Dinosaurian Osteology: Lecture 6

COELUROSAURS

October 5, scheduled quiz, postponed one week

For many years, small, hollow-boned carnivorous dinosaurs with small heads and long necks were called coelurosaurs - or "hollow-tailed-lizards"

They were characterised by their small size and agile appearance. During the past several decades they have been the object of a debate on whether they are big mimics or bird ancestors:

The terms "non-avian dinosaur" and "avian dinosaur" will not be used in this course, in spite of the fact that they are considered to be compatible with cladistic usage. The paraphyletic term "coelurosaur" will signify "non-avian dinosaur" and "bird" will signify the phyletically non-neutral term "avian dinosaur" (e.g. what would we call them if birds were not descended from dinosaurs?). Archaeopteryx, of course, is treated as the conceptual node belonging to both coelurosaurs and birds.

Watch for the presence/absence of furcula (also present in Allosaurus) — the fircula (or "wishbone") represents the fused clavicles. The furcula in coelurosaurs is simple, lacking the long process (the hypocleidium or "stem" of the wishbone "Y" in birds).

Hair structures have been reported in several coelurosaurs (compsognathids, therizinosaurs, dromaeosaurus) and in Early Jurassic coelophysoids (near a pubis "belly-print," Sabath and Gierlinski 1998, JVP 18, 3, Suppl. P. 73A; Kundrat 1998, JVP 18, 3, Suppl., p. 57A)- but pebbly, hair-free integuments occur in sauropods, ankylosaurs, ornithopods, stegosaurs, ceratopsians and several genera of large theropods. It would appear that hairlessness is correlated with large body size, as in mammals. Hair-like structures are probably widely distributed through the theropods. Their occurrence in remotely-related pterosaurs (Frey and Martill 1998, N. Jb. Geol. Palaont. Abh. 210,3, p. 421-441) suggests that hair- like structures might be widely distributed through dinosaurs in general.

Coelurosaurs possess relatively large brain cavities (usually judged by the external width of the back of the skull):

Coelurosaurs were arranged by Gauthier 1986 (Mem. Calif. Acad. Sci. 8) into a series of clades that were progressively more bird-like (see TEXT fig. 12.1, p. 270) The most interesting coelurosaurian specimens have been reported from in Early Chinese lake deposits over the last three years. These include the earliest good skeletal records of oviraptors, theirzinosaurs and dromaeosaurs, and compelling support for the concept of "feathered dinosaurs." These specimens include many skeletal attributes of birds, including: Feathers are preserved with skeletons of Protoarchaeopteryx and Caudipteryx (see below). Feather-like structures are also known from the small and peculiar Late Triassic archosaur Longisquama (Jones et al. 2000. Science 288: 2202- 2205).

The earliest records of various coelurosaur groups are in the range of 150-125 myr years old, all of which are equal to or younger than the earliest record of the animal considered to be the oldest bird (Archaeopteryx). [see JVP abstracts 1995, Britt, Holtz, Makovicky, Sereno; 1997, Holtz; studies in progress] There is an undocumented early to middle Jurassic radiation in which the true bird ancestors were involved, as well as acute danger of confounding homoplasies in deducing bird phylogeny from small dinosaurs postdating the origin of birds.

The ghost lineage solution (Sereno 1997. fig. 5) depicts most coelurosaur groups as antedating Archaeopteryx. This is justified to a large degree by the morphological divergence represented by the various groups of coelurosaurs.

JURASSIC FORMS, MORE LIKELY TO HAVE BEEN INVOLVED IN ORIGIN OF BIRDS

Compsognathus, 150 myr

Coelurus, 150 myr

Ornitholestes, 150 myr

Archaeopteryx, 150 myr

CRETACEOUS FORMS WITH RECORDS PROBABLY SPANNING 40 MYR

Dromaeosauridae, 128 myr

Oviraptoridae, 128 myr

Therizinosauridae, 128 myr

Tyrannosauridae, 125 myr

Ornithomimidae, 125 myr

Troodontidae, 125 myr

Avimimidae, 83 myr

Birds are adaptively and thus morphologically distinct from dinosaurs; it would be fair to consider them as highly modified dinosaurian derivatives (dinosaurs have feathers unsuited for flight, and did not fly). Birds had becomefully avian long before terrestrial coelurosaurian lineages became extinct at the end of the Cretaceous.
 
