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A juvenile Parasaurolophus (Ornithischia: Hadrosauridae) from the Upper Cretaceous Fruitland Formation of New Mexico

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Description
A juvenile Parasaurolophus (Ornithischia: Hadrosauridae) from the Upper Cretaceous Fruitland Formation of New Mexico
Transcript
  Lucas, S.G., and Heckert, A.B., eds., 2000, Dinosaurs of New Mexico. New Mexico Museum of Natuml History and Science Bulletin No. 17. A JUVENILE PARASAUROLOPHUS (ORNITHISCHIA: HADROSAURIDAE)FROMTHE UPPER CRETACEOUS FRUITLAND FORMATION OF NEW MEXICO ROBERT M. SULLIVAN and G.E. BENNETT, III Section of Paleontology and Geology, The State Museum of Pennsylvania, Third and North Streets, Harrisburg, PA 17108-1026 Abstract- An incomplete skull (SMP VP-1090) consisting of a nearly complete rightdentary includingteeth, both surangulars, left? quadratojugal, both jugals, left quadrate, ?left postorbital and unidentified bone fragments, was recovered from the Upper Cretaceous (upper Campanian) Fruitland Formation (Ah-shi-sle-pah Wash) during the summer of 1998. The specimen is a lambeosaurine based on: (1) an anteriorly bowed quadrate; and (2) a short edentulous rostral section of the dentary. Other characters permit identification to the generic level. These include: (1) elongate teeth with slightly sinuous median ridges; and (2) jugals distinguished, in part, by the near parallel ascending postorbital and posterior jugal processes that form a narrow lower temporal fenestra, aswell as a distinct broad anterior process. These features are consistent with those of Parasaurolophus, so we refer SMP VP-1090 to Parasaurolophus sp. Based on the size of the dental battery, we estimate its length to be one-third that of an adult Parasaurolophus tubicen (NMMNH P-25100), and conclude that this new specimen is a juvenile. Although other juvenile material has been referred to d. Parasaurolophus, this is the first bona fide juvenile of Parasaurolophus from New Mexico. INTRODUCTIONDuring the summer of 1998, the senior author discoveredthe disarticulated remains of a small (juvenile) hadrosaur skullweathering out of the Fruitland Formation in Ah-shi-sle-pah Wash(in the Ah-shi-sle-pah Wilderness Study Area). With the help offield assistants Fred Widmann and Michael Lamirata, the speci men (SMP VP-1090) was collected and brought back to The State Museum of Pennsylvania (SMP) for study. While the specimenlacks the distinctive narial crest (long or short variety) that is characteristic of Parasaurolophus, other recovered elements (see below)clearly indicate that the specimen is referable to this genus. Inthis paper, we document the first unequivocal juvenile specimenof Parasallrolophlls from New Mexico. GEOLOGIC SETTINGThe Fruitland Formation crops out throughout most of theAh-shi-sle-pah Wilderness Study Area. Although the upper and lower limits of this formation have yet to be mapped in detaillocally, Hunt and Lucas (1992) illustrated the dinosaur-bearing unit outcrops in this region. Extrapolation of the ash datum fromashes 2 and 4 of Fassett and Steiner (1997) to the west places the age of the Fruitland at about 74.5 million years in the Ah-shi-sle pah Wilderness Study Area.SYSTEMATIC PALEONTOLOGY DINOSAURIA 215 ORNITHISCHIA Seeley, 1887 HADROSAURIDAE Cope, 1869LAMBEOSAURINAE Parks, 1923 PARASAUROLOPHUS Parks, 1922 FIGURE 1. Parasallroiophlls sp., SMP VP-1090, right dentary in A, lateral, and B, medial views. Bar scale = 5 em. Parasaurolophus sp.(Figs. 1-3) New referred material-SMP VP-1090 (Figs. 1-3), incomplete juvenile skull consisting of a nearly complete right dentary including teeth, both jugals, ?left postorbital, left quadrate, left quadratojugal,incomplete left and nearly complete right surangular and miscellaneous bone fragments. Formation/Age-Fruitland Formation (late Campanian). Locality-Ah-shi-sle-pah Wash (Ah-shi-sle-pah Wilderness Study Area), SMP loco #365, SanJuan County, New Mexico. Collectors/Date-Robert M. Sullivan, Fred Widmann and Michael Larnirata, 5 August 1998.  