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To renew call Telephone Center, 333-8400 UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN w^osm L161— O-I096 ILLINCk y BT jrbamac paign AUG 1 3 n% FIELDIANA Geology NEW SERIES, NO. 31 The Genus Placodus: Systematics, Morphology, Paleobiogeography, and Paleobiology Olivier Rieppel CO CO CD «t) r~\ CO III THELIBPA'^vO^ tm Uu FEB - D '• -'n UNIVERS J -- w Published December 29, 1995 URBANA-( HAM' '►'«' Publication 1472 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY Information for Contributors to Fieldiana General: Fieldiana is primarily a journal for Field Museum staff members and research associates, although manuscripts from nonaffiliated authors may be considered as space permits. The Journal carries a page charge of $65.00 per printed page or fraction thereof. Payment of at least 50% of page charges qualifies a paper for expedited processing, which reduces the publication time. 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Text: Manuscripts must be typewritten double-spaced on standard-weight, 8V E E n = 50 B Placodus gigas [rFl Placodus andriani Placodus antiquhr H lower Muschelkalk 1 — I — I — I — h— t- 0.628 0.684 0.740 0.796 0.852 0.908 0.964 1.020 1.076 1.132 1.188 1.244 1.300 3rd palatine tooth long. / 3rd palatine tooth trans. Fig. 3. Porportions of the third (posterior) palatine tooth plate in 50 articulated Placodus i}alatal dentitions. himself (1858, p. 180) to be conspecific with Pla- codus andriani. The specimen shows little more than the three tooth plates, with a replacement tooth for the anteriormost tooth plate. The total length of the fragment is 135 mm. The specimen is not diagnostic of a separate species. Placodus pachygnathus (Owtn, 1858, PI. 10, Figs. 6, 7; BNfNH R-1641) consists of a very fragmentary lower jaw preserving parts of both rami (Fig. 6). The total length of the fragment is 123.5 mm. The specimen is not diagnostic of a separate species and was considered of questionable validity by Fig. 4. Holotype of Placodus bathygnathus Owen (bmnh R- 19677; original of Owen, 1858, PI. 11, Figs. 1-3). Upper Muschelkalk, Bayreuth. A, Left lateral view; B, occlusal view. Scale bar = 20 mm. RIEPPEL: THE GENUS PLACODUS Fig. 5. Holotype of Placodus bombidens Owen (bmnh R-1643; original of Owen, 1858, PI. 9, Figs. 3-6). Upper Muschelkalk, Bayreuth. A, Medial view; B, occlusal view. Scale bar = 20 mm. Meyer (1863, p. 177). Lydekker (1890) synony- mized Placodus pachygnathus with Placodus gi- gas. Placodus hypsiceps was described as a valid spe- cies by Meyer (1863; erroneously referred to as P. hypsicephalus [nomen nudum] by Braun, 1863, p. 10; first described as a skull oi Placodus gigas by Braun, 1862, p. 10), but considered a possible ju- nior synonym of Placodus gigas by Lydekker (1890). The specimen is kept at the Oberfrank- isches Erdgeschichtliches Museum, Bayreuth (bt, uncatalogued), and consists of the left half of a longitudinally split skull (Fig. 7). The specimen shows some morphological detail, which allows the identification of some derived characters shared with Placodus gigas. These include the anterior extent of the jugal, the anterior ascending process of the maxilla, which defines most of the antero- ventral margin of the external naris, and the ex- clusion of the frontal from the dorsal margin of the orbit by a contact of prefrontal with postfron- tal. Traces of a suture between the nasals (if not due to breakage) may be indicative of subadult age, as are the relatively small palatal teeth. How- ever, no suture between quadrat ojugal and squa- mosal can be identified in the posterior cheek re- gion, indicating (early?) fusion of the two elements (see discussion below). The aberrant proportions considered diagnostic by Meyer (1863) are due to deformation of the skull during fossilization. Placodus quinimolaris (Braun, 1863; Meyer, 1863, PI. 25, Figs. 2-4) is kept at the Oberfrank- isches Erdgeschichtliches Museum, Bayreuth (bt, uncatalogued), and consists of a very incomplete Fig. 6. Holotype of Placodus pachygnathus Owen (bmnh R-1641; original of Owen, 1858, PI. 10, Figs. 6, 7). Upper Muschelkalk, Bayreuth. A, Left lateral view; B, occlusal view. Scale bar = 20 mm. FIELDIANA: GEOLOGY Fig. 7. Holotype of Placodus hypsiceps Meyer (bt, uncatalogued; original of Meyer, 1863, PL 24, Figs. 1-3; PL 29, Fig. 4). Upper Muschelkalk, Bayreuth. Scale bar = 20 mm. skull, comprising little more than the two palatines and maxillae and fragments of the right jugal (Fig. 8). Most skulls referable to Placodus gigas have four maxillary teeth, whereas Placodus quinimo- laris has five. The species is listed as valid by Lydekker (1890). Jaekel (1907, PI. 4) figured a skull oi Placodus gigas (kept at the Staatliches Mu- seum fur Mineralogie und Geologic Dresden, ba Tr 43) with five teeth on the right maxilla and four teeth on the left; and on that basis he synonymized Placodus quinimolaris with Placodus gigas. In the Dresden skull, the five teeth on the right maxilla Fig. 8. Holotype of Placodus quinimolaris Braun (bt, uncatalogued; original of Meyer, 1863, PL 25, Figs. 2-4). Upper Muschelkalk, Bayreuth. Scale bar = 20 mm. RIEPPEL: THE GENUS PLACODUS Fig. 9. Placodus gigas Ag (ba Tr 43; original of Jaekel, 1907, PI. 4). Upper Muschelkalk, Bayreuth. A, Skull, palatal view; B, right maxilla, occlusal view. Scale bar = 20 mm. show an interesting size variation, in that the an- teriormost tooth is significantly smaller than the posterior four teeth and situated somewhat more medially (Figs. 9, 10). This suggests that a fifth tooth has been added to the usual four maxillary teeth during late ontogeny, an argument supported by the fact that the Dresden skull is somewhat smaller than the holotype of Placodus quinimo- laris, in which all five maxillary teeth are of more or less equal size. The first palatine tooth is 17.5 (17) mm long and 23 (23.9) mm wide in the Dres- den skull, whereas that of Placodus quinimolaris is 20 mm long and 26 mm wide. The third palatine tooth is 24 (23.5) mm long and 32.5 (32.2) mm wide in the Dresden skull, whereas that of Pla- codus quinimolaris is 30 (32) mm long and 34 (36) mm wide. (Values in parentheses refer to the left side of the skull.) An undescribed skull of Placodus gigas kept at the Naturkundemuseum Erfurt (Er, coll. Wagner, #78/235) shows only three maxillary teeth on the well-preserved right maxilla (Figs. 1 1 , 12). The Erfurt skull is slightly smaller than the Dresden skull, its first palatine tooth being 15.5 (16) mm long and 19 (21) mm wide, the third palatine tooth being 23 (23) mm long and 27.5 (26) mm wide. However, the skull of a small in- dividual kept at the Senckenberg Museum in Frankfurt am Main (smf R-4038) is of similar size as the Erfurt skull, yet shows four teeth on the maxilla. Ontogeny therefore offers only a partial explanation for individual variation found in max- illary tooth counts in Placodus, ranging from three to five. Variation in tooth counts on the maxilla corroborates the synonymy of Placodus quini- molaris with Placodus gigas. Whereas all material of Placodus from the upper Muschelkalk is referable to Placodus gigas, re- mains from the lower Muschelkalk have been re- ferred to a separate species, Placodus antiquior (Huene, 1936; Peyer & Kuhn-Schnyder, 1955; 10 FIELDIANA: GEOLOGY Kruckow, 1979; Pinna, 1990). The holotype of Placodus antiguior (Huene, 1936, Figs. 24a-c) from the Schaumkalk of Freyburg/Unstrut (upper lower Muschelkalk, Figs. 13, 14), as well as a fragmen- tary dentary (Huene, 1936, Figs. 25b; see also Huene, 1902, 1905) from the same locality and referred to the same species (Fig. 1 