 

SITUATION: ABSTRACTS IN ADVANCE OF SUPPORTING DATA

Changes in meaning of words during last ten years, beginning with those most deeply imbedded in the literature:

"Coelurosauria" - formerly small carnivorous dinosaurs (including small ceratosaurs), now a diverse group of more or less birdlike tetanurans

Characters after Holtz, 1994 (J. Paleo. 68: 1100-1117), Hutchison and Padian 1997 (Dinosaur Encyclopedia: 129- 133)

Characters after Sereno, 1997 - related to cursorality "Coelurosaurs" do seem to be related to each other

For classification ranks above that of the family, see Varrichio 1997, Padian and Hutchison 1997 (JVP 17, 3, Suppl, p. 68A), Makovicky and Sues 1998 (Am. Mus. Novitates 3240, 27 pp), Holtz 1999 (JVP 19, 3 Suppl, p. 52A), Padian et al. 1999 (JVP 19: 69-80).

According to Holtz 1999, a strict consensus produces a 7- part polytomy; higher categories are less stable than the family groups; for the purposes of this course family group names will be used.

Above-family ranks:

"Deinonychosauria" (Colbert and Russell 1969, AMNH Novitates 2380) second foot claw evolved:

The term is generally suspect, but is retained by Sereno 1997

"Maniraptora" as proposed by Gauthier (1986) formerly included all "coelurosaurs" except ornithomimids; it is now often restricted to dinosaurs presumed to be linked to the immediate ancestory of birds (generally including dromaeosaurs)

Maniraptoran characters after Holtz (1994)

Maniraptora after Sereno (1997) oviraptorosaurs, deinonychosaurs and birds Maniraptora after Sereno (1999) Ornithomimosauria (Alvarezsaurids, Therizinosaurids); Tyrannoraptorans (Tyrannosaurids, Oviraptorosauria, Deinonychosauria, Birds) "Arctometatarsalia" as proposed by Holtz (1993, JVP 14: 480-519) and defined by a slender proximal end of the third metatarsal, evolved: The term should be abandoned on the grounds of homoplasy, except in an anatomical sense.
 
 

LATE JURASSIC COELUROSAURS

Compsognathus, TEXT, p. 288- 292, often considered to be representitive of an archaic coelurosaur - see Sinosauropteryx, below

Monograph: Ostrom 1978 (Zitteliana. 4: 73- 118)

Chicken-sized (immature), 3-3.5 kg, 75 cm (30") long, swallowed a small lizard; adult 1 m long

Three digits in manus, first not preserved

Coelurus, TEXT, p. 288, the type material after 30 years has still not been reviewed, some material looks ornithomimoid (cf. fig. 13.10 and 8.2 vertebrae, fig. 13.11 and 8.8 pubis)

The dorsal centra are elongate and spool-shaped, as in ornithomimids (Norell and Makovicky 1999)

New specimen of Coelurus (Miles et al. 1998. JVP 18, 3, Suppl, p. 64A)

Ornitholestes, TEXT p. 292-294, not recently reviewed, 1-2 m long CRETACEOUS COELUROSAURS

Sinosauropteryx,(Chen et al. 1998, Nature 391: 147-152), a compsognathid dinosaur with the longest known tail (64 vertebrae) of any theropod, represented by two specimens averaging about the size of a large chicken

Compsognathidae Notes: A very primitive-looking little dinosaur
 
 

Protoarchaeopteryx,(Qiang et al. 1998, Nature 393: 753-761; Currie and Norell 1998. JVP 18, 3, Suppl. p. 36A) dichotomy with velociraptorines (note that the skull in the only known specimen is limited to the tip of the rostrum)

TYRANNOSAURIDAE, TEXT, chapter 6, cf. p. 189,

best known family of theropods, teeth reported 1856

5-15 m in length (1,500-4,500 kg re fem circ)

Used to be largest terrestrial predators - T. rex skull 150

cm long, Giganotosaurus skull 180 cm long; T. rex weight

6-8 tons, Giganotosaurus 8-10 tons

Completely known taxa:

Albertosaurus

Daspletosaurus

Tyrannosaurus

Characters after Holtz (1994)