216 DESCRIPTION Dentary (including the coronoid process)- The dentary(Fig. 1) is nearly complete, although the tooth dental battery itselfis only partly preserved. The length of the dentary is 270 mm, with a maximum depth of 83 mm measuredmidway along thedental battery to the ventral margin. In lateral aspect (also evi dentin medial aspect), the dentary is distinguished by a pronounced downturn of its anterior edentulous projection that servesfor the attachment of the predentary. A large, anteroventrally-projecting foramen is situated just below the dorsal anterior marginof the dentary. The nearly complete coronoid rises from thedentary posteriorly and is tilted forward. In medial view (Fig. lB), the anterior edentulous portiondescends ventromedially. There is a strong dentary symphysisthat bears a distinct, anteroventrally-directed groove at its posterior edge. The mandibular groove is well developed and extends along the ventral margin of the dentary, terminating just behind the anterior-most part of the dental battery. The articularsurface of the mandibular groove is well developed along theposterior ventral margin and expands posteriorly; forming a strongcontact for the angular. Posteriorly, below the coronoid process,there is the large vacuity of the Meckelian canal.The dental battery is incomplete, having been eroded awayposteriorly. At least 26 tooth rows are present, but a precise countcannot be made due to the missing teeth and condition of thedental battery. Special nutrient foramina are present along thelower margin of the dental battery. The teeth are best preserved in the anterior half of the dentary battery. Individual teeth areelongate, and some are distinguished by slightly sinuous carinae.In addition, many of the teeth bear denticles along the anterior and posterior margins of the tooth crowns. There are also faintsecondary ridges present on some of the medial surfaces of theteeth located between the medial ridge and the margins. Jugal-Both the left and right jugals (Fig. 2A-D) are preserved, with the right being the more complete. The right jugalmeasures 170 mm from the anterior-most tip to the posterior-mostexpanse of the quadratojugal process. The jugal appears to be relatively shorter anteroposteriorly than in hadrosaurines, but measurement of the left jugal is not possible because it is broken anteriorly and posteriorly. Both jugals are fractured, especially theregion of the quadratojugal process. The follOWing description isbased on the right jugal (Fig. 2A-B).The lateral surface of the jugal is smooth and bowed later ally below the lower temporal fenestra. The ascending quadratojugal and postorbital processes are sub-parallel, form ing a rather narrow lower supratemporal opening. The anteriorly positioned inferior edge of the orbital margin is broad posteriorly, and rises somewhat abruptly anteriorly. The anterior projection of the jugal, which largely articulates with the lacrimal, and possibly the palatine (dorsally) and the maxilla (ventrally), isvery broad and is nearly symmetrical, divided equally (above and below) by an anteriorly-directed process (see Discussion). Medially, this anterior region bears a strong sutural attachment, in part indicated by an irregular ridge posteriorly, that is positioned nearthe anterior-most end of the suborbital constriction of the jugalbar. The suborbital constriction of the jugal bar, thatpart of thejugal below the orbit, is relatively deep and short (anterior-posteriorly). The ventral surface of the suborbital constriction of thejugal bar is highly arched. Postorbital-There is a small, disarticulated element that we have questionably identified as the left postorbital (Fig. 3A D). This element, which was found adjacent to the ascending postorbital process of the leftjugal, is nearly complete and conformsto the shape of a hadrosaur postorbital. However, it seems rela-tively small compared to the other elements of the specimen,which makes us somewhat hesitant with this identification. Indorsal view, the postorbital is somewhat flat and bears a posteriorly-directed, wing-like process that articulates with the medial,anterior-directed process of the left squamosal. The posteriormargin of the postorbital forms the anterolateral part of the supratemporal fenestra. Medially, there is a strong articular surfacefor the prefrontal/ frontal contact (Sullivan and Williamson, 1999,fig. 17). The anterior margin of the postorbital forms the posterior-most extent of the orbital rim. Ventrally; the element descendsalong the lateral