5), are kept at the Institut fiir Geologische Wissenschaften der Martin-Luther Universitat, Halle (drawer M 5/1). Diagnostic characters of Placodus antiquior enu- merated by Huene ( 1 936, p. 1 34) are (as compared to Placodus gigas): relatively smaller size of the first palatine tooth plate; different shape of the orbit; narrow frontal bridge between the orbits; and different shape of the upper temporal fossa, which is also relatively shorter. In view of the very incomplete preservation of the skull, only two of these characters can be critically evaluated— the proportions of the first palatine tooth and the shape of the orbit. The left orbit shows a thickened an- terior edge and a slightly angulated anterodorsal comer as in Placodus gigas, and, as in the latter species, the jugal extends anteriorly to a level in front of the anterior margin of the orbit. Again, the maxilla forms an anterior ascending process that defines the anteroventral margin of the ex- ternal naris. There is no indication of the relative size and shape of the upper temporal opening, or of the relative width of the frontal bridge between the orbits. The teeth of Placodus antiquior (holo- type and left dentary) do not differ in any signif- icant degree from those of other remains of Pla- codus from either the lower or the upper Mu- schelkalk. In particular, the proportions (length/ width) of the first palatine tooth in Placodus an- tiquior are very closely comparable to those of the holotype of Placodus andriani and Placodus gigas (Fig. 1 6). Similarly, the proportions of the tooth plates on the left dentary attributed to Placodus antiquior by Huene (1936) fall squarely into the range of variability of Placodus gigas jaws from the upper Muschelkalk (Fig. 17). Placodus anti- quior turns out to be another junior synonym of Placodus gigas. Naming a different species for Pla- codus remains from the lower Muschelkalk reflects a general tendency of earlier authors to separate taxa from the lower and upper Muschelkalk for stratigraphic rather than morphological reasons. Synonymy of Placodus antiquior with Placodus gigas indicates that the taxon persisted throughout the Muschelkalk, although no Placodus material has yet been recorded from the middle Muschelk- alk. Two species of Placodus have been described Fig. 10. Placodus gigas Ag (ba Tr 43; original of Jaekel, 1907, PI. 4). Upper Muschelkalk, Bayreuth. Scale bar = 20 mm. Abbreviations: ec, ectopterygoid; in, in- ternal naris; pi, palatine; pm, premaxilla; pt, pterygoid. from deposits other than the Germanic Muschelk- alk. If valid as a species, Placodus impressus would represent the geologically earliest placodont. Ag- assiz (1833-1845) described the species from the top of the Lower Triassic (upper Buntsandstein) of Zweibriicken (Saarland, Germany) on the basis of isolated teeth. Owen ( 1 858, p. 1 7 1) commented: "The character on which Placodus impressus Ag- assiz is differentiated, *une impression ou une sorte de sillon longitudinal qui se voit au milieu de la couronne,' is one common to the newly-formed crushing teeth of all placodonts." Lydekker ( 1 890, p. 5) listed Placodus impressus as unrepresented in the British Museum (Natural History), but these collections today include five maxillary teeth (bmnh R- 1 328-9, with a maximal diameter of 1 2-1 4 mm) identified as Placodus impressus. An old museum label refers to these teeth as Placodus andriani, in better accordance with their provenience from the RIEPPEL: THE GENUS PLACODUS 11 Fig. 11. Placodus gigas Ag (Er 78/235). Upper Muschelkalk, Bayreuth. A, Skull, palatal view; B, skull, dorsal view. Scale bar = 20 mm. upper Muschelkalk of Bayreuth. Peyer and Kuhn- Schnyder (1955, p. 480) tentatively identified four teeth of the original material of Placodus impressus described by Agassiz (1833) as those of Sargodon tomicus, the remaining fifth tooth as a possible cyamodont. Placodus impressus is a nomen du- bium. In 1973, Jurcsak reported the discovery of an elongated placodont tooth (13 x 5.5 mm) from the Triassic (Anisian) of Alesd near Oradea, which he identified as a tooth of Paraplacodus. In 1976, Jurcsak described a lower jaw fragment from the same locality, which he referred to the genus Pla- codus, questioning whether it represented a juve- nile individual or a new but small species {''Pla- codus gracilis"?," Jurcsak, 1976, p. 75), close to Paraplacodus (Jurcsak, 1978, p. 41). In the figure caption (Jurcsak, 1976, Figs. 13-15; see also Jurc- sak, 1982, Fig. 16), the specimen is referred to as ''Placodus gracilis n.sp." To judge from the figures, however, the specimen does not represent Placo- dus or, indeed, a placodont. The mandibular sym- physis is narrow, and there are five slender anterior teeth with a narrow cylindrical base and broken tips; three anterior teeth are strongly procumbent. Placodus shows an elongated symphysis and two chisel-shaped anterior teeth, separated from pos- terior crushing teeth by a wide diastema. An elon- gated symphysis, and a wide diastema separating two conical and procumbent anterior teeth from the posterior crushing teeth, are also characteristic of Paraplacodus (Peyer, 1935). Placodus gracilis, therefore, is a nomen dubium, pending a revision of the original material. However, the occurrence of placodonts (mostly Cyamodontoidea, perhaps also Placodontoidea) in the Triassic (Anisian) of Transylvania is documented by other material (Jurcsak, 1977, 1978, 1982), all of which is too fragmentary to be diagnostic at the genus or species level. Skeletal Morphology of Placodus gigas Skull The skull of Placodus gigas is known from a number of specimens, most of which are incom- 12 FIELDIANA: GEOLOGY plete to a variable degree. The best specimen is the acid-prepared skull bt 1 3 from the Oberfrank- isches Erdgeschichtliches Museum in Bayreuth (Figs. 18, 19), recently described by Sues (1987). It will also form the basis of this description, sup- plemented by the other material listed in the Ap- pendix. The durophagous habits of Placodus are reflect- ed by the robust skull. The premaxillae form a spatulate rostrum, which is set off from the re- mainder of the skull by a distinct constriction of the snout at the level of the anterior margin of the external nares. The premaxillae form short pos- terior (nasal) processes that meet the nasal in be- tween the two external nares (i.e., not projecting beyond the posterior margin of the external nares). The premaxillary-maxillary suture extends from the anterolateral comer of the external naris in a lateroventral direction. The dorsal surface of the premaxilla is pierced by numerous small nutritive foramina, but also by larger foramina located im- mediately in front of the external naris as well as further anteriorly, close to the anterior margin of the premaxilla. Shallow grooves extending from those foramina indicate the course of nerves (branches of the medial ethmoidal nerve: Sues, 1987) and blood vessels that have supplied the snout (Fig. 20). The maxilla of Placodus is a massive element with a well-developed ascending process provid- ing lateral cover for the preorbital region of the skull and defining the posterior margin of the ex- ternal naris. The anterior end of the maxilla is deeply bifurcated (Fig. 1 9C). The lateral anterior process meets the premaxilla in an anterolaterally trending suture. The medial anterior process of the maxilla extends medial to the premaxilla along the ventral and anteroventral margin of the ex- ternal naris, meeting the nasal in the anterior mar- gin of the external naris. Posteriorly, the maxilla does not quite reach the level of the posterior mar- gin of the orbit. The