Dinosaurian Osteology: Lecture 4

Ancestral Dinosaurs: Major Groups

Introduction: the many characters described in this lecture should be visualized, and be sight-recognized instead of memorized

Stem-node definitions have recently been introduced in order to stabilize nomenclature

Norian - 223-208 myr
Carnian - 235-223 myr
Ladinian - 240-235 myr
Early Triassic - 240-245 myr
1. Lab on skull, forelimb

2. Basal groups of Dinosaurs:

Saurischia (Theropoda + Prosauropods and Sauropods)
Ornithischia
Problem of the ornithischia and the early evolution of dinosaurs.
None of these dinosaurs is unusually large by modern standards.
These are important dinosaurs, for they define ancestral attributes of dinosaurs
3. The Carnosaurs - large, primitive theropods

SAURISCHIA [Novas and Sereno 1994, JVP 13(4)]
The Saurischia were formerly defined on basis of tri-radiate pelvis, which is typical of most primitive archosaurs. They are weakly separated from protodinosaurs

A recent list of synapomorphies - Padian 1997, Dinosaur Encyclopedia, p. 648-9 (Herrerasauridae and Eoraptor are excluded):

Saurischia - all dinosaurs closer to birds than to Ornithischia
Ornithischia - all dinosaurs closer to Triceratops than to birds
Note the absence of characters pertaining to the hind limb

Padian et al. 1999. JVP 19(1): 69-80: Herrerasauridae and Eoraptor lack many synapomorphies of both theropods and saurischians, including:

Novas an Sereno 1994 list additional saurischian synapomorphies which also occur in Herrerasauridae and Eoraptor: Saurischians are also tend to be characterized by:
Presence of pleurocoels (pneumaticity) Presence of gastralia Absence of ossified tendons
Saurischian characters, Langer et al. 1999, C. R. Acad. Sci. Paris, Terre et Planetes 329: 511-517), resulting from stem prosauropod characters: A new stem saurischian, Bonaparte et al. 1999, Monogr. Natl. Sci. Mus.(Tokyo) 15: 89-109. Guaibasaurus, from the Carnian (Late Triassic) of Brazil, appears to be more primitive than theropods and prosauropods - a stem saurischian: Unfortunately the skull and forelimbs are not preserved. What is preserved looks rather intermediate between theropods and prosauropods, AND more primitive than herrerasaurids. Guaibasaurus supports a close relationship between the herrerasaurids ("theropods") and prosauropods.

THEROPODA: Eoraptor [Sereno et al. 1993, Nature 361: 64-66] nearly a basal saurischian; (1 m) middle Carnian (230 ma) of Argentina

Theropod distinctions from prosauropods: (Novas 1994, JVP 13: 400-423) Enlarged skull: Inertial turning: Raking manus: Powerful legs: Sereno 1999: most important theropod characters PROSAUROPODA: Azendohsaurus (Gauffre 1993, Palaeontology 36: 897-908) - from western High Atlas, near Marrakech, Carnian metoposaurs, temnospondyls, phytosaurs, 3 dicynodonts, procolophonids - (NC Implications), a prosauropod hand and foot has been collected from Arizona Saturnalia (in reference to "Carnival"), a prosauropod from Brazil as old as other basal dinosaurs (Langer et al. 1999, this recently described specimen is now the most completely known Carnian herbivorous dinosaur): All of the large prosauropods vanished at the Tr-Jr boundary (Olshevsky 1992, p. 106) - and rapidly re-radiated during the beginning of the Jurassic into groups of larger prosauropods\sauropods

ORNITHISCHIA: Oldest skeletal material (pertaining to "Fabrosauridae") occurs in the Carnian (Argentina, Pisanosaurus), Norian (latest Triassic, Texas, Technosaurus). Ornithischians very distinctive

Completely known taxa: Lesothosaurus, TEXT, pp. 416-425, see Sereno 1991 (JVP 11: 168-197): (1 m) Hettangian -?Sinemurian (early Jurassic), S Afr. (*) = shared with Padian 1997

Sereno 1997 Characters suggest nippers and chewers living in sunlight (where small plants grow) with short neck, small forelimbs, long rigid body, and strong (fleet) hind limbs

Primitive ornthischians are little "torpedoes," not dodgers; the tail is not longitudinally differentiated. Ornithischians

All of the specialized ornithischian groups (stegosaurs, ankylosaurs, ornithopods, pachycephalosaurs, ceratopsians) appeared after the end of the Triassic; the radiation was after (younger than) that of saurischians.

