The two runs per dataset were compared by calculating the Bayes gen. et sp. nov. was recovered from the middle Neva- factor, which is based on the difference between the harmonic disculites taylori ammonite zone (1), which constrains its geologic means of the log-likelihoods of each respective run multiplied by age to around 245 Mya. This age determination and the others two. It provides a measure that helps in assessing whether variation given in this section are based on ref. 2, which provides detailed in the rates of character change improves the fit of the data to the geochronological information for the Triassic based on U/Pb model. Generally, a Bayes factor >10 is considered to reflect dates in its supporting online material, section 3.1. The age of a strong support for the gamma shape distribution (8, 9). The the /Triassic (P/T) boundary (Changshingian/Griesba- current analyses yielded a Bayes factor of 7.38. Hence, we chose to chian boundary) is 252.6 ± 0.2 Ma. Because there is no radio- present the analysis without gamma shape distribution and its metric age for the Nevadisculites taylori zone, the youngest possible associated posterior probability values (Fig. 3), even though the absolute age for Thalattoarchon is given by the radiometric age of resulting tree topology of both analyses was identical. the overlying latest middle Anisian Balatonites shoshonensis zone, The parsimony-based analysis was conducted with PAUP 4.0b10 which is dated as 244.6 ± 0.36 Ma. Thus, Thalattoarchon had on a MacIntosh computer. Five taxa were assigned outgroup status evolved no later than 8 ± 0.28 Ma after the P/T boundary and only (Petrolacosaurus, Thadeosaurus, Claudiosaurus, Hovasaurus,and about 4 My later than the first secondarily marine tetrapods in the ). All characters were treated as unordered, assigned late Spathian (248.12 ± 0.28 Ma). equal weight, and were parsimony informative. Gaps were treated The stratigraphic position of Himalayasaurus tibetensis is only as missing data, and multistate taxa were interpreted as uncertainty. stated to be “Norian” (3). Thus, its oldest possible geological age The reference taxon of the analysis was . The search is about 231 Ma, based on a radiometric date for the latest Late mode was heuristic and used exactly the same settings as in the of 230.91 ± 0.33 Ma. This makes it at least 13 My younger original analyses. The analysis found 44 most parsimonious trees than Thalattoarchon and 21 My younger than the P/T boundary. (MPTs) 279 steps in length, which were optimized both under DELTRAN and ACCTRAN character optimization. The strict Preservation of Specimen. When the specimen was discovered, the consensus of the MPTs has a consistency index of 0.514, a rescaled snout and a section of the posteriormost dorsal and anterior caudal consistency index of 0.401, and a retention index of 0.792. vertebral column had already been lost to weathering. Excavation The parsimony-based analysis found T. saurophagis to be more recovered the greater part of the skull, the vertebral column largely derived than Mixosauridae and and represents in articulation (except for the missing part of the posteriormost a basal merriamosaur falling into an unresolved polytomy with dorsal and anterior caudal column), and scattered parts of the Besanosaurus, Californosaurus, , and more derived pelvic girdle and hind limbs. Except for an isolated phalanx, no merriamosaurs. A list of apomorphies found in the PAUP anal- were found of the pectoral girdle and forelimb. The spec- ysis that diagnose the described taxon is provided in Table S3. imen was deposited on the sea floor with its ventral side up as seen in some other large specimens from Nevada (4) and SI Anatomical Descriptions the Lower (5). The cervical and anterior dorsal vertebral Skull Anatomy. Whereas weathering removed the premaxillae, the column was telescoped into the skull and the region behind the anterior part of the maxillae and nasals, the external nares, and the skull when the specimen arrived on the sea floor, belly up and fi tooth-bearing part of the lower jaw, the skull is well preserved, and head rst. The missing limb elements may have been lost from the most sutures are easily discerned (Fig. 1 A–D). Beginning with the carcass during the drifting phase or been removed by bottom anteriormost preserved bones, the maxillae bear large teeth to the currents or scavengers before burial. The skull and some verte- posterior tip of the . There are large nutritive foramina on brae were preserved in a concretion and are less deformed than the lateral side of the maxillae, dorsal to the teeth. The nasals the rest of the skeleton. Nevertheless, the skull was flattened are very extensive, contacting the postorbitals posteriorly, but they considerably by sediment compaction. are not participating in the upper temporal opening. The frontals Phylogenetic Analysis. The phylogenetic framework for this study are small, forming