nasals are fused along the dorsal midline of the skull (Fig. 19 A). Anteriorly, the nasal forms slender processes that line the dorsal margin of the external naris, thus embracing the posterior (nasal) processes of the premaxillae. Laterally, the nasal forms relatively slender lateral processes that ex- tend onto the lateral surface of the prefacial skull, entering between the ascending process of the maxilla and the prefrontal. Posteriorly, the nasal extends in between well-defined anterolateral pro- cesses of the frontal to about the level between the first and second third of the longitudinal diameter Fig. 12. Placodus gigas Ag (Er 78/235). Upper Mu- schelkalk, Bayreuth. Skull in palatal view. Scale bar = 20 mm. of the orbit. The posterior part of the nasal is broad, and terminates in a slightly concave pos- terior suture with the frontal. The frontals again are fused along the dorsal midline of the skull, a longitudinal ridge indicating the line of fusion (the frontals remain separate in the skull BSP 1925 I 16, described by BroiH, 1912). The frontal is a relatively broad plate, embracing the posterior end of the nasal with relatively short anterolateral processes, whereas equally short but well-defined posterolateral processes embrace the anterior end of the parietal. In the specimen de- scribed by Broili ( 1 9 1 2, PI. 1 4, Figs. 1-4; bsp 1 968 I 75), the frontal and parietal meet in a deeply interdigitating suture, and posterolateral processes of the frontal are not distinct (Fig. 21). The prefrontal is a large element and defines the anterodorsal, anterior, and anteroventral margin of the orbit. The anterodorsal comer of the orbit is developed into a thickened ridge. Dorsally, the prefrontal meets the postfrontal in the dorsal mar- gin of the orbit. Ventrally, the prefrontal is in ex- tensive contact with the jugal. Medially, the pre- frontal forms a relatively narrow descending pro- cess that contacts the palatine and thus defines the RIEPPEL: THE GENUS PLACODUS 13 Fig. 13. Holotype of Placodus antiquior Huene (Ha, M5/1; original of Huene, 1936, Figs. 24a-c). Schaum- kalk (upper lower Muschelkalk), Freyburg/Unstrut. A, Ventral view; B, dorsal view; C, left lateral view. Scale bar = 20 mm. anterior wall of the orbit. This descending process of the prefrontal is pierced by the large lacrimal foramen (also seen in smf R-4 1 26). A lacrimal is absent in Placodus. The postfrontal defines the posterodorsal mar- gin of the orbit as well as part of the anterior margin of the upper temporal fossa. A tapering ventral process forms a broadly overlapping con- tact with the postorbital along the caudal margin of the orbit. The postorbital defines the posterior and pos- teroventral margin of the orbit. The element is broadly exposed in the dorsal part of the dermal covering of the temporal region, extending pos- teriorly to a level slightly behind the midpoint of the upper temporal region. In its dorsal part, the 14 FIELDIANA: GEOLOGY Fig. 14. Holotype of Placodus antiquior Huene (Ha, M5/1; original of Huene, 1936, Figs. 24a-c). Schaumkalk (upper lower Muschelkalk), Freyburg/Unstrut. A, Skull, left lateral view; B, skull, palatal view. Scale bar = 20 mm. Abbreviations: ju, jugal; m, maxilla; prf, prefrontal; pt, pterygoid. postorbital forms a distinct spine that projects into the posterodorsal comer of the orbit. Antero- ventrally, the postorbital forms a distinct step in the lower margin of the orbit. Between this step and the posterodorsal spine, the postorbital forms a shallow depression that extends backward as a shallow groove across the temporal arch in a pos- terodorsal direction. The jugal is a large element that is broadly ex- posed in the anterior ventral part of the temporal arch, extending posteriorly to the same level as the postorbital. Anteriorly, it narrows as it lines the dorsal margin of the maxilla and defines part of the ventral margin of the orbit. A narrow anterior process of the jugal extends between maxilla and prefrontal to a level well in front of the anterior margin of the orbit. The interpretation of the posterior temporal re- gion of the skull oi Placodus remains controversial to the present day. Broili (1912) believed the qua- RIEPPEL: THE GENUS PLACODUS 15 Fig. 15. Fragmentary dentary, referred to Placodus antiguior (Ha, M5/1; original of Huene, 1936, Fig. 25b). Scale bar = 20 mm. dratojugal to be absent in Placodus, or fused to the squamosal; the squamosal would define most of the lateral, the posterior, and the posteromedial margin of the upper temporal fossa and would extend ventrally to cover the quadrate in lateral view. Huene (1911) had been unable to identify a suture line in the posterior part of the upper tem- poral arch that would separate the squamosal from the quadratojugal (his supratemporal and squa- mosal). The hatched line indicated by Huene (1911) was eventually confirmed as the suture between quadratojugal and squamosal by Sues (1987), a claim that was disputed by Pinna (1989). Follow- ing the latter author, the squamosal would be re- stricted to the posterior and posteromedial margin of the upper temporal fossa, whereas the quadra- tojugal would be of extraordinary size and would enter the posterolateral margin of the upper tem- poral fossa. This interpretation of the temporal region of the skull of Placodus was influenced by a comparison with Nosotti and Pinna's (1993a) reconstruction of the sutural pattern in Cyamodus. A critical discussion of the cranial anatomy of Cy- amodus is beyond the scope of this paper, although personal inspection of the skulls of Cyamodus kuhnschnyderi (Nosotti &. Pinna, 1993a; smns 15855 and SNfNS 16270) did not convince me that the quadratojugal does, indeed, broadly enter the dorsal margin of the upper temporal fossa. In smns 16270, both temporal arches have been largely replaced with resin; only their posterior part is preserved and encrusted with osteoderms. In smns 15855, the contact between postorbital and squa- mosal appears identifiable in the dorsal margin of the upper temporal arch, and the contours of the quadratojugal can be followed on the medial sur- face of the temporal arch along its ventral margin. As far as it can be identified, this sutural pattern 20 0) e o Q. 10- e C -\ — h i -\ — I — h n = 42 H Placodus gigas lin Placodus andriani 23 Placodus antiquior H lower Muschelkalk ■i — I — h S^ -i — I — h -i — I — h 0.62 0.66 0.70 0.74 0.78 0.82 0.87 0.91 0.95 0.99 1.03 1.07 1.11 1st palatine tooth long. /1st palatine tooth trans. Fig. 16. Proportions of the first (anterior) palatine tooth plate in 42 articulated Placodus dentary dentitions. 16 FIELDIANA: GEOLOGY 12^ 10- co c a> e o 0) Q. CO 9 E 8- 4- 2- i^v.'