THE PROBLEM OF PISANOSAURUS

See Sereno 1991, 1997
Skull Postcranium Association (Sereno 1991, p. 143) The Consensus is that Dinosaurs are Monophyletic

The saurischian-like characters of the pelvis and ankle, and ornithischian-like jaws in Pisanosaurus appear to support a close saurischian-ornithischian relationship among the oldest known dinosaurs.

Hans-Dieter Sues (pers. com. September, 2000) accepts the association of the Pisanosaurus material, noting that the scapula is always small in primitive ornthischians

Langer (pers. com. September, 2000) accepts the association of the material, but also notes that only one of his 203 postcranial characters supports an ornithischian affinity: "It's not even a very strong one. Pisanosaurus shares with "other" ornithischians a calcaneum that presents a posterior elevation in the articular surface for the fibula. Whereas other basal dinos have this area nearly flat. It is true, however, that this feature also apears in Tetanurans."

The cranial fragments of Pisanosaurus are much more ornithischian-like than the postcranial characters, and the possible mixing of the fragmentary materials is a possibility. The alternatives:
1. Pisanosaurus is representative of stem ornithischians, and its similarities to saurischians are evidence of proximity to the common ancestor of dinosaurs
2. Pisanosaurus is representative of stem ornithischians, and after the saurischian material is removed from the type, it closely resembles Lesothosaurus; the common ancestor of dinosaurs is more remote (unstated)

We need more skeletal material of Triassic ornithischians!!

How would the case for dinosaurian monophyly be weakend?

Apart from the special case of Pisanosaurus, the prevailing presumption is that Lesothosaurus-like ornithischians were present among the oldest dinosaurs 230 myr ago, and that they possessed all of the defining characters of ornithischians

We evidently have between 5 and 10 million years to close the gap between ornithischian and saurischian dinosaurs before they become protodinosaurs. Given the ambiguity about Herrearasaurus and Eoraptor, the gap may be closer to 5 myr than to 10 myr.

Recall that during this 5 (or 10) myr, a high degree of adaptation to bipedality must have been acquired, for this defines the Dinosauria. After this high degree of bipedality is acquired, then the synapomorphies defining primitive saurischians and primtive ornithischians must have appeared.

This may be shown graphically: 

230 million years -     Saurischians            Ornithischians

                                        .                       .

                                        .                       .

?  million years -              . .stem dinosaurs. .

                                                    .

                                                    .

240 million years -                protodinosaurs
The degree of specializations of ornithischians are strikingly different from those of saurischians and "protodinosaurs" (e.g. herrerasaurids, Eoraptor) - the task is to: Thus, according to convention, protodinosaurs would have become specialized bipeds, AND THEN ornithischians would have acquired their defining characteristics, all within perhaps as little as 5 million years.

According to some, ornithischians (being dinosaurs) would have appeared after the branching off of the clades leading to Herrerasaurus and Eoraptor. In such cladograms, these clades are very close to the Dinosauria. They are also very close to the Saurischia.

Do you believe that archaic ornithischians are closer to the Dinosauria than they are to Herrerasaurus and Eoraptor?

BUT, it is known (indeed, it is obvious) that adaptation to bipedalism continued among both saurischian and ornithischian lineages AFTER 230 myr ago (cf. length of leg/length of body, and reduction of lateral toes in ornithomimids and ornithopods.

Why could not adaptations to bipedality have been acquired at the same time as saurischian and ornithischian synapomorphies, PRIOR TO 230 myr?

This seems to be a very reasonable hypothesis, given the small amount of time available between stem dinosaurs and protodinosaurs.

If so:
1. - some of the characters presently defining dinosaurs (and a high degree of bipedality) would become homoplasies.
2. - a branch-node definition of dinosaurs would force the recognition of dinosaurs back in time to include groups that are presently considered protodinosaurs
3. - it would not be astonishing if the Dinosauriformes, Dinosauromorphs, Ornithodira and EVEN the Avemetatarsalia were ultimately incorporated into the Dinosauria!
4. Scleromochlus might then approximate the appearance of "Eodinosaurus," the ancestral dinosaur (as easily as it now approximates the appearance of "Eopterosaurus."

It is almost certain that:

But don't worry, we do have a pretty good idea of what saurischians and ornithischians are - and theropods and prosauropods are very closely related to each other.

