The Affinities and Ecology of Triassic Ichthyosaurs

The affinities and ecology of Triassic ichthyosaurs JACK M CALLAWAY I department of Geological Sciences, University of Rochester, Rochester, New York 14627 ABSTRACT Many current classifications of Class Reptilia the nasals in most of the earliest Triassic ichthy- place the ichthyosaurs in their own subclass, the osaurs, but the nasals were much longer than the The resolution of the problem of ichthy- Ichthyopterygia (Carroll, 1988; Ostrom and frontals in later forms (Fig. 1). At least some osaurian affinities has been complicated and Carroll, 1979; Romer, 1966). This emphasizes Triassic ichthyosaurs, specifically Mixosaurus prolonged by an emphasis on the morphology their uniqueness but avoids the problem of their and Grippia, possessed supratemporal bones of Jurassic ichthyosaurs, some 30 to 40 m.y. affinities. (Callaway, 1989), a pair of small bones at the younger than the earliest known species. Nearly every group of reptiles has been sug- back of the skull, which are absent in Jurassic Morphology of the skull and axial skeleton of gested as a possible ichthyosaurian ancestor, in- and later ichthyosaurs (McGowan, 1973; Triassic ichthyosaurs is the key to resolving cluding Sphenodon-like reptiles (Baur, 1887), Romer, 1968). The temporal fenestra of the the problem. The ichthyosaurs are included Rhynchocephalia (Osborn, 1903), phytosaurs most primitive ichthyosaur was thus bordered here in the Subclass Diapsida on the basis of (McGregor, 1906), Mesosaurus (von Huene, by the parietal, postfrontal, postorbital, squa- skull roof and palatal features. Within the Di- 1922,1923), protorosaurs and squamates (Wil- mosal, and supratemporal (Fig. 1 A); in later ich- apsida, ichthyosaurs appear to be most liston, 1925), pelycosaurs (Romer, 1948), turtles thyosaurs, it was bordered only by the parietal, closely related to the Younginiformes of the (Appleby, 1959,1961), lizards (Tarsitano, 1982; postfrontal, and squamosal (Fig. IB). The pineal Infraclass Lepidosauromorpha, with which Williston, 1917), sauropterygians and placo- foramen was farther back on the skull in Early they may have shared a common ancestor. donts (Mazin, 1982), eosuchians (Tarsitano, Triassic ichthyosaurs, especially in Grippia, Triassic ichthyosaurs were considerably 1983), and thecodonts (Carroll, 1985, 1988, where it was entirely within the parietals (Fig. different ecologically from their Jurassic de- p. 254), but there has never been a consensus. 1 A). In Jurassic ichthyosaurs, the pineal opening scendants. Many Triassic ichthyosaurs were Von Huene (1937) even suggested a separate was largely within the frontals (Fig. IB), a longer bodied and had long, broad tails in origin for the ichthyosaurs from the amphibians. change that may have also affected the shape of contrast to the deep, streamlined bodies and The problem has probably persisted for so long the frontal-parietal suture. Triassic ichthyosaurs lunate tails of Jurassic species. As a result, because most previous hypotheses have been had a fairly slender stapes (ear bone) in compar- Triassic ichthyosaurs probably tended to be based on the morphology of the better known, ison to the robust, heavy stapes of Jurassic ambush predators, whereas Jurassic ichthy- well-preserved Jurassic species; yet the oldest forms. The heavy stapes was used by Romer osaurs were mainly pursuit predators. Early ichthyosaurs of the Early Triassic predate the (1948) to argue for synapsid affinities for the ichthyosaurs may have been more general- earliest Jurassic forms by 30 to 40 m.y. The ichthyosaurs, but as that trait is a later innova- ized in their prey preference, as suggested by morphology of Triassic species is critical for un- tion in ichthyosaurs, his argument is invalid. heterodonty in many Early and Middle Trias- derstanding ichthyosaurian ancestry and rela- Primitively, ichthyosaurs have a broad cheek re- sic species. tionships with other reptiles. gion (postorbital, squamosal, and quadratojugal; Figs. 2A, 2B), whereas more derived ichthy- INTRODUCTION MORPHOLOGY OF TRIASSIC osaurs have a narrow cheek region (Fig. 2C). ICHTHYOSAURS Considering these differences in skull morphol- Order Ichthyosauria were important marine ogy, it is not surprising that analyses focusing on predators that ranged in age from Early Triassic Early Triassic ichthyosaurs are considerably Jurassic specimens have resulted in conflicting (Smithian) to Late Cretaceous (Santonian). different in skull morphology from the better hypotheses on ichthyosaurian affinities. Even the oldest known ichthyosaurs were well known Jurassic forms. The differences are im- Triassic ichthyosaurs as a group combined adapted to an aquatic habitat and show no close portant in deciphering ichthyosaurian relation- primitive and advanced