-s jyyyy^ Placodus antiquior Winterswijk jaw ^ n=21 0.60 0.65 0.70 0.75 0.90 0.95 1.00 1.05 1.10 0.80 0.85 4th dentary tooth long. / 4th dentaty tooth trans. Fig. 17. Proportions of the foxirth (posterior) dentary tooth plate in 21 articulated Placodus dentary dentitions. corresponds to the sutures observed in the tem- poral arch of Macroplacus raeticus (Schubert- Klempnauer, 1975), with the squamosal defining the entire posterior and most of the lateral margin of the upper temporal fossa, while the quadrato- jugal remains restricted to the ventral aspect of the posterior part of the temporal arch (Fig. 22). These observations contradict Nosotti and Pinna's (1993a) reconstruction of the temporal region in Cyamodus and renders the genus obsolete as a model for the reconstruction of temporal sutures in Placodus. Whereas ridges appear to indicate a squamosal-quadratojugal suture on the occiput of Placodus, as drawn by Sues (1987) and accepted by Pinna (1989), there is no unequivocal evidence for the lateral termination of the squamosal at the posterolateral comer of the upper temporal fossa, as reconstructed by Pinna ( 1 989). A shallow groove seems to restrict the squamosal to that position on the left side of skull bt 13, but no such groove or suture is distinct on the right side. In addition, a fragmentary skull (smns 59434; Fig. 23) shows a well-preserved posterolateral margin of the up- per temporal fossa, but no indication of a squa- mosal-quadratojugal suture as indicated by Pinna (1989). However, I agree with Pinna (1989) that the suture line separating the squamosal from the qua- dratojugal as drawn by Sues (1987) is erroneous. Sues (1987) probably took the lower edge of the shallow depression extending from the orbit across the temporal arch in a posterodorsal direction as a suture. The absence of such a suture can be es- tablished beyond doubt by microscopical inspec- tion of the bone surface and by the use of bone surface ornamentation as a guide to the suture pattern (Fig. 24). The absence of a squamosal- quadratojugal suture as indicated by Sues (1987) is furthermore well demonstrated by the holotype of "Placodus hypsiceps" (Fig. 25), by the skull de- scribed by BroiU (1912, bsp 1968 I 75) (Fig. 26), and by the fragmentary skull smns 59434. Un- equivocal evidence for the presence of a quadra- tojugal in Placodus is missing. The bone may be absent or fused with the squamosal, as postulated by BroiU (1912). The parietals are fused in bt 13 (paired in bsp 1968 I 75) and form a relatively broad parietal skull table. The frontoparietal suture is somewhat obscured in bt 13 by partial fusion of the bones and by a break that traverses the skull at that level. The lateral margins of the parietal are concave, as they define the medial margin of the upper tem- poral fossa. Posteriorly, the parietal skull table is deeply excavated. The pineal foramen lies some- what in front of the center of the skull table in bt 13 (in BSP 1968 I 75, the pineal foramen lies close to the deeply interdigitating frontoparietal suture; Fig. 21). The dermal palate of Placodus is completely RIEPPEL: THE GENUS PLACODUS 17 Fig. 18. Placodus gigas Ag (bt 13, original of Sues, 1987). Upper Muschelkalk, Bayreuth. A, Skull, dorsal view; B, skull, ventral view; C, skull, left lateral view. Scale bar = 50 mm. closed except for the confluent internal nares (Figs. 18, 19). The internal nares are separated at a some- what deeper level by the fused vomer, which meets the premaxillae anteriorly. The broad palatines have expanded anteriorly at a level below the orig- inal position of the internal nares, partially ob- scuring the latter in ventral view, thus forming the posterior and lateral margins of the single medial opening for the internal nares. Lateral to that opening, the palatine meets the premaxilla be- tween the vomer (medially) and the maxilla (lat- erally). The palatines have also expanded posteriorly in correlation with the development of large crushing tooth plates (see below). The palatines meet in an extended ventromedial suture and restrict the pterygoids to a posterior position entirely behind the level of the anterior margin of the subtemporal FIELDIANA: GEOLOGY fossa. As a consequence thereof, the pterygoids remain widely separated from the maxilla. The anteromedial margin of the subtemporal fossa is formed by the narrow ectopterygoid, which pos- teriorly extends into the anterior part of the pter- ygoid flange. The pterygoids meet in an interdig- itating ventromedial suture (their posterior edges are incomplete in bt 1 3). Posterolaterally, the pter- ygoid extends into a deep quadrate ramus that forms a prominent pterygoid flange oriented more or less longitudinally and serves as the area of origin for the large pterygoideus musculature. The occipital view of the skull shows a broad occipital exposure of the parietal and squamosal. Ridges indicate a supposed suture between squa- mosal and quadratojugal as indicated by Sues (1987) and Pinna (1989) (see discussion above). The parietal meets the supraoccipital in an ex- tended suture enclosing a small foramen at the anterolateral comers of the supraoccipital (a vas- cular foramen according to Sues, 1987). The oc- ciput of Placodus is deeply concave, and shows slender yet distinct paroccipital processes formed by the exoccipital and opisthotic (Fig. 27). Small posttemporal fossae are bordered ventrally by the paroccipital processes, dorsally by the parietal and squamosal. The distal tips of the paroccipital pro- cesses terminate freely and seem to have been cap- ped by cartilage (intercalary cartilage, an epihyal derivative [Bellairs & Kamal, 1981]) in life, con- necting the paroccipital process to the dorsal part of the quadrate close to the junction of quadrate, squamosal, and pterygoid. The deep quadrate ra- mus of the pterygoid establishes an interdigitating sutural contact with the posteromedial aspect of the quadrate along the latter's entire height. The splanchnocranium is represented by the os- sified epipterygoid and quadrate. The latter bone is deeply concave posteriorly, in dorsal contact with the squamosal and in anteromedial contact with the pterygoid. It is covered in lateral view by the squamosal (see the discussion of the quadra- tojugal above). The mandibular condyle is broad transversely and subdivided to fit the saddle-shaped articular facet of the lower jaw. The epipterygoid of Placodus is a distinct ele- ment with a broad base, sutured to the dorsolateral aspect of the pterygoid and reaching the posteri- orly expanded palatine with its anterior portion. The anteromedial margin of the epipterygoid is lined by an ascending process, which Broili (1912) interpreted as part of the palatine but which more probably is part of the pterygoid. The dorsal part of the epipterygoid is narrower than its base, and contacts the prootic and/or the descending flange of the parietal (the two elements are difficult to distinguish in this region in bt 13). The cavum epiptericum opens anteriorly through a gap locat- ed between the clinoid process of the basisphenoid medially and the pterygoid process lining the an- teromedial edge of the epipterygoid laterally. The basicranium of Placodus deserves special discussion, because morphological relations have changed significantly due to the posterior expan- sion of the palatines. As can be seen in an occipital view of the skull (Fig. 27), the basioccipital con- dyle is formed by the basioccipital only; the ex- occipitals do not meet on the dorsal surface of the occipital condyle. The occipital condyle is pierced by a distinct notochordal pit. The exoccipital is pierced by one large foramen for the passage of the hypoglossal nerve into the jugular foramen (SMF R-359 and R-4038). A well-defined jugular foramen (metotic foramen) is located between the exoccipital and the opisthotic. Ventrally, the op- isthotic appears fused with large tubers extending ventrolaterally from the basicranium in front of and below the occipital condyle (Fig. 28). These tubers have been described as basipterygoid pro- cesses by Sues (1987), but they are a composite structure, formed by the basioccipital posteriorly and the basisphenoid anteriorly, and are better termed palatobasal tubers (Broili, 1912; Zanon, 1989; Nosotti & Pinna, 1993b). The basioccipital- basisphenoid suture can be identified on the an- terolateral aspect of these palatobasal tubers (Fig. 27A; see also Nosotti & Pinna, 1993b), extending dorsally into the anteroventral comer of the fe- nestra ovalis. Accordingly, the basioccipital broadly participates in the formation of the ventral margin