Ancestral tyrannosaur, Siamotyrannus (Buffetaut et al. 1996, Nature 381: 689-691), pre-Aptian. 125 myr Tyrannosaurid synapomorphies with coelurosaurs (Holtz 1994, Sereno 1997) (compare the restoration of an immature tyrannosaur with that of Compsognathus) Tyrannosaurids resemble ornithomimids Notes on Tyrannosaurus: Habits (after MEA 120 course):

head

body forelimbs hindlimbs ORNITHOMIMIDAE, TEXT, chapter 8

Aptian-Albian-Maastrichtian

Completely known taxa:

Struthiomimus

Gallimimus

Characters (see also Osmolska, Din. Enc. p. 499)

No pleurocoels on dorsal vertebrae

Beaked Mesozoic ratites, but with scraping claws/forelimbs an ostrich could use. Gastroliths in 12 skeletons of Mongolian ornithomimids, Kobayashi et al. 1999. Nature 402: 480-481

Deinocheirus TEXT p. 292, giant ornithomimisaur, Late Cretaceous of Mongolia, forelimbs 2.4 m long

SMALL, COELUROSAUR-LIKE SPINOSAURS

Pelicanimimus (Perez-Moreno et al. 1994, Nature 370: 363-367)

antebrachium as in ornithomimids

manus as in ornithomimids, subequal metacarpals, digits

pouch beneath throat

skull, neck, forelimb proportions OK

small, pteranodont-like crest

220 teeth: 7 pmx-30 mx/75 dent

Irritator (Martill et al. 1996, J. Geol. Soc. London 153: 5-8)

crest, recessed nares

Angaturama (Kellner and Campos 1996, N. Jb. Geol. Palaont. Abh. 199: 151-166),

recessed nares, 7 pmx teeth
 
 

TROODONTIDAE, (Russell and Dong 1993B, Can. J. Earth Sci. 30: 2163-2173; Varricchio 1997, Din. Enc. p. 749); TEXT chapter 11

Troodon oldest valid dinosaur name from Western Hemisphere

Albian?-Maastrichtian, Sinornithoides (1 m, 2.5 kg);

Troodon (2 m, 45 kg). Beautiful little animals.

troodonts resemble ornithomimids (Varricchio 1997): Notes: OVIRAPTORIDAE, TEXT, chapters 9, 10 (Russell and Dong 1993A, Barsbold 1997. Din. Enc. p. 505; Sues 1997. JVP 17: 698-716; Sereno 1997, 1999; Makovicky and Sues 1998. Am. Mus. Novitates 3240, 27 p., Barsbold et al. 2000. Acta Palaeontologica Polonica 45: 97-106)

1-2 m long, terminal Cretaceous caenagnathid from the Hell Creek Formation is very large — at least 2 m high at the hips (undescribed)

Albian-Maastrichtian

Caudipteryx is a basal oviraptorosaur (Qiang et al. 1998. Nature 393: 753-761. Currie and Norell 1998. JVP 18, 3, Suppl., p. 36A; Barsbold et al. 2000, Nature 403: 155- 156; Sereno 1999, p. 2143) Notes: The nesting oviraptorid (Clark et al. 1999, Am. Mus. Novitates 3265, 36 p.; see also Norell et al. 1995, Nature 378: 774-776; Norell et al. 1994, Science 266: 779-782) previously unrecorded similarities to birds: Notes: AVIMIMIDAE, Avimimus TEXT p. 281- 287, relatively minor group, best available description in English THERIZINOSAURIDAE, Therizinosaurus, TEXT p. 294; Segnosauria

chapter 18 (Russell and Dong 1993A; Clark et al. 1994):

Neocomian-Maastrichtian

Notes: Beipiaosaurus (Xu et al. 1999, Nature 399: 350- 354) has integumentary filaments (5-7 cm long) similar to those in Sinosauropteryx. Largest known theropod from the Liaoxi lakes, length 2.2 m. DROMAEOSAURIDAE, for which a putative relationship with Archaeopteryx has been much discussed. These are the famous dinosaurian raptors (dinosaurian "bobcats" of Jurassic Park, with very flexible clawed hands and a balancing tail spar. In general their skeletons are reminiscent of small, derived allosauromorphs

Completely known taxa:

Deinonychus, TEXT: Chapter 12 (2.5 m, 60 kg)

Velociraptor, complete skeleton associated with

Protoceratops, not described, over 1.5 m long Mongograph:

Ostrom 1969, Bull. Peabody Mus. Nat. Hist. Yale, 30

Notes:

