Evolution of Jaw Mechanisms in Ornithopod Dinosaurs

118 Pages·1984·3.692 MB·English

by  David B. Weishampel (auth.)

Advances in Anatomy
Embryology and Cell Biology
Vol. 87
Editors
F. Beck, Leicester W. Hild, Galveston
R. Ortmann, KOln J.E. Pauly, Little Rock
T.H. Schiebler, Wiirzburg
David B.Weishampel
Evolution of Jaw Mechanisms
in Ornithopod Dinosaurs
With 20 Figures
Springer-Verlag
Berlin Heidelberg N ewYork Tokyo
1984
DAVID B. WEISHAMPEL
Assistant Professor, Biological Sciences,
College of Arts & Sciences,
Florida International University,
Miami, Florida 33199 U.S.A.
ISBN-13: 978-3-540-13114-4 e-ISBN-13: 978-3-642-69533-9
DOl: 10.1007/978-3-642-69533-9
Library of Congress Cataloging in Publication Data
Weishampel, David B., 1952-. Evolution of jaw mechanism in ornithopod dinosaurs.
(Advances in anatomy, embryology and cell biology; vol. 87) Bibliography: p. Inclu
des index.
1. Ornithischia. 2. Jaws. I. Title. II. Series: Advances in anatomy, embryology and
cell biology; v. 87.
QL801.E67 vol. 87 [QE462.065] 574.4s [567.9'7] 83-20412
ISBN-13: 978-3-540-13114-4
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To Judy
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still living
Contents
1 Introduction 1
2 Ornithopod Jaw Mechanics 3
3 Materials and Methods 6
4 Comparative Arthrology 11
4.1 Introduction 11
4.2 Mechanical Units 12
5 Neurocranial Segment 14
5.1 Laterosphenoid-Skull Roof Joint 14
5.2 Basisphenoid-Pterygoid Joint 14
6 Maxillary Segment 22
6.1 Parietal Unit 22
6.1.1 Frontal-Parietal Joint 22
6.1.2 Postorbital-Frontal Joint 23
6.1.3 Postorbital-Parietal Joint 23
6.1.4 Postorbital-Squamosal Joint 23
6.2 Quadrate Unit 24
6.2.1 Quadrate-Squamosal Joint 24
6.2.2 Quadrate-Pterygoid Joint 26
6.2.3 Quadrate-Quadratojugal Joint 26
6.2.4 Jugal-Quadrate Joint 27
6.2.5 Quadrate-Mandible Joint 28
6.3 Muzzle Unit 29
6.3.1 Frontal-Frontal Joint 29
6.3.2 Nasal-Frontal Joint 30
6.3.3 Premaxilla-Frontal Joint 30
6.3.4 Prefrontal-Frontal Joint 31
6.3.5 Prefrontal-Lacrimal Joint 31
6.3.6 Lacrimal-Nasal Joint 32
6.3.7 Prefrontal-Premaxilla Joint 32
6.3.8 Nasal-Nasal Joint 32
6.3.9 Nasal-Premaxilla Joint 33
6.3.10 Nasal-Maxilla Joint 33
6.3.11 Premaxilla-Premaxilla Joint 34
6.3.12 Maxilla-Premaxilla Joint 34
6.3.13 Premaxilla-Lacrimal Joint 35
6.3.14 Nasal-Prefrontal Joint 36
6.3.15 Postorbital-Prefrontal Joint 36
VII
6.4 Maxillopalatal Unit ...... . 36
6.4.1 Palatine-Pterygoid Joint 37
6.4.2 Pterygoid-Ectopterygoid Joint 37
6.4.3 Maxilla-Ectopterygoid Joint 38
6.4.4 Maxilla-Palatine Joint 38
6.4.5 Maxilla-Pterygoid Joint 39
6.4.6 Jugal-Maxilla Joint 39
6.4.7 Maxilla-Lacrimal Joint 40
6.4.8 Jugal-Lacrimal Joint . 41
6.4.9 Lacrimal-Palatine Joint 41
6.4.10 Jugal-Quadratojugal Joint 42
6.4.11 Jugal-Ectopterygoid Joint 42
6.4.12 Postorbital-Jugal Joint . 43
6.4.13 Squamosal-Quadratojugal Joint 44
6.4.14 Jugal-Palatine Joint .... 44
6.4.15 Palatine-Ectopterygoid Joint 44
6.4.16 Vomer-Palatine Joint . 44
6.4.17 Vomer-Pterygoid Joint 44
6.4.18 Vomer-Premaxilla Joint 45
7 Mandibular Segment . . . . 46
7.1 Dentary-Dentary Joint 46
7.2 Predentary-Dentary Joint 47
8 Dental Morphology . . . 49
8.1 Fabrosauridae . . . 51
8.2 Heterodontosauridae 53
8.3 Hypsilophodontidae 55
8.4 Iguanodontidae 57
8.5 Hadrosauridae . . . 59
9 Kinematic Analyses of Ornithopod Jaw Mechanisms 63
9.1 Predictions .............. . 65
9.2 Documentation and Evolutionary Implications 72
10 Discussion and Conclusions 82
11 Summary .... 87
12 Acknowledgments 100
Note added in proof 102
13 References 103
14 SUbject Index 110
VIII
1 Introduction
The Ornithopoda, one of five suborders within the Ornithischia, was originally
proposed by Marsh in 1881 to include those bipedal dinosaurs possessing a
predentary bone fitted over the rostral end of the mandibles. Ornithopods as
recognized today can be further characterized by moderately long facial skele
tons equipped with well-developed, often toothless premaxillae and moderate
to large external nares. Maxillary and dentary dentitions vary but usually consist
of at least one replacement series beneath the functional set; some have many
rows of successional teeth. Tooth morphology suggests ornithopods were suc
cessful herbivores but, as will be discussed, the precise way(s) in which ornitho
pods chewed their food, hence lending important information about their tro
phic position, has not been settled. Postcranially, ornithopods show specializa
tion for bipedality in hindlimb construction and lack well-developed protective
structures on their flanks, back, and tail.