of the fenestra ovalis. The palatobasal tubers articulate with the pos- terior and medial aspects of the diverging quadrate rami of the pterygoids. In posterior view, the ba- sisphenoid is exposed between the palatobasal tu- bers and above the pterygoids (Fig. 28B; see also Nosotti & Pinna, 1 993b). The cranioquadrate pas- sage extends between the pterygoid and the dor- solateral aspect of the palatobasal tubers into the postero ventral part of the cavum epiptericum deep to the epipterygoid. Before entering the cranio- quadrate passage, the internal carotid must have given rise to the stapedial (temporal) artery, which entered the temporal region through a distinct slit- like gap between the quadrate ramus of the pter- ygoid laterally and the otic capsule, as well as the descending flange of the parietal medially (Fig. 1 9 A). The otic capsule is composed of the prootic RIEPPEL: THE GENUS PLACODUS 19 Fig. 19. Placodus gigas Ag (bt 13, original of Sues, 1987). Upper Muschelkalk, Bayreuth. A, Skull, dorsal view; B, skull, palatal view; C, skull, left lateral view. Scale bar = 20 mm. Abbreviations: bo, basioccipital; bs, basisphenoid; ep, epipterygoid; f, frontal; ju, jugal; m, maxilla; n, nasal; op, opisthotic; pm, premaxilla; p, parietal; pi, palatine; po, postorbital; pof, postfrontal; prf, prefrontal; pt, pterygoid; q, quadrate; so, supraoccipital; sq, squamosal; v, vomer. anteriorly and the opisthotic posteriorly. Its me- dial wall remains unossified. Details of the course of the facialis nerve are difficult to identify in the skull BT 1 3. SMF R-4038 (original of Edinger, 1 928) shows a rather large opening in the lateral wall of the otic capsule, the irregular shape of which sug- gests an original subdivision of the foramen by a horizontal bony bridge now lost. If that interpre- tation is correct (and it corresponds to Edinger's, 1928, reconstruction of the roots of the facialis nerve), then the dorsal part of the opening would correspond to the vestibular fenestra and the ven- tral portion would serve as the exit of the facialis nerve through a foramen located immediately an- teroventral to the vesitbular fenestra. The trigem- inal nerve preserved its classic relations to the epipterygoid (Goodrich, 1930), the Gasserian gan- glion being housed in a deep prootic incisure sit- uated in front of the prootic and epipterygoid and limited dorsally by the "alisphenoid bridge" (Bro- ili, 1912). The "alisphenoid bridge" is an autapomorphic character of Placodus; its precise derivation re- mains obscure at this time. It appears to be an ossification of the primary braincase, forming a transverse bony bridge in front of the prootics and underlying the tractus olfactorii. An alternative interpretation would be to compare the bony bridge to an ossified subiculum infundibuli (Bellairs & Kamal, 1981), but the space between it and the dermal skull roof appears to be too narrow to ac- commodate the cerebral hemispheres. Lower Jaw The lower jaw of Placodus is characterized by a much elongated symphysis accommodating the roots and replacement teeth for the strongly pro- cumbent incisors. The dentaries always contribute to the mandibular symphysis with a deeply inter- digitating suture. The degree to which they do so 20 FIELDIANA: GEOLOGY RIEPPEL: THE GENUS PLACODUS 21 Fig. 20. Placodus gigas Ag (bt 1 3, original of Sues, 1987), premaxillary rostrum in dorsal view. Upper Mu- schelkalk, Bayreuth. Scale bar = 10 mm. is variable, however. In smf R-1035 and smf R-4 1 1 2 (Fig. 29 A), the dentaries remain separate from one another in their anterior part (i.e., be- tween the alveoli for the incisors), whereas in other jaws (such as smf R-41 10 [Fig. 29B] and bsp AS VII 1208) the dentaries are in contact with each other up to the anterior margin of the mandibular symphysis. The degree to which the anterior parts of the dentaries fuse may be subject to late onto- genetic variation because the posterior crushing teeth are somewhat larger in smf R-4 1 1 (with fully fused dentaries), as opposed to smf R-4 1 1 2 (see discussion of dentition below). Behind the symphysis and lateral to the posterior crushing teeth, the dentary develops a broad lateral shelf for the insertion of the superficial jaw adductor muscle fibers. Posteriorly, the dentary extends into a large ascending process that covers most of the lateral surface of the high coronoid process. The broad splenial closes Meckel's canal me- dially, gaining a ventral and, anteriorly, a narrow ventrolateral exposure on the lower jaw. Anteri- FiG. 21. Placodus gigas Ag (bsp 1968 I 75; original of Broili, 1912, PI. 14, Figs. 1-4). Upper Muschelkalk, Hegnabrunn near Kulmbach. A, Skull, dorsal view; B, frontal and parietal bones, dorsal view. Scale bar = 20 mm. Abbreviations: f, frontal; p, parietal. 22 HELDIANA: GEOLOGY Fig. 22. Macroplacus raeticus Schubert-KJempnauer (holotype, bsp 1967 I 324; original of Schubert and Klemp- nauer, 1975, Pis. 4, 5). Rhaetic (upper Triassic), Hinterstein near Hindelang, Bavarian Alps. Scale bar = 20 mm. Abbreviations: f, frontal; ju, jugal; m, maxilla; n, nasal; p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; pt, pterygoid; qj, quadratojugal; sq, squamosal. orly, the splenials of the two mandibular rami meet in an interdigitating medioventral suture behind the dentaries, thus contributing to the anteropos- terior elongation of the mandibular symphysis. Posteriorly, the splenial meets the prearticular and angular in a suture enclosing a large mylohyoid foramen. The coronoid process of Placodus is very high, but incompletely preserved in most specimens ex- cept for that described by Drevermann (1 933), the original of which is on permanent exhibit and hence inaccessible for detailed investigation. The den- tary forms a large posterodorsal process that cov- ers most of the lateral surface of the coronoid pro- cess, meeting the surangular posteriorly (Fig. 30). The coronoid bone has a very limited lateral ex- posure on the coronoid process (Huene, 1936, 1943). The element is largely restricted to the an- teromedial aspect of the coronoid process, defining the anterior margin of the deep adductor fossa. This is a derived (autapomorphic) character of Placodus, contrasting with the broad lateral ex- posure of the coronoid in Paraplacodus and in cyamodontids (Drevermann, 1928) (Fig. 31). The adductor fossa in the lower jaw oi Placodus is wide and deep, allowing an anterior expansion of the jaw adductor musculature into Meckel's canal (m. intramandibularis: see Rieppel, 1990, for a dis- cussion). A fragmentary jaw (smf R-365) shows a vertically oriented bony ridge projecting from the dorsal surface of the angular into the floor of the adductor fossa; it must have intersected the intra- mandibular muscle, thereby providing an im- proved area for fiber insertion. The surangular covers the posterolateral aspect of the coronoid process, from where it extends in an anteroventral direction betwen the dentary and the angular. The angular is a broad, cup-shaped Fig. 23. Placodus gigas Ag (smns 59434), incomplete skull, right lateral view. Upper Muschelkalk, Bayreuth. Scale bar = 20 mm. RIEPPEL: THE GENUS PLACODUS 23 Fig. 24. Placodus gigas Ag (bt 13, original of Sues, 1987). Upper Muschelkalk, Bayreuth. A, Posterior (temporal) region of skull in left lateral view; B, posterior (temporal) region of skull in right lateral view. Scale bar = 20 mm. element that wraps around the posteroventral part of the lower jaw. The retroarticular process is well developed and covered by the large articular- prearticular medially and dorsally. The prearti- cular meets the angular in a suture that runs along the laterodorsal edge of the retroarticular process. Dentition Placodus is characterized by strongly procum- bent, chisel-shaped anterior teeth, separated by a diastema from the posterior crushing teeth. The premaxillae of Placodus each bear three incisors (Braun, 1836). The maxillae bear four rounded crushing teeth in most specimens, with variation ranging from three to five (''Placodus quinimolar- is"). The palatines are expanded posteriorly and bear three large tooth plates each, which increase in size from front to back. Each dentary bears two strongly procumbent in- cisors opposing the premaxillary incisors. Three large tooth plates are typically situated on the broad posterior part of the dentary, separated from the incisors by a distinct diastema. The posteriormost tooth plate is the largest, and is positioned im- FiG. 25. Holotype of Placodus hypsiceps Meyer (bt, uncatalogued; original of Meyer, 1863, PI. 24, Figs. 1-3; PI. 29, Fig. 4). Upper Muschelkalk, Bayreuth. Scale bar = 20 mm. Abbreviations: ju, jugal; m, maxilla; n, nasal; pm, premaxilla; po, postorbital; pof, postfrontal; prf, prefrontal; sq, squamosal. 