Characters (also after Currie 1997, Din. Enc. p. 194; Norell and Makovicky 1997. American Museum Novitates 3215, 28 p.; Barsbold and Osmolska 1999. Acta Palaeontologica Polonica 44: 189-219): Dromaeosaurines Velocitaptorines The skull of Velociraptor differs from that of Dromaeosaurus (Barsbold and Osmolska 1999; Norell 1998. JVP 18, 3, Suppl, p. 66): Characters shared with other coelurosaurs: New taxa:

Sinornithosaurus (Xu et al. 1999. Nature 401: 262-266) is the most complete and one of the most ancient dromaeosaurs known, from the early Cretaceous of northeastern China. The authors find that the form is more closely related to birds than to troodontids; Protoarchaeopteryx and Caudipteryx more remote from birds than are troodonts. In addition to dromaeosaurid synapomorphies are:

Achillobatar (giant dromaeosaurid) from the Late Cretaceous of Mongolia (Perle et al. 1999. National University of Mongolia, Contrib. 101, Mongol. Am. Mus. Paleon. Proj., 105 p.). Animal estimated to have been 7.5 m (about 25 ft) long.

Bambiraptor (immature dromaeosaurid) from the Late Cretaceous of North America (Burnham et al. 2000. Univ. Kansas Paleontol. Contrib. 13, 14 p.)

CROSSING THE RUBICON (NODE) FROM COELUROSAURS TO BIRDS






The focus shifts to Archaeopteryx and beyond into creatures that for the most part resemble coelurosaurs — which continue to change, at seemingly relatively slower rates than birds did, through Mesozoic time.

What is a bird?

The characteristics we think of as defining modern birds are not limited to birds; many are inherited from bird ancestors (plesiomorphies) and others have appeared in other lineages not ancestral to birds (homoplasies), e.g.

Birds at their origin acquired the ability to fly. They are characterized by a side-facing shoulder, wing feathers that are asymmetric and suited for flight, and by a reversed "big" toe for perching. Archaeopteryx possessed all of these characters, molded onto the skeletal frame of a crow-sized dinosaur with flight feathers, large hands, a relatively short tail (for a dinosaur) and a foot that could grasp a branch. The morphological "take- off" was probably through adaptations for simultaneously running and flapping (Burger and Chiappe 1999. Nature 399: 60-62). Assuming only a reptilian metabolic rate, by running and flapping within three seconds the animal could attain a speed of 25.6 ft/s (19 mph) and become airpborne - lizards sustain bursts of activity much longer. The thrust from flapping is preadaptive to flight for it accelerates an animal and enhances its ability to jump, making it difficult to capture.
 
 

145 myr: Feathers and Archaeopteryx, the "First Bird"

According to Feduccia (1996. The origin and evolution of birds, Yale), feathers are the most complex epidermal structures known and are adapted for aerodynamic function (which degenerates in flightless birds). The consensus is that feathers evolved only once.

Seven specimens of Archaeopteryx have been collected: 1861, 1876, 1956, 1970 (1855), 1973 (1951), 1987, 1992, from Solnhofen lithographic limestones in southern Germany dated at approximately 145 myr. The creature resembles what a missing-link bird might have looked like and it possessed feathers suited for flight; everyone agrees that it was a bird:

Skeletal attributes inherited from small theropods (cf. Ostrom 1976, Biol. J. Linn. Soc. 8: 91-182; Gauthier 1986, Mem. Calif. Acad. Sci. 8) include: Avian characters listed by Chiappe 1995 (Nature 378: 349- 355) include: A close relationship between Archaeopteryx and coelurosaurs is contested by Fecuccia (1996) on the basis of: Feather-like structures are known to be associated with small, peculiar archosaurs by the end of Triassic time (Jones et al. 2000) and with coelophysoid theropods by the beginning of the Jurassic (Sabath, K. and G. Gierlinski 1998; Kundrat, M. 1998). The morphological diversity represented by Archaeopteryx and feathered coelurosaurs during late Jurassic-early Cretaceous time suggests that feathers were widespread among theropods much earlier (cf. Sereno 1997). These feathers were probably not primarily adapted for flight, but for thermoregulation or display.