The Ornithopoda can itself be divided into five families: Fabrosauridae, He
terodontosauridae, Hypsilophodontidae, 19uanodontidae, and Hadrosauridae
(subdivided into the subfamilies Hadrosaurinae and Lambeosaurinae). Both
fabrosaurids and heterodontosaurids, first known from the Late Triassic and
Early Jurassic of Argentina and South Africa, were small animals differing
in details of cranial, dental, and appendicular anatomy. Fabrosaurids are be
lieved to represent the basal ornithopod stock (Galton 1972, 1978; Thulborn
1970a, 1972). During the Jurassic, ornithopods underwent major radiations
that included the medium- to large-sized Hypsilophodontidae and the large
bodied Iguanodontidae, both of which survived into the Cretaceous. Both hypsi
lophodontids and iguanodontids are known from Europe, Africa, and North
America; iguanodontids are additionally known from Asia and Australia. Also
during the later part of the Cretaceous, especially in eastern Asia, North Amer
ica, China, and Europe, ornithopods are represented by the Hadrosauridae,
popularly known as duck-bills. These large-bodied ornithopods and the few
remaining iguanodontids and hypsilophodontids became extinct at the close
of the Cretaceous.
Although ornithopods have been known for quite some time (the second
dinosaur to be named was Iguanodon by Mantell in 1825), many recent advances
in our knowledge of these animals have improved ideas about their anatomy,
diversity, and evolution (Galton 1974a, b, 1978; Dodson 1975, 1980; Taquet
1975, 1976; Bonaparte 1976; Homer and Makela 1979; Norman 1980; Sues
1980; Colbert 1981; Homer 1983). Of primary importance here are the works
on ornithopod jaw movement, a subject first studied in detail by Versluys (1910),
originator of the term cranial kinesis in his work on reptilian jaw mechanics.
Major recent studies of jaw mechanics and cranial myology in ornithopods
have been conducted by Ostrom (1961 b), Galton (1974a), and Thulborn
(1971 b), among other less detailed works. These and earlier studies of cranial
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anatomy in ornithopods have produced a plethora of hypotheses concerning
ornithopod jaw mechanics, each involving the potential for movement at the
jaw joint and between the palatoquadrate elements and other cranial bones,
ultimately involving the joint between the dorsal head of the quadrate and
the squamosal.
Despite these studies, no one has yet analyzed jaw mechanics throughout
the Ornithopoda. By using current taxonomic and phylogenetic studies of the
ornithopods as a framework, it is possible to analyze the evolution of mastica
tory mechanics within this clade of Mesozoic herbivores and, in doing so, ad
dress problems of functional novelty and adaptive radiation.
2
2 Orn ithopod Jaw Meehan ies
How animals move their jaws, whether for ingestion and/or mastication, has
been of long-standing interest in vertebrate biology and is presently a dominant
research programme in and of itself. Analyses of jaw systems in modem verte
brates, such as those by Nobiling (1977) on sharks, Anker (1974) and Liem
(1980) on teleost fishes, Throckmorton (1976), Gorniak et al. (1982), Rieppel
and Labhardt (1979), and Smith (1982) on reptiles, Zweers (1974, 1982) and
Buhler (1981) on birds, and Greaves (1978), Hiiemae (1978), Crompton et al.