24 FIELDIANA: GEOLOGY Fig. 26. Placodus gigas Ag (bsp 1968 I 75; original of Broili, 1912, PI. 14, Figs. 1^). Upper Muschelkalk, Hegnabrunn near Kulmbach. Posterior (temporal) re- gion of skull in left lateral view. Scale bar = 20 mm. Abbreviations: ju,jugal; po, postorbital; pof, postfrontal; q, quadrate; sq, squamosal. mediately in front of the coronoid process, where the load arm relative to the force arm is shortest. The crushing tooth plates on the dentary are typ- ically much larger than the maxillary teeth, and occlude against the medial part of the latter as well as the lateral aspect of the palatine tooth plates. Variation of maxillary tooth counts was dis- cussed in the systematic section above, with re- spect to the questionable validity of Placodus qui- nimolaris. Variation of tooth counts can also be seen in the posterior dentition of the lower jaw, with three large tooth plates being the norm. In a number of individuals, a distinctly smaller and rounded (fourth) crushing tooth is positioned im- mediately in front of the (three) large dentary tooth plates. Such is the case in a fragmentary dentary from the lower Muschelkalk of Winterswijk (Oos- terink, 1978; Fig. 1 in this paper), as well as in several sjjecimens from the upp>er Muschelkalk (bsp 1968 I 76, SMF R-363, smf R-1035, smf R-41 10, SMNS 58021, SMNS 17572). Late ontogenetic ad- dition of a fourth tooth to the posterior dentary tooth row is suggested by a comparison of smf R-41 12 and smf R-41 10 (Fig. 32). The first spec- imen (smf R-4 112) is smaller than the second (transverse diameter of fourth dentary tooth plate: 27.5 [25] mm), the anterior parts of the dentaries remain separate, and only three tooth plates are located on the dentary. smf R-41 10 is somewhat larger (transverse diameter of fourth dentary tooth plate: 31.5 [33.2] mm), the anterior parts of the dentary have completely fused, and a fourth ele- ment has been added to the posterior dentary tooth row on the left lower jaw ramus. It should be noted, however, that of a total of 24 articulated dentary Fig. 27. Placodus gigas Ag, occipital view of skull. A, BT 13 (original of Sues, 1987), upper Muschelkalk, Bayreuth; B, bsp 1968 I 75 (original of Broih, 1912, PI. 14, Figs. 1-4), upper Muschelkalk, Hegnabrunn near Kulmbach. Scale bar = 20 mm. Abbreviations: bo, ba- sioccipital; eo, exoccipital; p, parietal; pt, pterygoid; q, quadrate; so, supraoccipital; sq, squamosal. dentitions, the seven specimens that show four posterior dentary teeth span the entire size range (Fig. 33). They include the smallest specimen of all, the right dentary from the lowermost Mus- chelkalk of Winterswijk (Fig. 34). Conversely, the two largest available jaws bear only three dentary tooth plates. As is the case with maxillary teeth (see discussion above), ontogeny offers only a par- tial explanation of the variation of tooth counts in the posterior dentary tooth row. Tooth replacement in Placodus was shown by Broili (1912) to be by vertical succession. This is documented by many specimens, including sec- tions, for the maxillary, palatine, and posterior dentary teeth. In the maxillary, palatine, and pos- terior dentary bones, the replacement teeth de- velop directly above, or below, the functional tooth. Transverse sections through the palatines, as well as parasagittal sections through the maxilla and/ or dentary, show replacement teeth at different stages of maturation. The limited material avail- able indicates that at least the large tooth plates RIEPPEL: the GENUS PLACODUS 25 Fig. 28. Placodus gigas Ag, basioccipital tubers. A, Right posterolateral view (smf R-4038; original of Edinger, 1 928, PI. 24, Fig. 1 ), upper Muschelkalk, Bayreuth; B, posterior view (bt 1 3; original of Sues, 1987), upper Muschelkalk, Bayreuth. Scale bar = 20 mm. on the palatines and dentaries are replaced one by one, maintaining the dentition in a continuously functional condition. The anterior chisel-shaped incisors show hori- zontal tooth replacement, which, due to the strongly procumbent position of the functional teeth, mim- ics vertical replacement. Replacement teeth for the premaxillary (bmnh R-41096, Fig. 3 5 A; smns 18641) and anterior dentary teeth (bsp AS VII 1 209, Fig. 35B) develop in a position posterior (medial morphologically) to the functional tooth, within a replacement pit that will eventually migrate an- teriorly (morphologically laterally) in completion of the replacement cycle. In his description of the skull bt 1 3, Sues (1987) described a slitlike infraorbital fenestra (Fig. 3 6 A). In fact, this specimen shows similar openings all along the maxillary-palatine suture, each posi- tioned lateral to a palatine tooth plate. The pos- terior foramen (the infraorbital fenestra of Sues, 1 987), located lateral to the posterior palatine tooth plate, is also well developed in a fragmentary skull (smns 18641), which shows that the opening has no connection to the floor of the orbit. In the ho- lotype of Placodus gigas (bsp AS VII 1208), the replacement for the left posterior palatine tooth plate can be seen through the foramen (Fig. 36B). I therefore concur with G. Pinna (pers. comm.) that the foramina located lateral to the palatine tooth plates (including the infraorbital fenestra of Fig. 29. Placodus gigas Ag, mandibular symphysis, ventral view. A, smf R-4112 (original of Huene, 1936, Fig. 23a), upper Muschelkalk, Bayreuth; B, smf R-41 10 (orginal of Huene, 1936, Fig. 23a), upper Muschelkalk, Bayreuth. Scale bar = 20 mm. 26 HELDIANA: GEOLOGY sang par Fig. 30. Placodus gigas Ag, right lower jaw ramus (SMF R-41 12). Upper Muschelkalk, Bayreuth). A, Right lateral view; B, medial view (partially reconstructed). Scale bar = 20 mm. Abbreviations: ang, angular; c, cor- onoid; d, dentary; par, prearticular; sang, surangular; sp, splenial. Sues, 1987) are, in fact, dental lamina openings relating to the development of replacement teeth for the palatal and maxillary dentition. Postcranial Skeleton The postcranial skeleton of Placodus has been described in a beautifully illustrated monograph by Drevermann (1933; published posthumously). The functional anatomy of the skeleton of Pla- codus has been dealt with by Vogt (1983). The following description will be based mainly on Drevermann's (1933) sp>ecimen unless otherwise noted. The vertebral column of Placodus comprises 8 cervicals, 20 dorsals, 3 sacrals, and 40-50 caudals. The vertebrae are characterized by a deeply am- phicoelous and notochordal centrum (Fig. 37). The neural canal is high and almost