Middle Jurassic coelurosaurs (including the unknown ancestors of Archaeopteryx) have never really been sought and have never been found. However, there is a snap- shop of feathered creatures preserved in the deep, early Cretaceous lakes of northeastern China: for a list of Cretaceous birds there (and elsewhere) see:

http://www.isgs.uiuc.edu/dinos/dml/names/aves.h tm

For the infamous "Archaeoraptor"hoax, see

http://www.ngnews.com/news/2000/01/01212000/fea thereddino_9321.asp

http://www.ngnews.com/news/2000/04/04072000/raptor_11794. asp

http://www.sciencenews.org/20000115/fob6.asp

[Coelurosaur skeletons preserved in the China lakes include Sinosauropteryx (Compsognathidae, with filaments), Protoarchaeopteryx (cf. Dromaeosauridae, with filaments and feathers), Sinornithosaurus (Dromaeosauridae, with filaments), Beipiaosaurus (Therizinosauridae, with filaments) and Caudipterxy (Oviraptoridae, with filaments and feathers). In no case are the feathers adapted for flight.]

The lake deposits postdate the occurrence of Archaeopteryx by about 20 myr, by which times birds had diminished to an average size well below that of coelurosaurs (Sereno 1999); some were as small as sparrows and hummingbirds.

Among the birds preserved are two major groups:

Enantiornithenes ("backwards birds" that became extinct at the end of the Cretaceous):

Ornithurenes (ancestral to modern birds): The confuciusornithids, which are related to the ancestry of both enantiornithines and ornithurines are represented in great abundance in the Chinese lake deposits; for a recent review see Chiappe et al. 1999. (Bull. Am. Mus. Nat. Hist. 242, 89 p.). However, confuciusornithids were relatively small (for coelurosaurs) and obviously capable of flight — they are universally regarded as birds. There remain several taxa the affinities of which have been suggested as related to the ancestry of birds:

Protoavis, late Triassic of Texas

Unenlagia, early Late Cretaceous of Argentina

Alvarezsauridae, Late Cretaceous, Argentina and Asia

Rahonavis, late Cretaceous of Madagascar

These are discussed seriatem:

215 myr: Protoavis, (Chatterjee 1991. Phil. Trans. Roy. Soc. London, Ser. B, 332: 277-342; 1999. Palaeontographica Abt. A, 254: 1-100), Norian deposits - nearly as old as the oldest dinosaurs, preceeds Archaeopteryx by 75 myr. According to several authors (e.g. Ostrom 1991. Nature 353: 212; 1996. Archaeopteryx 14: 39-42; Sereno 1997. p. 460; 1999. P. 2143) the material may represent several taxa. According to Chatterjee, Archaeopteryx is was a living fossil, unrelated to the ancestry of modern birds.

Among the highly advanced characters are:

Among the extremely primitive characters are: Conclusion: This is a very odd animal. The strata in which the material was found should be thoroughly prospected to support the association.

88 myr: Unenlagia (Novas and Puerta 1997. Nature 387: 390-392), Turonian-Coniacian 88 myr of Argentina - Mapuche word for half-bird, 2 m long; several characters are shared with dromaeosaurids, including pleurocoels on dorsals, unlike in troodontids and Archaeopteryx)

Archaeopteryx-shared characters Archaeopteryx characters that are lacking (humerus/femur ratio greater in Archaeopteryx) Notes: Conclusion: Unenlagia should be more broadly compared to coelurosaurs (suggested by Norell and Makovicky 1999)

87-70 myr: ALVAREZSAURIDAE, Bonaparte 1991, 1996. These small bipedal raptors exhibit characteristics of both coelurosaurs and birds. The type specimen of Alvarezsaurus consists of a vertebral column, sacrum and ilium, scap-cor and hind limb, approximately Coniacian (87 myr) and originally considered to be an abbarent Gondwana "ornithomimid-mimic"

Mononykus (Perle et al. 1993, Nature 362: 623-626; 1994 AMNH Novitates 3105; Chiappe et al. 1996 (Mem. Queensland Mus. 39(3): 557-582), a turkey sized "coelurosaur" 1 m long.