traits in limb morphol- morphological similarities to any other group of ships. Grippia, from the Spathian of Svalbard, is ogy. Many Early and Middle Triassic ichthy- reptiles. The absence of transitional forms has the most primitive in both skull and limb mor- osaurs, especially Grippia, Chaohusaurus, and historically minimized the problem of what to phology (Mazin, 1981). Grippia and the slightly Utatsusaurus (Mazin, 1986,1981; Shikama and include within the order. It has, however, posed younger Chaohusaurus from the Spathian of others, 1978; Yang and Dong, 1972), retained a major problem in understanding the origin and China had considerably shorter snouts, with the primitive condition of an elongated radius affinities of the ichthyosaurs. Cuvier (1814) first fairly short premaxillae and dentaries, in con- and ulna. Utatsusaurus and Grippia had elon- recognized that ichthyosaurs were reptiles, and trast to the greatly lengthened premaxillae and gated metacarpals as well (Figs. 3A, 3B). This although that became firmly established by the dentaries of more advanced ichthyosaurs (Figs. suggests that there may have been a wider range mid-1800s, their evolutionary relationship to 1, 2). In addition, the frontals were approxi- of movement possible in the limb, especially in other reptiles continued to be problematic. mately equal in length or somewhat shorter than comparison to the stiff paddles of post-Triassic Geological Society of America Bulletin, v. 102, p. 409-416,6 figs., April 1990. 409 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/4/409/3380760/i0016-7606-102-4-409.pdf by guest on 27 September 2021 410 MASSARE AND CALLAWAY Figure I. Top (dorsal) view of ichthyosaur skulls. A. Early Triassic Grippia (after THE SYSTEMATIC POSITION OF Callaway, 1989). The exact shape and position of the frontal-parietal suture are uncertain, but THE ICHTHYOSAURIA the suture was well anterior to the pineal opening. B. Early Jurassic Ichthyosaurus (after Romer, 1956). Post-orbi- In discussing ancestry and affinities, the distal was present but is not tinction between derived and primitive traits is a visible in dorsal view. critical one. The presence of primitive traits Abbreviations: f, frontal; identifies organisms as having had a common j, jugal; 1, lacrimal; m, ancestor at some time in the past, but it does not maxilla; n, nasal; p, parie- provide information on the most recent ances- tal; pf, postfrontal; pn, tors, nor the closest affinities. Derived characters pineal foramen; po, post- are the key to this information, because a partic- orbital; pr, prefrontal; px, ular derived trait, especially an unusual or com- premaxilla; q, quadrate; plicated one, is most likely to have evolved only qj, quadratojugal; sq, once in a lineage rather than evolving several squamosal; st, supratem- times independently. Thus organisms that share poral. the same derived traits are more closely related to each other than to organisms that do not pos- sess the derived trait. This idea is articulated more completely in the philosophy of phyloge- netic systematica (cladistics) espoused by Hennig (1966), and modified by numerous subsequent systematists (for example, Cracraft and Eldredge, 1979; Wiley, 1981). Many previous workers have suggested diap- trial reptiles is a problem, because homologies sid affinities for the ichthyosaurs (Osborn, 1903; are difficult to ascertain (see Callaway, 1989, for McGregor, 1906; Williston, 1917, 1925; Col- discussion of homologies). Because limb fea- bert, 1980, p. 167; Tarsitano, 1982, 1983; forms (Fig. 3F). Many Triassic ichthyosaurs had tures, especially ankle morphology, are widely Mazin, 1982; Carroll, 1985, 1988, p. 253), but elongated, notched phalanges, but in Chaohu- used in defining higher taxa of reptiles, the de- few substantiated their suggestions with any saurus and Grippia (Fig. 3A), the finger bones rived limbs of ichthyosaurs have always pre- kind of character analysis. It is only recently that retained a cylindrical shape, comparable to that sented a problem in fitting them into existing a detailed definition of the Diapsida has been of terrestrial tetrapods. In Mixosaurus of the classifications. developed against which to test these hypoth- Early and Middle Triassic, the metacarpals and phalanges became less elongate and lost their distinctiveness (Fig. 3C). In later Triassic ichthyosaurs, the proximal limb bones, as well as the distal bones, were short and wide (Fig. 3E), more like the Jurassic condition (Fig. 3F); however, the Late Triassic Merriamia continued to retain relatively long upper limb bones (Fig. 3D). The hindlimbs are similar in morphology to the forelimbs in the few Triassic ichthyosaurs in which they are well known. The Early and Middle Triassic ichthyosaurs

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