contact. The squamosal contact tapers posteriorly in lateral view (Fig. 3D). The frontal/prefrontal contact (Fig.3C) is not as large and is highly rugose, indicating a strong sutural attachment. There is some breakage of the anterior portionsof both the squamosal and frontal/prefrontal contact. The extentof the breakage cannot be determined with certainty, but we believe the missing portions to be negligible. Quadrate-Only the left quadrate (Fig. 2E-F) was recovered. It is nearly complete and measures 185 mm tall from its inferior end, where it articulates with the articular and surangular,to its superior end where it articulates with the squamosal. Inlateral view, the quadrate is distinctly curved posteriorly. Thiscurvature is especially pronounced along the upper posterior halfof the quadrate.There are two anteriorly-directed wings of the quadrate thatextend from the shaft: (1) an incipient external lateral wing (Fig. 2E) that articulates with the quadratojugal and jugal; and (2) awell-developed medial, or internal, wing that articulates with thepterygOid. Viewed anteriorly, the two wings of the quadrate forma rather deep "V" shape that is directed anteroposteriorly. Thelateral surface of the external wing is, in part, marked by the jugalshelf, which lies along the superior anterior edge. Its upper limitlies 35 mm below the top of the squamosal process and extends down to the top of the quadratojugal notch. Below this notch thereis a continuation of the jugal-quadratojugal-quadrate contact thatextends for about 17 mm, measured parallel to the long axis ofthe quadrate. The region below this contact is broken, but the surface appears to taper to the dentary-surangular process.The medial surface of the quadrate is characterized by avery large, anteriorly-directed, medial or internal wing (Fig. 2F). The posterior medial surface of the wing, along the main shaft ofthe quadrate, is marked by two laterally depressed areas. Thelarger of the two is centrally located, with its deepest point lyingadjacent to the mid- point of the quadrate's shaft. This depression expands across nearly the entire medial surface of the internal wing. A minor depression lies directly below it. The lower part of the quadrate'Sshaft is marked byan oblique, anteroventrally-directed, ridge. The superior surface of the medial wing tapers posterior to the squamosal process of the quadrate. The superior medial surface of the squamosal process of thequadrate is slightly broken. Quadratojugal-A small, quasi-rectangular-shaped bonerepresents the left quadratojugal (Fig. 3E-F). The bone is extremelythin, especially along its anterior lateral margin where it articulates with the medial surface of the posterior process of the jugal.It is broken, in part, along the medial ventral margin. There is araised lateral surface along the posterior margin, which is the thickest part of this element, and represents the exposed lateral surface of the quadratojugal (when in articulation with the jugal and quadrate). The medial surface is very smoothwith only a fainttextural pattern. Surangular- The anteromedial part of the left surangularwas recovered, but it is too fragmentary to describe, so our description of the surangular is based entirely on the right element.The right surangular (Fig. 3G-J) is nearly complete, but it is slightly  217 FIGURE 2. Parasallr%phllS sp. SMP VP-I090, jugals (A-D) and left quadrate (E-F). A, right jugal, lateral view; B, right jugal, medial view; C, left jugal;medial view; and D, left jugal, lateral view; and left quadrate in E, lateral and F, medial views. Bar scale = 5 em. crushed and distorted. The anterior edge of this element is broken. Numerous fractures are present, especiallyalong the dorsomedial marginandwhere the articular rests atop the distalend. The anterodorsallateral ascending process is missing, so the bone is incomplete anteriorly. In dorsal view (Fig. 3H), there is a narrow channel that marks the "floor" of the surangular that lies between the anterodorsallateral ascending process and the medial flange that articulates with the splenial. The medial flangerises anterodorsally, overhangs the medial surface and is directed somewhat medially with respect to theventral part ofthe surangular. Posteriorly, the articular surface is distinguished, in part, by a raised