DERIVATIVE THEROPODS ("NEOTHEROPODA")

Large-headed carnivores
Pneumatic spaces in skull (lightening skull)
Hind limbs become longer
Proximal articulation mtt I lost
New material being described - new cladistical analyses under way (particularly one by Holtz, to be published in Gaia)
According to Sereno 1999 (Science), neotheropods include:
Ceratosauroids ( = Ceratosauria of TEXT)
Tetanurae
Spinosauroids (included with allosauroids)
Allosauroids ( = Carnosauria of TEXT)
Coelurosauria ( = Coelurosauria of TEXT)
The consensus position was described in the "Symposium on Phylogeny and Biology of Carnivorous Dinosaurs" Pittsburgh SVP, 1995 by Holtz (55 taxa, >250 characters) and Holtz and Padian 1995 (JVP Abstracts, p. 35A): there are two major subgroups: 
        Clade             Defining synapomorphy

        Carnosauria       closer to Allosaurus than to birds

        Coelurosauria     closer to birds than to Allosaurus
CERATOSAURIA: (TEXT, Chapter 5): Group recognized by Gauthier at Berkeley 15 yrs ago; first triumph of cladistics, late Carnian to end of Cretaceous - now the Ceratosauria is falling before new caldistic revisions, relagating them to basal neotheropods (and thereby to basal carnosaurs).

Monographs:

Colbert on Coelophysis (Mus. N. Ariz. Bull. 57)
Welles on Dilophosaurus (Palaeontogr. Abt. A 185: 85-180)
Bonaparte on Carnotaurus (1990 LACM Contrib. Knowl. 416(4), 41p.)
Madsen et al. on Ceratosaurus (in press)
Completely known taxa (4):
Ceratosaurus
Coelophysis
Dilophosaurus
Syntarsus
Famous small- to medium-sized theropods:
Ceratosaurus first theropod to be reconstructed (1892), 6.6 m, 675 kg,
Coelophysis most completely known primitive theropod, 2.5 m, 70 kg
Dilophosaurus "spitting" with flared neck in Jurassic Park, 5.5 m, 350 kg
Chinese "Dilophosaurus" belongs to a new and distinct genus; Lamanna et al. 1998. JVP 18(3, suppl.): p. 57A
Carnotaurus - excellent life-reconstruction by Steve Czerkas
Gregarious:
Syntarsus fluvial lens in eolian deposits early Jurassic Fundy, S. Africa,lacrimal crests similar to Dilophosaurus
Coelophysis quarry fluvial, few complete specimens, most slightly dissociated - several hundred individuals
Synapomorphies listed in TEXT (note that these were compiled before good cranial material of abelisaurids had been described): Plesiomorphies: Other characters Homoplasies (with birds):
  • raptor hand of Syntarsus (TEXT cf. fig. 5.6 and 12.5, noting curved mtc III in latter)
  • synsacrum, fused pelvis (8 sacrals in Ceratosaurus)
  • ankle; proximal tarsals fused to crus, distal tarsals fused to mtt
  • much co-ossification

    • Homoplasies (with ostrich dinosaurs): Definitions (Padian et al. 1999. JVP 19(1): 69-80):
    Ceratosauria - all neotheropods closer to
    Ceratosaurus than to Neornithes
    Coelophysoidea - Dilophosaurus and Coelophysidae
    Ceratosauroidea - Ceratosaurus, Abelisauridae and possibly Elaphrosaurus
    Major Groups
    Coelophysoids, Sereno 1997 Abelisaurids; the Cretaceous "carnosaurs" of the Southern Hemisphere, cf. Carnotaurus, Bonaparte 1990: Apomorphies of Carnotaurus: Abelisauridae (skulls of Carnotaurus from Argentina, Majungatholus from Madagascar), synapomorphies after Sampson et al. 1998. Science 280: 969-1156 Abelosauroids (Abelisauridae + Naosauridae), synapomorphies after Coria and Salgado 1999. JVP 19 (3, Suppl.) Phylogeny

    Both Rauhut 1998 JVP 18(3, suppl.)and Carrano 1999 JVP 19 (3, suppl.) agree that the Ceratosauria represents a paraphyletic assemblage of basal theropods, with coelophysoids, abelisauroids and Ceratosaurus representing successively closer outgroups to tetanurans. These references are to abstracts; the major studies have not yet been published.

    Paraphyly - excludes a descdendant of a most recent common ancestor

    TETANURANS: The "stiff-tailed" theropods, see Holtz 1994 J. Paleo. 68(5): 1100-1117, Sereno et al. 1996 Science 272: 986-991

    Definitions (Padian et al. 1999. JVP 19(1): 69-80):
    Tetanurae - all neotheropods more closely related to Neornithes than to Ceratosaurus
    Avetheropoda - most recent common ancestor of Neornithes and Allosaurus and all descendants of that ancestor

    "Some taxa, including Torvosaurus, Spinosaurus, Baryonyx, Afrovenator and poorly understood "megalosaurs" such as Piatnitzkysaurus, Eustreptospondylus, Poikilopleuron, and Megalosaurus, may fall outsidethe Avetheropoda but are still usually considered basal members of the Tetanurae."