rectangular in cross section. The pedicles of the neural arch sit on nar- row ridges, which define the neural canal on the dorsal surface of the centrum. The neurocentral suture remains visible throughout the vertebral column, but the neural arch rarely separates from the centrum during fossilization. The transverse processes are slender and elongate throughout the dorsal region. Intervertebral articulations are strengthened by the development of hyposphene and hypantrum (Figs. 37, 38). These are weakly develop)ed in the cervical region, but distinct sang ang Fig. 3 1 . Cyamodus sp., lower jaw (smf R-4040; orig- inal of Drevermann, 1928, PI. 23, Figs. 3a-d); upper Muschelkalk, Bayreuth. Scale bar = 20 mm. Abbrevi- ations: ang, angular; ar, articular; c, coronoid; d, dentary; sang, surangular; sp, splenial. throughout the dorsal region and absent in the caudal region. The hyposphene is a posterior pro- jection on the neural arch, situated just above the neural canal but below the postzygapophyses; it articulates with the hypantrum, a groove located below the prezygapophyses of the succeeding ver- tebra. The hyposphene-hypantrum articulation differs fundamentally in its topological relations from the zygosphene-zygantrum articulation of other sauropterygians (eosauropterygians: Riep- pel, 1 994a). The zygosphene is an anteror projec- tion of the neural arch located above the prezy- gapophyses. It articulates with the zygantrum on the posterior surface of the preceding vertebra, again dorsal to the postzygapophyses. The centra of the cervical vertebrae are keeled ventrally. If present, the rudimentary hyposphene and hypantrum do not engage in intervertebral articulation. The articular surfaces of the pre- and postzygapophyses show a tendency toward in- creased inclination along an anteroposterior gra- dient. In the third cervical vertebra, the articular surface of the prezygapophysis is inclined by ap- proximately 20° from the horizontal (facing dor- somedially), in the seventh element, the inclina- tion has increased to approximately 35° from the horizontal. The first cervical rib is associated with the axis. The cervical ribs are dichocephalous and carry a free anterior process in addition to tuber- culum and capitulum. The diapophysis is formed by the base of the neural arch facing ventrolater- ally, and develops into a distinct free-ending trans- verse process in the last cervical. The parapo- physis, facing laterally, develops into a distinct projection close to the anterior margin of the cen- trum in the posterior cervical vertebrae. Hyposphene and hypantrum are prominently developed and engage in intervertebral articula- tion in the first dorsal. The diapophysis expands RIEPPEL: THE GENUS PLACODUS 27 Fig. 32. Placodus gigas Ag, lower jaw, occlusal view. A, smf R-41 10 (original of Huene, 1936, Fig. 23a), upper Muschelkalk, Bayreuth; B, smf R-41 12 (original of Huene, 1936, Fig. 23a), upper Muschelkalk, Bayreuth. Scale bar = 50 mm. into elongated transverse processes in the anterior dorsal region, whereas all traces of the parapo- physis are lost on the second dorsal element. The dorsal ribs are holocephalous. The trend toward increased inclination of the articular surface of pre- and postzygapophysis continues into the posterior dorsal region, where it becomes reversed. In the eighth dorsal vertebra, the articular surface of the prezygapophysis is inclined by approximately 45° from the horizontal (facing dorsomedially), in the 1 3th element, the inclination has increased to ap- proximately 55° from the horizontal, but in the 1 5th dorsal, the inclination has decreased to ap- proximately 30° from the horizontal. In the 18th dorsal vertebra, the inclination of the articular sur- faces has decreased to approximately 1 5° from the horizontal, whereas in the sacral vertebrae, the inclination of the articular surfaces of pre- and postzygapophyses is approximately 10-15° from the horizontal. The sacrum of Placodus comprises three ver- tebrae, but Drevermann's (1933, PI. 9, Figs. 49a- c) specimen shows an interesting partial sacrali- zation of the last dorsal ("lumbar") vertebra. Gradual reduction in the length of transverse pro- cesses starts with the 1 6th dorsal element. The last dorsal vertebra retains a short transverse process on the left side but shows a deep and only weakly projecting articular facet on its right side. Expan- sion of the articular facet across the neurocentral suture onto the centrum is characterisitic of sacral vertebrae. The sacral ribs are not fused to the sa- cral vertebrae, and are characterized by distinct proximal and distal expansions. The distal expan- sion is least expressed in the first sacral rib, and most distinct in the third sacral rib. The proximal caudal vertebrae show an in- creased inclination of the articular surfaces of the pre- and postzygapophyses again, approximately 35° from the horizontal in the first element and approximately 55° from the horizontal in the third element. Caudal ribs are not fused to their re- spective centrum and may have extended to the 10th or 12th element; the ribs of the first caudal vertebra trend in an anterior direction (toward the ilium) and show a distal expansion, as do the sacral 28 FIELDIANA: GEOLOGY 'op5, ein neuer Placodon- tier. Palaeontographica, A, 84: 99-148. 1943. Fragmenta Sauropterygiana. Neues Jahrbuch fiir Mineralogie, Geologie und Palaontolo- gie, Monatshefte 1943, Abt. B, 248-255. 1 949. Eine biologische Museums-Aufstellung . 1956. Palaontologie und Phylogenie der nied- eren Tetrapoden. G. Fischer, Jena, xii + 716 pp. Jaekel, O. 1907. Placochelys placodonta aus der Ob- ertrias des Bakony. In Resultate der wissenschaftlich- en Erforschung des Balatonsees, 1 . 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Die Triasfauna der Tessiner Kalkalpen. VIII. Weitere Placodontierfunde. Abhandlungen der Schweizerischen Palaontologischen Gesellschaft, 55: 1-26. . 1968. Comparative Odontology. Translated and edited by Rainer Zangerl. University of Chicago Press, Chicago, xiv + 347 pp. Peyer, B., AND E. KuHN-ScHNYDER. 1955. Placodon- tia. In Piveteau, J., ed., Traite de Paleontologie. 5: 459-486. Masson, Paris. Pinna, G. 1 989. Sulla regione temporo-jugale dei rettili placodonti e sulle relazioni fra placodonti e ittiotterigi. Atti della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, 130: 149- 158. . 1990. Notes on stratigraphy and geological dis- tribution of placodonts. Atti della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, 131: 145-156. Pinna, G., and J.-M. Mazin. 1993. Stratigraphy and paleobiogeography of the Placodontia. In Mazin, J.- M., and G. Pinna, eds., Evolution, Ecology and Bio- geography of the Triassic Reptiles. Paleontologia Lombarda, n.s., 2: 125-130. RiEPPEL, O. 1989. A new pachypleurosaur (Reptilia: Sauropterygia) from the Middle Triassic of Monte San Giorgio, Switzerland. Philosophical Transactions of the Royal Society of London, B, 323: 1-73. . 1990. The structure and development of the jaw adductor musculature in the turtle Chelydra ser- pentina. Zoological Journal of the Linnean Society, 90: 27-62. 1 994a. Osteology of Simosaurus gaillardoti and the relationships of stem-group Sauropterygia. Field- iana (Geology), n.s. 28: 1-85. . 1994b. The braincasesof 5/mo5<3Mrusand A^o- thosaurus: monophyly of the Nothosauridae (Reptilia: Sauropterygia). Journal of Vertebrate Paleontology, 14: 9-23. RoMER, A. S., and L. I. Price. 1940. Review of the Pelycosauria. Geological Society of America Special Papers, 28: 1-538. Schmidt, M. 1928. Die Lebewelt unserer Trias. 