area that is situated medially. Lateroventrally,the surangular is smooth and deep at the anterior end (Fig. 3I).The ventral surface of the posterior articular region is marked by an indentation that corresponds to the underside of the raisedarea described previously (Fig. 3G). The medial surface is rough and broken along its ventroanterior-most extent where a rugosity or protuberance rests, based on comparison to the fragmentaryleft surangular that preserves this region. Miscellaneous elements-A number of miscellaneous bone  218 A D FIGURE 3. ParasaurolopllllS sp. SMP VP-I090, ?left postorbital, left quadratojugal and right surangular. A-D, ?left postorbital in A,dorsal; and B, ventral views; C, medial view showing articular surface for attachment with the frontal/prefrontal; and D, lateral viewshowing articular surface for attachment with the left squamosal. E-F, left quadratojugal. In E, lateral view; F, medial view; and G-J, right surangular. G, ventral view; H, dorsal view; I, lateral view; and J, medial view. Bar scale = 1 em. fragments, some from the dental battery and/ or other tooth bear ing portions of the maxillae, were recovered from the site. Thesefragments were eroding at the surface of the outcrop and indicated the presence of the elements described above.  DISCUSSION This specimen (SMP VP-1090) is noteworthy because it isthe first unequivocal juvenile lambeosaurine that can be identified to the generic level. Therefore, its documentation is of importance andmay assist future workers in the identification of partial hadrosaurid skull remains.Some features of this specimen indicate its lambeosaurineaffinities. These include the strong ventral deflection of the anterior part of the dentary, and tooth crown margins possessing denticles on the superior part of the tooth. To a lesser extent, the morphology of the dentary, surangular and quadrate are consistent with its generic assignment, although arguably these are only indicative of the subfamily.The teeth and morphology of the jugal allow reference to Parasaurolophus, and are nearly identical to those of Parasaurolophus tubicen (NMMNH P-25100) described by Sullivan and Williamson(1999). The teeth have slightly sinuous carinae as reported for P. tubicen (Sullivan and Williamson, 1999). The occurrence of denticles along the margins of the anterior teeth is also consistent with ParasaurolophliS tubicen. The presence of faint vertical sec ondary ridges, as reported by Hall (1993) on teeth belonging tojuvenile hadrosaurines, are also present on some of the teeth of our specimen. Because these ridges are now known to occur in both juvenile hadrosaurines and lambeosaurines, their presenceis clearly a primitive feature and therefore has no taxonomic significance. However, secondary ridges may have ontogenetic significance, indicating an immature developmental stage. Moreover, the elevated tooth crown margins cited by Hall (1993) are known in all members of the Hadrosauridae and therefore have no utility in subfamilial or generic identification within this clade.The number of teeth in the dental battery is not known exactly. However, we estimate that 26 vertical series of teeth werepresent, each bearing two to three functional teeth per verticalseries. The adult Parasaurolophus has three functional teeth per vertical series and approximately 43 vertical series of dentary teeth based on NMMNH P-25100.The morphology of the jugals compares readily to that of Parasaurolophus. In both P. walkeri and P. tubicen Qugals are not known for P. cyrtocristatus) the jugals arerelatively short anteroposteriorly and are further distinguished by a near parallelpostorbital and posterior jugal processes resulting in rather nar row lower temporal fenestrae. It should be noted that while most hadrosaurines depart from this morphology, some have nearlyparallel jugal processes. Horner (1988) reported that the jugal in the hadrosaurine Brachylophosaurus canadensis, in part, is characterized by quadratojugal and postorbital processes that are near parallel, forming a nearly perfect V-shape temporal fenestra.Moreover, the anterior projecting process of the jugal is symmetrical, resulting in the equal participation with the lacrimal above and the maxilla below. This latter condition is primitive, occur ring also in the basal