Diagnosis of Alvarezsauridae: Characters shared with Archaeopteryx Characters shared with with higher birds, not in Archaeopteryx: Characters shared with higher birds, unknown in Archaeopteryx Patagonykus Novas 1996 (Mem. Queensland Mus. 39(3): 675-702); 1997 (JVP 17: 137-166). Alvarezsaurus (A), Patagonykus(P) in same group as Mononykus (M), but pre-Campanian (Turonian- Santonian); Novas suggests that the group may have originated in Gondwana and dispersed to Asia. There is a lot of skeletal variation for these genera to all be included in a single family: Resolved: Alvarezsaurids are related to birds (Chiappe et al. 1998. Nature 392: 275-278); the skull of Shuvuuia (Late Cretaceous of Mongolia) is positioned phylogenetically between Archaeopteryx and Enantiornithines. A major implication is that alvarezsaurs secondarily adopted flightless, terrestial habits. Resolved: Alvarezsaurids are related to ornithomimids: (Sereno 1997, p. 460; 1999, JVP 19, 3, Suppl. p. 75A)

They lack the following avian characteristics:

They share the following ornithomimid characteristics: Conclusion: The streptostyly is so pronounced and so similar to that of birds that the case for avian rather than ornithomimid relationships appears stronger.

70 myr: Rahonavis (Forster et al. 1998. Science 279: 1915-1919; Science 280: 185; see also Sampson et al. 1997. Natural History 1997(3): 24-27; Forster et al. 1996, DinoFest II Abstracts), raven-sized archaic bird only slightly larger than the London Archaeopteryx, from the Upper Cretaceous quarry (65- 70 myr) of Madagascar. Approximately intermediate between Archaeopteryx and alvarezsaurids.

Bird characters, more advanced than in Archaeopteryx

Coelurosaur characters Note: Conclusion: The animal is a coelurosaur pertaining to a previously unknown family group related both to troodontids and dromaeosaurids, and that has just re- invented flight in late Cretaceous time. It is a Cretaceous ecomorph for the Jurassic Archaeopteryx and a stunning example of parallel evolution.

Coeval to the occurrence of Rahonavis in Madagascar is that of an ostrich-sized ground bird (Gargantuavis) from southern France (Buffetaut and Le Loeuff 1998. J. Geol. Soc. London 155: 1-4). Rahonavis is far too late in time (75 myr) to have remained plesiomorphic since the time of Archaeopteryx.

ARCHAEOPTERYX RECONSIDERED

COMPARED TO DROMAEOSAURS ARCHAEOPTERYX IS LESS DERIVED:

COMPARED TO DROMAEOSAURS ARCHAEOPTERYX IS MORE SPECIALIZED: The prevailing view is that dromaeosaurs are closely related to the ancestry of birds (cf. Chiappe, Novas, Sereno). Sereno (1997, p. 458) states that an eviscerating second toe occurs in some primitive birds, but is not well developed in Archaeopteryx. Currie 1995 (JVP 15: 576- 591) on the other hand notes, with regard to the skull of Dromaeosaurus, that members of the group "...lack many of the theropod-avian synapomorphies found in other theropod families, and have too many specializations to be plausible avian ancestors (p. 587)."

Norell (1998. JVP 18, 3, Suppl, p. 66) cites postcranial characters linking dromaeosaurs to birds:

Can dromaeosaurs be derived from Archaeopteryx through a reversal to flightlessness over 20 myr? The number and morphological complexity of reversals seems excessive

A word from Alan Feduccia (pers. com., September 2000), a critic of bird-dinosaur relationships: "The skull of Archaeopteryx resembles vaguely some anatomical points seen in oviraptorids, but not dromaeosaurs, where there is almost no similarity. The postcranial skeleton has some resemblance to dromaeosaurs, but nothing else. Too, when we look at teeth, tooth replacement, fingers, ascending processes to the tibia,and a host of other features, we have to place a square peg in a round hole. Interestingly, the current poster child for the dinosaurian origin of birds is (the dromaeosaur) Bambiraptor, some 70 million years younger than Archaeopteryx, and the entire assemblage of Chinese fossils, showing (???) the origins of everything from feathers to flight, is some 25-30 million years post- Archaeopteryx. This is the signature of CONVERGENCE!"

COELUROSAURS (EXCEPT TYRANNOSAURS AND DROMAEOSAURS) RESEMBLE ARCHAEOPTERYX:

THESE COELUROSAURS ARE ALSO MORE DERIVED THAN ARCHAEOPTERYX

All possess a slender, medially recurved dentary symphasis (the symphyseal contact is straight in Archaeopteryx, and as individual groups the following apomorphies:

Ornithomimids:

Therizinosauridae: Troodontids: Oviraptors: Avimimidae: CONCLUSIONS

Where are we?