    CARNOSAURIA: Current consensus (Padian and Hutchison, Hutchison and Padian, Encyclopedia of Dinosaurs, 1997) differs from TEXT, Ch. 6, 14; there are new skeletons of known forms, new forms):
    Carnosaurs are basal tetanurans - exclusive of coelurosaurs (and thus tyrannosaurids)
    Carnosaurs are allosauroids of Sereno,/blockquote>

    Definitions (Padian et al. 1999. JVP 19(1): 69-80):
    Carnosauria - all neotheropods closer to Allosaurus than to Neornithes

    [Coelurosauria - all neotheropods closer to Neornithes than to Allosaurus]
    Preliminary attempt to trace clades within the Carnosauria:
    Cryolophosaurus more basal carnosaur
    Monolophosaurus sister taxon to Allosauridae
    Giganotosaurus and Carcharodontosaurus closer to Allosauridae than Sinraptoridae
    Allosauroidea - node connecting Allosauridae with Sinraptoridae (also Currie and Carpenter 2000)

    Other carnosaurian taxa remain to be studied

    Carnosaurs by tradition have included:
    Megalosauridae - short powerful forearm, heavy manus ungual I, poorly known European forms
    Torvosauridae - grouped with spinosaurs as an early side-branch (Sereno 1990), poorly known and powerfully constructed (North America)
    Spinosauridae - crocodile-like skulls, hooking thumb claw (Africa, Europe, South America)
    Sinraptoridae - Cryolophosaurus (Antarctica), Monolophosaurus, Sinraptor (China)
    Allosauridae - basal Avitheropoda, with furcula, Holtz 1996, JVP Abs. p. 42A (North America, Europe, Africa)
    Carcharodontidae - (closer to Allosaurus than Sinraptor, Holtz 1996); eustachian system tripartite as in crocodiles (Larsson 1996, JVP Abs. p. 47A), (Africa, South America)
    Monographs:
    Madsen 1976 on Allosaurus, Utah Geol. Min. Surv. Bull 109, 163 p.
    Currie & Zhao 1993 on Sinraptor, Can. J. Earth Sci. 30: 2027-2081.
    Milner 1997 on Baryonyx, Bull. Nat. Hist. Mus. Lond., Geol 53 (1): 11-70.
    Currie & Carpenter 2000 on Acrocanthosaurus, Geodiversitas 22 (2): 207-246.
    Chure on allosaurids, PhD Thesis
    Completely known taxa:
    Allosaurus
    Sinraptor (nearly complete)
    Carnosaur synapomorphies (Holtz 1994; "Allosauroid" synapomorphies of Sereno et al. 1996) Spinosauridae - Cristatusaurus (Taquet and Russell 1998, C. R. Acad. Sci. Paris, Science de la Terre et des Planetes, 1998, 327: 347-353; = "Suchomimus," Sereno et al. 1998. Science 282: 1298-1302), Baryonyx and Irritator. Torvosaurids included within the spinosauroidea

    Spinosaurs (Baryonyx, Charig and Milner 1997) lack synapomorphies characteristics of Allosaurus and higher tetanurans (neotetanurans) such as:

    Carcharodontosauridae - synapomorphies after Alcober et al. 1998. JVP 18(3, suppl.): A curious resemblance to abelisaurids:

    Characters suggesting relationship between abelisaurids and carcharodontosaurids (Novas 1997, Encyclopedia of Dinosaurs, p. 1):

    Similarities between abelisaurids and carcharodontosaurids (Alcober et al. 1998), relationship between groups uncertain, could be due to parallel evolution) Currie and Carpenter 2000: Carcharodontosauridae closer to Abelisauridae than Allosauridae

    Acrocanthosaurus and the allosaurs: (Currie and Carpenter 2000): Acrocanthosaurus differs from Allosaurus:

    Acrocanthosaurus differs from carcharodontosaurids Tom Holtz (personal communication, August 2000): "I think that the fairest resolution is probably a three-way split: an Allosaurus-Neovenator clade, an Acrocanthosaurus clade, and a carcharodontosaur clade. Either or both of the latter clades could concievably come out of an Allosaurus-like form. However, Oliver Rauhut has mentioned that he thinks some of the Tendaguru teeth may be proto-carcharodontosaur, suggesting that the split between the Allosaurus line and the Carc.-Gig. line had already occurred by the Late Jurassic."

    We await a review of theropods by Holtz in press, Gaia.

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