(1977), and DeVree and Gans (1976) on mammals form one aspect of the
mechanics and dynamics of vertebrate jaw motion. Another focus on the evolu
tion of vertebrate jaw systems is furnished by studies of a variety of fossil
groups: Barghusen (1973), DeMar and Barghusen (1972), and McGowan (1973)
on reptiles, and Greaves (1972), Kay and Hiiemae (1974b), Rensberger (1978),
and Krause (1982) on mammals. Similarly, the present study is based on fossils
in order to understand the mechanics of jaw motion in the diverse Mesozoic
herbivores known as ornithopods. Most commonly, particularly in work on
mammals, vertebrate jaw mechanisms are modeled as class III levers. The class
III lever model considers that the mandibular condyle acts as a fulcrum and
masticatory muscles apply force between this position and the point of resis
tance, the bite point. The simplest mechanism in which the lever model is used
is the hinge-like motion (rotation) of the mandibular rami at the jaw joints
to close the jaws. No additional movement at the occlusal surface is imparted
from the bones of the facial skeleton. Vertical adduction produces unidirectional
shearing or crushing, depending upon tooth shape and apposition of tooth
rows. Translational movement, if possible at the jaw joint, produces fore-aft
or lateral-medial motion. Jaw joint rotation or rotation plus translation charac
terizes all mammals and some reptiles. Cranial kinesis (Versluys 1910, 1912)
is another type of mechanical system in which elements of the facial skeleton
and the palate move more or less as a unit with respect to the braincase and
is commonly found in lizards, snakes, and birds. Mobility of the quadrate on
the braincase is known as streptostyly, while immobility is termed monimostyly
(Stannius 1856). Among ornithopods, it is on the implications of the difference
between monimostyly and streptostyly, and the possible presence of some form
of continued cranial kinesis, that much work on jaw mechanics (and subsequent
conflict) has been based.
The earliest comments to be made on cranial functional morphology in ornith
opods were by Marsh (1893), in a study of early discovered hadrosaurid material
(yPM 618). He suggested that the quadrate may have been free to move against
the squamosal, but did not comment further on the significance of this joint
to hadrosaurid jaw mechanics. Among other early North American workers,
Lambe (1920) rejected quadrate-squamosal movement in hadrosaurids and sug
gested that the jaw mechanism consisted of simple adduction of the lower jaws,
3
wIth concoIDltant sheanng 01 the dentary teeth past those of the maxilla. Jaw
adduction as the sole jaw mechanism in hadrosaurids was followed by Lull
and Wright (1942) in their monographic treatment of hadrosaurid taxonomy
and biology.
Workers in Europe interpreted hadrosauridjaw mechanics in a vastly different
fashion. In 1900, Nopcsa described cranial material of Telmatosaurus transsyl
vanicus (BMNH R3386) and inferred that the squamosal-quadrate joint had
considerable freedom of movement (much like Marsh's interpretation) and that
other joints between the quadrate and palate and cheek region were equally
free. Thus, Nopcsa believed that the hadrosaurid quadrate was capable of swing
ing in a fore and aft direction. Versluys, in his initial studies on intracranial
movement in reptiles (Versluys 1910, 1912), rejected quadrate-squamosal move
ment in hadrosaurids, but later (Versluys 1923) supported Nopcsa's idea of
fore and aft rotation of the quadrate-squamosal joint. Versluys also suggested
that the mandibles rotated laterally about their long axes during mastication.
Von Kripp (1933) reexamined the material upon which Versluys based his stu
dies on quadrate movement in hadrosaurids (Edmontosaurus regalis NS R4036)
and rejected fore and aft mobility of the quadrate, based on joint restrictions.
In contrast to both Nopcsa and Versluys, von Kripp hypothesized that jaw
mechanics included lateromedial rotation of the quadrate-squamosal articula
tion, augmented by medial rotation of the mandibles about their long axes.
Work on ornithopod jaw mechanics resumed in 1961, when Ostrom detailed
hadrosaurid cranial anatomy, positing yet another jaw mechanism. Based on
muscle action vectors and tooth wear characters, Ostrom suggested that the
quadrate-squamosal joint was fixed and masticatory movement occurred by
means of propalinal translation of the mandibles against the lower head of
the quadrate. Hopson (1980) questioned the mechanism described by Ostrom
and suggested that the mandibles moved side-to-side relative to the maxillae,
based on tooth wear characters.
Although virtually all work on ornithopod jaw mechanics is based on hadro
saurid skull material, there are studies of jaw mechanisms in other ornithopods.
In his anatomic and taxonomic work on fabrosaurids, Thulborn (1971 b) sug
gested that jaw action consisted of shearing contact between maxillary and
dentary teeth by simple adduction of the mandibles. Hopson (1980) examined
the jaw mechanics in heterodontosaurids, concluding that mandibular move
ment was oriented side-to-side, based on tooth wear characters and joint mor
phology. Lastly, Galton (1974a) and Sues (1980) discussed jaw systems in hypsi
lophodontids (e.g., HypsilophodonJoxii and Zephyrosaurus schaJJi, respectively)
and concluded that the dentary teeth moved side-to-side against those in the
maxillae, much like Hopson's hypothesis of heterodontosaurid mastication, but
based on cranial musculature reconstructions and bony anatomy.
Clearly, there is no lack of interpretation of jaw mechanisms in ornithopods.
Most workers have relied extensively on cranial arthrology and reconstructions
of masticatory musculature, without due consideration for alternative jaw mech
anisms. In order to test both proposed and alternative jaw mechanisms, each
can be reduced to its component parts and modeled as kinematic linkage systems
by means of three-dimensional computer simulation. A significant feature of
computer modeling is in making predictions independent of the data used in
constructing the model, i.e., tooth-to-tooth wear for each mechanism. Thus,
4