the anteromedial margin of the upper temporal was obtained through a phylogenetic analysis using both parsimony opening, but fully surround the pineal foramen. There is a low and and Bayesian approaches. The matrix analyzed (Table S2) was small sagittal crest in front of the pineal foramen, changing to based on the matrix of Sander et al. (6), which, in turn, is a broad plateau posteriorly. The parietals are even smaller than the a modified and extended version of the matrix of Motani (7). Our frontals and only participate medially in the upper temporal open- matrix has 37 taxa, including the newly added T. saurophagis gen. ing. The latter shows a shallow but distinct anterior terrace that in- et sp. nov., and contains 111 characters. No new characters were corporates the posterolateral process of the nasal. The occipital added to this matrix. region of the skull is not exposed, partially because of the far pos- The Bayesian analysis was conducted using Mr. Bayes 3.2.1 teriorly drawn out upper temporal openings. The posteromedial to under application of the Mk model, which represents the only posterolateral margin of the upper temporal opening is formed by currently available model for morphological datasets. The anal- the supratemporal, thereby excluding the squamosal from the upper ysis was performed with four chains in two independent runs with temporal opening. The supratemporal lacks a ventral process. The 10 million generations and tree sampling at every 100 generations. anterolateral margin of the upper temporal opening is formed by the A 25% burn-in was disregarded for subsequent analysis. It was large anteromedial process of the triradiate postorbital. This bone tested whether the Markov chain Monte Carlo (MCMC) chains also forms the posterior margin of the orbit. The dorsal margin of the have reached stationarity by plotting log-likelihood values against orbit is formed by the fused prefrontal and postfrontal, which does numbers of generations. Moreover, the SD of split frquencies is not participate in the upper temporal opening. The lacrimal and well below the threshold of 0.01, which is a very good indication of jugal are narrow and slender.

Fröbisch et al. www.pnas.org/cgi/content/short/1216750110 1of5 The palate is only partially prepared yet, but it seems to be dorsal portion tapers to a point, suggesting a weak connection dominated by the large pterygoids, which appear widely separated with the sacral ribs. Two more bones are informative, a femur and at the skull midline, exposing the basisphenoid and the base of the a zeugopodial element. The femur (Fig. S1C)isofthetypical braincase. The separation of the pterygoids has been accentuated ichthyosaurian shape, flattened and wide with a slightly constricted by diagenetic flattening of the skull. The lower jaws are preserved shaft and an expanded distal end. Compared with that of C. petrinus in articulation with the skull and accordingly have lost their UCMP 9947 (ref. 11, plate 12, figure 5), it is of approximately the anterior parts to weathering. The preserved parts are massive and same length (23 cm is preserved) but wider. The flattened zeugo- deep. Only the dentary and the surangular can be identified with podial element is either a tibia or a fibula. It has expanded ends and certainty. The former extends as far back as the maxilla but one concave margin. Most importantly, it is wider than long. apparently lacked teeth, which means that at least the five or six Hence, the hindlimb and pelvic girdle were disproportionally small posteriormost maxillary teeth lacked antagonists in the preserved in Thalattoarchon, with the length of the femur being only twice the region of the rostrum. The surangular has a distinct coronoid height of the caudal vertebrae. eminence below the posterior third of the orbit. The dentition of Thalattoarchon is described in the main text, but SI Length Estimate and Proportions here we provide some additional information. Following Massare, Our minimum length estimate of 8.6 m for the holotype skeleton the cut feeding guild is characterized by the presence of cutting of Thalattoarchon is based on the following reasoning. The pre- edges and a certain type of tooth wear. Tooth wear is not observ- served skull length is around 930 mm, but the entire rostrum is able in the available specimen, but the cutting edges strongly sug- missing. On the basis of our skull reconstruction, the preserved gest Thalattoarchon was a member of the cut feeding guild. In skull length is probably similar but slightly shorter than postnarial addition, both tooth shape and size are congruent with the cut skull length, i.e., the distance from the posterior end of the extranal guild. The tooth shape index in preserved teeth is ∼2.5, and the naris to the posterior end of the skull. For comparison, Cymbo- tooth size index is >0.07, which is a minimum estimate because of spondylus petrinus (UCMP 9950) has a total skull length