There has been a recent explosion of information, but small coelurosaurian dinosaurs are still undercollected. And available phylogenetic data is still insufficient to define relationships between groups. The requirements of flight are sufficiently stringent to have produced convergent evolution to an important degree in modern birds — it would be expected to be present to a significant extent in bird ancestors. The simple presence of feathers does not seem as important as it once was in defining avian relationships — animals as distinct as Archaeopteryx and Caudipteryx possessed feathers.

Groups of coelurosaurs

compsognathids

tyrannosaurs

ornithomimids

troodontids — (formerly allied to pachycephalosaurs)

oviraptorids — currently research (formerly allied to

ornithomimids)

avimimids

therizinosaurids — (formerly allied to prosauropods)

dromaeosaurs — (allied by Ostrom to Archaeopteryx)

Groups of archaic birds

Archaeopteryx

Protoavis(?)

Inenlagia

Alvarezsaurids

Rahonavis

It seems almost certain that homoplasy is of major importance in the acquisition of skeletal specializations by small Cretaceous coelurosaurs. The similarity between Archaeopteryx and Rahonavis, in spite of their separation in time of 75 myr is an amazing example of convergence.

About 40 myr separates Archaeopteryx from late Liassic time. The span is adequate to accommodate the retroflexion of the pubis, the changes in the ischium and the reversal of pedal digit I in a small feathered allosauromorph to produce an Archaeopteryx-like skeleton. It might also be sufficient to accommodate the transformation of symmetrical feathers into flight feathers. The skeleton of Archaeopteryx is relatively unspecialized, and pre-Archaeopteryx avian affinities will best be demonstrated through feather morphology. The best place to search for the mid-Jurassic record of proto- birds is in rift-lake deposits of early and middle Jurassic age, perhaps around the margins of the Caribbean Basin.
 
 

Quotations supporting the unimportance of homoplasy:

Ricqles et al. 1999. JVP 19 (3 Suppl. p. 70A- 71A)

"[The bones of] Birds and other theropod dinosaurs, however, share derived histological features that are not typically found in other reptiles and that are linked to high growth rates and high metabolic levels. Some features reported in these basal birds may reflect (inter alia) factors of ontogeny or life-history stratety, but do not preclude high metabolic levels and rapid growth rates. Basal birds probably did not grow exactly like living birds in all prespects, but they dikd not grow like non-dinosaurian reptiles."

Paul 1999. JVP 19 (3 Suppl. p. 68A)

"The hypothesis that birds descended from derived predatory dinosaurs is as well founded as is the therapsid- mammal link. The skeletons of basal birds and advanced theropods share multitudes of detailed derived features in the cranial sinuses, palates, braincases, forelimbs, pelves, hindlimbs and the rest of the skeleton. Available evidence favors the progressive evolution of a preavian pulmonary air- sac complex in theropods. Theropods even possessed feathers and bird-like egg shell microstructures. Convergence cannot explain such an extreme degree of similarity. Dromaeosaurs, troodonts, caudipterygians, oviraptorosaurs, therizinosaurs and protoarchaeopterygians further exhibit ossified uncinate processes and sternal ribs, horizontal scapula blades, sharpley reflexed coracoids that articulate via a hinge joint with large, plate-like sterna, folding arms with a semi-lunate carpal block, tails that are either very reduced or are similar to those of pterosaurs and urvogels, and symmetrical brachial feathers. Some of these avian features are not found in Archaeopteryx, but they are common to secondarily flightless birds. The above dinosaurs also possess some other avian features not present in archaeopterygiformes. A number of predatory dinosaurs may have evolved from ancestors whose flight ability approached, matched or even exceeded that of Archaeopteryx. It is possible that such dinosaurs were closer to modern birds than the original bird."

--

Sabath, K. and G. Gierlinski 1998. (JVP 18, 3, Suppl. P. 73A). Feathers of nonavian dinosaur. Early Liassic feather imprints 50 myr older than Archaeopteryx associated with a medium-sized squatting theropod - along its belly (ischium- pubis prints) brush-like semiplumes, incorrectly referred to feet by Gierlinski in 1996. - propubic theropod

Kundrat, M. 1998. (JVP 18, 3, Suppl., p. 57A) Comments on the significance of integumentary impression of the Early Jurassic theropod Eubrontes minusculus.

Rod-like pubic shaft as in Coelophysis and Syntarsus - a coelophysoid ceratosaur

Hypopophyses occur in Ornitholestes, troodontids oviraptorids, dromaeosaurs, Ornitholestes, and some alvarezsaurids

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