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Con- tinental Breakup and the Origin of the Atlantic Ocean and Passive Margins, pp. 71 1-755. Elsevier, Amster- dam. ZiTTEL, K. A.v. 1887-1890. Handbuch der Palaeon- tologie, 1 . Abtheilung. Palaeozoologie, III. Band. Ver- tebrata (Pisces, Amphibia, Aves). R. Oldenbourg, Miinchen und Leipzig, xii + 900 pp. Appendix: Material Included in This Study Institutional Abbreviations Bundesanstalt fur Geowissenchaften und Roh- stoffe, Berlin (bgr; only type material or otherwise published and figured specimens are catalogued in this institution. Other specimens are referred to by drawers. The prefix S specifies the stratigraphic collection. Each cabinet has two rows of drawers [left and right] numbered from top to bottom); The Natural History Museum, London: bmnh; Bayerische Staatssammlung flir Palaontologie und historische Geologic, Munich: bsp; Naturkun- demuseum Erfurt: Er; Institute fiir Geologische Wissenschaften der Martin-Luther Universitat, Halle: Ha; Senckenberg Museum, Frankfurt a.M.: SMF, Staatliches Museum fiir Mineralogie und Geo- logic, Dresden: ba Tr; Staatliches Museum far Na- | turkunde, Stuttgart: smns; Muschelkalk Museum | Ingelfingen, Sammlung H. Hagdom: shg; Ober- frankisches Erdgeschichtliches Museum, Bayreuth: BT, 42 HELDIANA: GEOLOGY Material Cyamodus: smns 15855, 16270 (upper Muschel- kalk, Tiefenbach near Crailsheim; skulls); smns 59825 (upper Muschelkalk, Hegnabrunn; dorsal vertebra); smns 15937 (upper Muschelkalk, Hel- denmiihle near Crailsheim; humerus); smns 17872 (upper Muschelkalk, Heldenmiihle near Crails- heim; humerus); smns 18057 (upper Muschelkalk, Heldenmiihle near Crailsheim; humerus); smns coll. M. Wild #1070 (upper Muschelkalk, Unterrodach. Original of Vogt, 1983, Fig. 2d); smns uncatalogued (upper Muschelkalk, Zuffenhausen— Stuttgart); Placodus gigas: Partially articulated skeleton— smf R-1035 (upper Muschelkalk, Steinsfurt near Heidelberg; original of Drevermann, 1931, 1933). Skulls or skull fragments— bmnh R-1642, upper Muschelkalk, Bayreuth (original of Owen, 1858, PI. 10, Figs. 2-5); bmnh R-35868, upper Muschelkalk, Bayreuth; bmnh R-41096, upper Muschelkalk, Bayreuth (original of Seeley, 1889); BSP AS VII 1208, upper Muschelkalk, Bayreuth (holotype of Placodus gigas); bsp AS VII 1211, upper Muschelkalk, Bayreuth; bsp 1968 I 75, up- per Muschelkalk, Hegnabrunn near Kulmbach (original of Broili, 1912, PI. 14, Figs. 1^); bt, uncatalogued, upper Muschelkalk, Bayreuth (ho- lotype of Placodus andriani, Placodus hypsiceps, Placodus quinimolaris, and several skull frag- ments); BT 13, upper Muschelkalk, Bayreuth; ba Tr 43, upper Muschelkalk, Bayreuth; Er 78/235, upper Muschelkalk, Bad Suiza; Er 78/23 5a, upper Muschelkalk, Bad Suiza; Ha, uncatalogued, lower Muschelkalk, Freyburg/Unstrut (holotype and as- signed material oi Placodus antiquior); smf R-359 a, b, upper Muschelkalk, Bayreuth (original of Bro- ili, 1912; Edinger, 1928); smf R-360, upper Mu- schelkalk, Bayreuth (original of Broili, 1 9 1 2, p. 151); SMF R-366, upper Muschelkalk, Bayreuth; smf R-1035, upper Muschelkalk, Bayreuth; smf R-4038, upper Muschelkalk, Bayreuth (original of Edinger, 1928); smf R-4038, upper Muschelkalk, Bayreuth; smf R-4162, upper Muschelkalk, Bay- reuth; SMNS 12679, upper Muschelkalk, Miinster; smns 1 864 1 , upper Muschelkalk, Crailsheim; smns 59434, upper Muschelkalk, Bayreuth; Lower jaws or fragments thereof— bmnh R-1641, upper Muschelkalk, Bayreuth (holotype of Placodus pachygnathus); bmnh R-1643, upper Muschelkalk, Bayreuth (holotype of Placodus bombidens); bmnh R- 19677, upper Muschelkalk, Bayreuth (holotype of Placodus bathygnathus); bsp 1925 1 16, upper Muschelkalk, Bayreuth; bsp 1968 I 76, upper Muschelkalk, Hegnabrunn near Kulm- bach; bsp as VII 1209, upper Muschelkalk, Bay- reuth; Er 78/278, upper Muschelkalk, Bad Suiza; Ha, uncatalogued, lower Muschelkalk, Freyburg/ Unstrut; smf R-359, upper Muschelkalk, Bay- reuth; SMF R-362, upper Muschelkalk, Bayreuth; SMF R-363, upper Muschelkalk, Bayreuth; smf R-364, upper Muschelkalk, Bayreuth; smf R-365, upper Muschelkalk, Bayreuth (original of Drev- ermann, 1933, p. 334); smf R-367, upper Mu- schelkalk, Bayreuth; smf R-368, upper Muschel- kalk, Bayreuth; smf R-492, upper Muschelkalk, Bayreuth; smf R-41 10, upper Muschelkalk, Bay- reuth; smf R-41 12, upper Muschelkalk, Bayreuth; SMNS 1 7572, upper Muschelkalk, Stengelberg; smns 58021, upper Muschelkalk, Lauchringen; smns uncatalogued, coll. M. Wild #98, upper Muschel- kalk, Hegnabrunn near Kulmbach. Vertebrae-smf R-576, R-578, R-579, R-2000, R-2003, upper Muschelkalk (dorsal vertebrae); smf R-2001, upper Muschelkalk, Bayreuth (two cer- vical vertebrae); smns 53006, lower Muschelkalk, Bodigheim (caudal vertebra); smns 59370, lower Muschelkalk, Freudenstadt (dorsal vertebra); smns coll. M. Wild #8 1 , upper Muschelkalk, Bindlach (sacral vertebra). Humerus— SMF R-672, upper Muschelkalk, Bayreuth (possibly Cyamodus, R. Wild, in litt., 3 November 1977); smns 15891, upper Muschel- kalk, Tiefenbach near Crailsheim; smns 59827, upper Muschelkalk, Hegnabrunn near Kulmbach (original of Vogt, 1983, Fig. 2b); smns uncata- logued, coll. M. Wild #1365, upper Muschelkalk, Hegnabrunn near Kulmbach). Femur— SMF R-86, upper Muschelkalk, Bay- reuth; SMF R-88, upper Muschelkalk, Bayreuth; SMF R-760, upper Muschelkalk, Bayreuth (proxi- mal end only); smns uncatalogued, coll. M. Wild #1789, upper Muschelkalk, Bindlach near Bay- reuth. Isolated Tooth Plates— bmnh R- 1328-9, up- per Muschelkalk, Bayreuth; bgr X-06153, upper Muschelkalk, Tamowitz, Upper Silesia (original of Meyer, 1851b, PI. 29, Fig. 51); bgr X-06154, upper Muschelkalk, Rybina, Upper Silesia (orig- inal of Meyer, 1851, PI. 29, Fig. 52); brg S 55-R 14, upper Muschelkalk, Riidersdorf near Berlin; BRG S 55-R 15, upper Muschelkalk, Riidersdorf near Berlin; brg S 55-L 02, lower Muschelkalk, Riidersdorf near Berlin; brg S 54-R 02, upper Muschelkalk, Jena; brg S 54-R 09, lower Mu- schelkalk, Freyburg/Unstrut; brg S 54-L 09, lower Muschelkalk, Jena; brg S 56-R 02, lower Mu- RIEPPEL: THE GENUS PLACODUS 43 schelkalk, Riidersdorf near Berlin; brg S 56-R 03, Muschelkalk, Jena; smns 15992, lower Muschel- lower Muschelkalk, Riidersdorf near Berlin; brg kalk, Jena; smns 56313, lower Muschelkalk, Lam- S 56-L 11, lower and middle Muschelkalk, Rud- bach-Sulzbad; smns 13665, lower Muschelkalk, ersdorf near Berlin; bsp 1959 XIII 25, lower Mu- Sondershausen. schelkalk, Freyburg/Unstrut; smns 13666a, lower 44 FIELDIANA: GEOLOGY A Selected Listing of Other Fieldiana: Geology Titles Available A Preliminary Survey of Fossil Leaves and Weil-Preserved Reproductive Structures from the Sentinel Butte Formation (Paleocene) near Almont, North Dakota. By Peter R. Crane. Steven R. Manchester, and David L. Dilcher. Fieldiana: Geology, n.s., no. 20, 1990. 63 pages, 36 illus. Publication 1418, $13.00 Comparative Microscopic Dental Anatomy in the Petalodontida (Chondrichthyes, Elasmobranchii). By Rainer Zangerl, H. Frank Winter, and Michael C. Hansen. Fieldiana: Geology, n.s., no. 26, 1993. 43 pages, 35 illus. Publication 1445, $16.00 Status of the Pachypleurosauroid Psilotrachelosaurus toeplitschi Nopcsa (Reptilia, Sauropterygia), from the Middle Triassic of Austria. By Olivier Rieppel. Fieldiana: Zoology, n.s., no. 27, 1993. 17 pages, 9 illus. Publication 1448, $10.00 Osteology of Simosaurus gaillardoti and the Relationships of Stem-Group Sauropterygia. By Olivier Rieppel. Fieldiana: Geology, n.s., no. 28, 1994. 85 pages, 71 illus.. Publication 1462, $18.00 Revised Phylogeny and Functional Interpretation of the Edrioasteroidea Based on New Taxa from the Early and Middle Ordovician of Western Utah. By Thomas E. 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