of 1,170 the flattening of the skull and the possible presence of larger teeth mm, with a postnarial skull length of ∼530 mm, which is only about more anteriorly. In contrast, Cymbospondylus petrinus was probably 57% of the same in Thalattoarchon. Skull length of Thalattoarchon a member of the smashing or general feeding guild. must have been well over 1,200 mm as a very conservative estimate. Assuming there were 60 presacral vertebrae, each 60 mm in Postcranial Anatomy. A total of 32 anterior and middle dorsal length (see above), and there was an intervertebral spacing of vertebrae are prepared. The anterior dorsals are recognized as such 5 mm, the length of the presacral column would have been 3,900 based on wide single-headed ribs that are associated with them (as mm. Because the actual number of presacral vertebrae is un- opposed to double-headed ribs in the cervical region of the vertebral known, the combined length of the unprepared blocks of pre- column). These vertebrae are nearly round in anteroposterior view, sacral column of 1,540 mm can be added to the length of the 32 and they are ∼100 mm in diameter and 60 mm long. Remarkably, prepared presacral vertebrae: 32 × 65 = 2,015 mm, making the this value for centrum length remains rather constant throughout length of the preserved presacral column 3,685 mm. the presacral column and in the anteriormost preserved caudals. We then added to this the length of the tail, which is preserved in The neural spines of the anterior dorsal vertebrae are 130–140 mm articulation to its tip. Of the tail, 1,400 mm remain unprepared, but tall and thus slightly higher than the vertebrae. They are laterally the 19 already prepared vertebrae amounted to 1,060 mm. The flattened throughout and do not leave a gap between them in lat- total preserved tail length thus is 2,460 mm, giving a minimum eral view. The neural arches in this region further have distinct preserved length estimate of 6,745 mm. However, considering the paired zygapophyses. Toward the middorsal region of the vertebral gap in the posterior trunk and anterior tail region, this value greatly column (Fig. S1A), vertebral height increases to ∼120 mm, with the underestimates the true length of the postcranial skeleton. length remaining the same. The anterior to middle dorsals have an If we very conservatively assume a skull length of 1,200 mm and elongate rib articular facet that dorsally is connected to the neural take the estimate of 3,900 mm for the presacral column and 3,500 arch facet, running down the side of the centrum in an anterior mm for the tail, we arrive at 8,600 mm or 8.6 m as a conservative direction. The rib articular facets are not truncated by the anterior length estimate of the Thalattoarchon holotype. This length is in margin of the centrum, unlike in Cymbospondylus.Thefacetdoes agreement with the measurements made in the field during ex- sometimes touch the anterior margin but is never confluent or cavation of the skeleton. truncated (Fig. S1A). However, the sources of uncertainties in our length estimate can The posterior dorsal vertebrae retain the same height and clearly be identified as the missing rostrum, the missing posterior length as the middle dorsals but have a flattened ventral surface. dorsal and anterior caudal vertebral column, and the unknown Their most striking feature is the double rib articulation, which is number of presacral vertebrae. Ongoing preparation and future also seen in basal ( and ) and finds will refine this estimate. in mixosaurs outside Merriamosauria. Among Merriamosauria, this character only occurs in Toretocnemus and in the latest SI Fauna and Food Web of the Fossil Hill Member Triassic to Neoichthyosauria (7, 10). The Fossil Hill Member presents the rare situation in which The anterior-most preserved caudal vertebrae have a triangular a stratigraphic member belongs to more than one stratigraphic shape with a strong but rounded double ventral keel and large formation. Specifically, the Fossil Hill Member is part of the Prida facets for the chevrons. In the middle caudals, the single rib ar- Formation, which crops out in the Humboldt Range of central ticular facets are low on the centrum in the more anterior ones but Nevada, and is also part of the Favret Formation, which crops out gradually rise to the middle of the centrum to be lost in the in the Augusta Mountains, about 50 km to the east (12). Although posterior caudals. All of the middle and posterior caudal vertebrae the western outcrop of the Fossil Hill Member in the Humboldt are laterally flattened and roughly twice as tall as wide while Range is the source of the classical ichthyosaur gradually decreasing in length. The tail appears rather straight fauna described by Merriam (8), vertebrate fossils were described without a downturned tip or tail bend. from the eastern outcrop in the Augusta Mountains only much The only completely preserved bone of the pelvic girdle and later (12, 13). hindlimb is the right ilium (Fig. S1B). Its acetabular portion is The invertebrate fauna of the Fossil Hill Member is devoid of similar in shape and size to the one figured by Merriam (ref. 11, benthic forms, with the possible exception of the halobiid bivalves plate 12, figures 1 and 4) for Cymbospondylus petrinus University that are very abundant in the upper part (12). Free-swimming of California Museum of Paleontology (UCMP) 9947, but the invertebrates are represented exclusively by , pri-