hadrosaurines Brachylophosallrus and Maiasaura (Horner, 1988). In Parasaurolophus, the anterior jugalprocess is broader, and the bones that articulate with the upper 219 edge differ. They include not only the lacrimal, but may also include the prefrontal (Sullivan and Williamson, 1999). The jugal of Parasaurolophus is further distinguished by a relatively short anteroposterior length and a highly arched ventral surface of thesuborbital jugal bar. In summary, the combination of these fea tures-(l) subparallel quadratojugal and postorbital processes; (2) broad, symmetrical anterior jugal process; (3) deep and short suborbital jugal bar; and (4) highly arched ventral surface of sub orbital jugal bar-distinguish the jugals of Parasallrolophus fromthose of all other hadrosaurine and lambeosaurine dinosaurs. It is worth noting that Horner (1988, p. 319) stated thatthe jugal can be used in identifying hadrosaurine genera because it does not change shape during ontogeny. We would also argue that the sameapplies to lambeosaurines-that the jugals have a consistent mor phology irrespective of size-based onour study of Parasallrolophus (Sullivan and Williamson, 1999). To a lesser extent, the surangular and quadrate also support reference to Parasaurolophus. The surangular, albeit somewhatcrushed and distorted, conforms to that of P. tubicen (NMMNH P-25100), although it is much smaller. An incomplete right quad rate, consisting of the lower half, is known from the holotype of P. tllbicen (Sullivan and Williamson, 1999, fig. 8f-h) and only differsslightly from our juvenile specimen.While juvenile hadrosaurids are perhaps rather common,recognition of juvenile lambeosaurine dinosaurs is indeed rare.Few juvenile lambeosaurine specimens have been recognized, and those that have, have been referred with question at the genericlevel (Sullivan and Williamson, 1999). We note that there has been some ambiguity regarding what is a "juvenile" versus" sub adult," as the latter term includes specimens within the former. Some workers equate, and seeminglyrestrict, the term "juvenile" with "hatchling" or "nestling" (seeHall,1993). Horner et al. (2000) recently tried to clarify the termsassociated with various developmental stages. They recognizedsix growth stages based on size and osteohistologic features. Theseare: adult, subadult, large juvenile, small juvenile, large nestling, and small nestling (Horner et al., 2000).The dentary of SMP VP-1090 is approximately one-third the length of adult P. tubicen (NMMNH P-25100). Assuming that thereis a general correlation of skull size to femoral size, our specimenfalls between the size ranges of small and large juveniles (Horneret al., 2000). Therefore, we are confident that the specimen is ajuvenile and likely represents an individual that is one to two years old. ACKNOWLEDGMENTS RMS thanks field assistants Fred Widmann and MichaelLamirata for help collecting the specimen. Thanks are extended to F. Michael O'Neill and the Bureau of Land Management (Albuquerque) for issuing the necessary permits to collect vertebratefossils on BLM administered lands. The specimen was collected under permit no. SMP-8270-RS/WA/WSA-98-C issued to RMS which is gratefully acknowledged. G. E. Bennett III prepared thespecimen for this report. REFERENCES Fassett, J.E. and Steiner, M.B., 1997, Precise age of C33N-C32R magneticpolarity reversal, San Juan Basin, New Mexico and Colorado: New Mexico Geological Society, Guidebook 48, p. 239-247.Hall, J.P., 1993, A juvenile hadrosaurid from New Mexico: Journal ofVertebrate Paleontology, v. 13, p. 367-369. Homer, J.R., 1988, A new hadrosaur (Reptilia, Omithischia) from the Up per Cretaceous Judith River Formation of Montana: Journal of Vertebrate Paleontology, v. 8, p. 314-321.Homer, J.R., 1990, Evidence of diphyletic srcination of the hadrosaurian (Reptilia: Ornithischia) dinosaurs; in Carpenter, K., and Currie, P. J.,eds., Dinosaur Systematics: Approaches and Perspectives. Cambridge,Cambridge University Press, p. 179-187.Homer, J.R., de Ricqles, A. and Padian, K., 2000, Long bone histology of the hadrosaurid dinosaur Maiasallra peeblesonlm: growth dynamics and physiology based on a series of skeletal elements: Journal of Vertebrate Paleontology v. 20, p. 115-129.
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