Fröbisch et al. www.pnas.org/cgi/content/short/1216750110 2of5 marily a great diversity of ammonites (1, 12), but also by coleoid maturity at a total body length of <3,000 mm is inconsistent with cephalopods of the Atractites group. The mode of life of the the individual pertaining to Cymbospondylus. halobiid bivalves is controversial: one hypothesis is that they had The enigmatic durophagous ichthyosaur was a pseudoplanctonic lifestyle (being attached to some floating first described from the Humbold Range in 1906 (24) and has seaweed), whereas the other is that they were able to live on the since been recorded from Middle Triassic marine deposits from seafloor under dysoxic conditions by harboring chemosymbionts around the Northern Hemisphere (25). It has an extremely un- (14). At any rate, it is clear from the sediments and fauna that usual crushing dentition apparently adapted for feeding on am- the seafloor was a hostile environment during the deposition of monites (25). Like Omphalosaurus, the other previously known the Fossil Hill Member. tetrapod taxa from the Fossil Hill Member are recorded from However, both the extremely diverse ammonite fauna (1) and offshore deposits of Middle Triassic age across the Northern the rich marine fauna (Table S4) suggest that the surface Hemisphere, principally those of Spitsbergen (Norway), Monte waters were productive and teaming with life. Both in the western San Giorgio (Switzerland and Italy), and Guizhou Province and the eastern outcrop, fishes are relatively rare. Wemple (15) (China). Thus, Thalattoarchon is the only marine tetrapod taxon described a small shark fauna from the Fossil Hill Member of the unique to the Fossill Hill Member. Humboldt Range, whereas the shark fauna described by Cuny The authochthonous preservation of the fauna of the Fossil Hill et al. (16) is from a bonebed at the top of the shallow water lower Member in black shale and the trophic specializations of the inferred from functional morphology permits the re- member of the Favret Formation, not from the Fossil Hill Member construction of the food web (Fig. S2). We are aware that the proper. Bony fishes from the Fossil Hill Member are only known trophic relationships in this food web do not have the same re- from the Augusta Mountains outcrop, including Saurichthys and liability as would be offered by fossilized stomach contents. The a small indeterminate osteichthyan (17). In addition, the complete > primary producer was the phytoplankton drifting in the water skeleton of a large coelacanth ( 1,500 mm total length) was re- column that was consumed by zooplankton and presumably the covered but remains undescribed. halobiid bivalves. The zooplankton would have been the food The tetrapod fauna of the Fossil Hill Member lacks any in- resource for the cephalopods and the small actinopterygians. The dication of the proximity of a shoreline and is overwhelmingly durophagous members of the fauna (i.e., Phalarodon dominated by ichthyosaurs despite the great diversity of other and particularly Omphalosaurus) would have preyed on these. marine reptile lineages occurring in the Triassic (18). The only Smaller predators among the marine vertebrate fauna and unequivocal nonichthyosaur is the sauropterygian Augustasaurus predators specializing on small prey were the actinopterygian (19, 20). The ichthyosaur fauna is dominated by two large-bodied Saurichthys, the undescribed coelacanth, the medium-sized ich- Cymbospondylus (4, 11), to which the majority of all finds thyosaur, and Augustasaurus. By virtue of its large size, Cymbo- pertain. Small ichthyosaurs of the Phalarodon (P. callawayi spondylus would have been able to prey on the smaller marine and P. fraasi) are rare (21–23) but more common in the eastern , but its generalized ichthyosaurian dentition, smaller outcrop, possible reflecting greater proximity to the paleo- skull, and long snout suggest that fish and squid also contributed shoreline (17). Fragmentary material also indicates the pres- heavily to its diet. At the top of the food chain was the macro- ence of a medium-sized ichthyosaur, the taxonomic distinct- phagous apex predator described in this report, T. saurophagis, ness of which from Cymbospondylus was recently revealed by the having specialized on prey items such as medium-sized marine discovery of a partial skeleton of a pregnant female. Sexual reptiles and juvenile Cymbospondylus (Fig. S2).

1. Bucher H (1988) A new middle Anisian (Middle Triassic) ammonoid zone from 14. Schatz W (2005) Paleoecology of the Triassic black shale bivalves Daonella: New northwestern Nevada (USA). Eclogae Geol Helv 81(3):723–762. insights into an old controversy. Palaeogeogr Palaeoclimatol Palaeoecol 216:189–201. 2. Brayard A, et al. (2009) Good genes and good luck: Ammonoid diversity and the end- 15. Wemple EM (1906) New cestraciont teeth from the West-American Triassic. Univ Calif Permian mass . Science 325(5944):1118–1121. Publ Bull Dept Geol 5(4):71–73. 3. Motani R, Manabe M, Dong ZM (1999) The status of Himalayasaurus tibetensis 16. Cuny G, Rieppel O, Sander PM (2001) The shark fauna from the Middle Triassic (). Paludicola 2(2):174–181. (Anisian) of North-Western Nevada. Zool J Linn Soc 133(3):285–301. 4. Fröbisch N, Sander PM, Rieppel O (2006) A new species of Cymbospondylus (Diapsida, 17. Sander PM, Rieppel OC, Bucher H (1994) New marine vertebrate fauna from the Ichthyosauria) from the Middle Triassic of Nevada and a re-evaluation of the skull Middle Triassic of Nevada. J Paleontol 68(3):676–680. osteology of the genus. Zool J Lin Soc Lond 147(4):515–538. 18. Motani R (2009) The evolution of marine reptiles. Evolution 2:224–235. 5. Martill DM (1987) A taphonomic and diagenetic case study of a partially articulated 19. Sander PM, Rieppel OC, Bucher H (1997) A new pistosaurid (Reptilia: ) ichthyosaur. Palaeontology 30(3):543–555. from the Middle Triassic of Nevada and its implications for the origin of plesiosaurs. 6. Sander PM, Chen X, Cheng L, Wang X (2011) Short-snouted toothless ichthyosaur J Vertebr Paleontol 17(3):526–533. from China suggests diversification of suction feeding ichthyosaurs. PLoS 20. Rieppel O, Sander PM, Storrs GW (2002) The skull of the pistosaur Augustasaurus One 6(5):e19480. from the Middle Triassic of northwestern Nevada. J Vertebr Paleontol 22(3):577–592. 7. Motani R (1999) Phylogeny of the Ichthyopterygia. J Vertebr Paleontol 19(3):473–496. 21. Schmitz L, Sander PM, Storrs GW, Rieppel O (2004) New Mixosauridae (Ichthyosauria) 8. Kass RE, Raftery AE (1995) Bayes factors. J Am Stat Assoc 90:773–795. from the Middle Triassic of the Augusta Mountains (Nevada, USA) and their 9. Müller J, Reisz RR (2006) The phylogeny of early eureptiles: Comparing parsimony and implications for mixosaur . Palaeontographica A 270(4-6):133–162. Bayesian approaches in the investigation of a basal fossil clade. Syst Biol 55(3):503–511. 22. Schmitz L (2005) The taxonomic status of nordenskioeldii (Ichthyosauria). 10. Sander PM (2000) Ichthyosauria: Their diversity, distribution, and phylogeny. Paläont Z J Vertebr Paleontol 25(4):983–985. 74(1/2):1–35. 23. Jiang D, Schmitz L, Hao W, Sun Y (2006) A new mixosaurid ichthyosaur from the 11. Merriam JC (1908) Triassic Ichthyosauria, with special reference to the American Middle Triassic of China. J Vertebr Paleontol 26(1):60–69. forms. Mem Univ Calif 1(1):1–155. 24. Merriam JC (1906) Preliminary note on a new marine reptile from the Middle Triassic 12. Nichols KM, Silberling NJ (1977) Stratigraphy and depositional history of the Star Peak of Nevada. Univ Calif Publ Bull Dept Geol 5(5):5–79. Group (Triassic), northwestern Nevada. Spec Pap Geol Soc Am 178:1–73. 25. Sander PM, Faber C (2003) The Alpine specimen of the enigmatic durophagous 13. Sander PM, Bucher H (1990) On the presence of Mixosaurus (Ichthyopterygia: Reptilia) ichthyosaur Omphalosaurus and its jaw anatomy. J Vertebr Paleontol 23(4): in the Middle Triassic of Nevada. J Paleontol 64(1):161–164. 799–816.

Fröbisch et al. www.pnas.org/cgi/content/short/1216750110 3of5 Fig. S1. T. saurophagis gen. et sp. nov. FMNH PR 3032 from the middle Anisian (Middle Triassic) part of the Fossil Hill Member of the Favret Formation, Favret Canyon, Augusta Mountains, Nevada. Bones of the postcranial skeleton. (A) Eight middorsal vertebrae in left-lateral view. Note the single rib articular facets that extend from the base of the neural arch to the anterior margin of the centrum without being truncated by it. Particularly the vertebrae on the left have suffered from plastic deformation during fossilization. (B) Ilium. (C) Femur, which is missing the distal end. (Scale bar: 20 mm.)

Fröbisch et al. www.pnas.org/cgi/content/short/1216750110 4of5 Fig. S2. Hypothetical food web of the Anisian (Middle Triassic) Fossil Hill Member of the Prida and Favret Formations, Nevada. The arrows depict only the major trophic relationships (see SI Fauna and Food Web of the Fossil Hill Member for details). Ichthyosaurs were only resolved at the genus level. The mac- rophagous apex predator T. saurophagis is an ecological novelty not seen in the marine realm before and in other Middle Triassic marine reptile faunas.

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