Characters of Triassic Archosaurs Leading to Dinosaurs 2006


-antorbital fenestra in snout wall between nostril and orbit

- teeth with serations

-ossified laterosphenoid in braincase

-lateral mandibular fenestra in posterior lower jaw bones

-postfrontal reduced to less than half the dimensions of the postorbital or absent

-The prootic midline contact on the endocranial cavity floor

Some other characters which change in intermediate groups leading to next (some may be challenged but these have been historically mentioned in analyses)

-distal tarsals 1 and 2absent*

-antorbital fossa surrounding the fenestra*

-parietal foramen absent*

-supratemporals absent* -pubis and ilium elongated

-palatal teeth on pterygoid, palatine and vomer absent*

-calcaneal tuber (heel) directed at more than 45 backwards and sideways*

-S-shaped "swan"-like neck

-forelimb less than half the length of the hindlimb

-calcaneum reduced to a small block of bone less than 1/3 the width of the astragalus

-canal between astragalus and calcaneum absent

-a least one dorsal vertebra added to sacrum

-hand digit 4 reduced

-cnemial crest on tibia


-forelimb-hindlimb ratio less than 0.55

-Interclavicle absent

-manual digits I-III with a long pentultimate phalanx with trenchant unguals

-tibia-femur ratio equal to or more than 1.0

-fibula tapering and calcaneum reduced in size

-compact metatrasus with metatrasal I-V tightly bunched

-metatrsals II-V more than 50% tibial length


-astragalar caudal groove absent

-deltopectoral crest on humerus is subrectangular

-calcaneal tuber rudimentary or absent

Some other characters which show changes intermediate groups before next

-anterior cervical vertebrae longer than mid-dorsals

-interclavicle absent

-clavicles rudimentary or absent

-tibia longer than femur

-metatarsals 1-4 tightly bunched together

-metatarsals 2-4 elongated

-dorsal body armor absent


-femoral proximal head subrectangular and distinctly offset

-astragalar craniomedial corner acute

-calcaneal distal articular face less than 35% that of astragalus

-articular facet for metatarsal V less than half of lateral surface of distal tarsal 4

-midshaft diameters of metatarsals I and V less than those of II-V

-metatrsal V has no "hooked"proximal end

-articular face for distal tarsal 4 is subparallel to shaft axis


-Centrum shape of cervical centra 6-9 parallelogram-shaped

-acetabular antitrochanter on ilium and ischium

- femoral head articular surface extends under head

-fossa trachanterica on proximal face of femoral head

-femoral cranial trochanter weakly developed


- tibia with caudolateral flange and receiving depression of dorsal aspect of astrgalus

-postfrontal absent

-brevis shelf on ventral surface of postacetabular part of ilium

-deltopectoral crest runs down 38% or more of length of humerus

-tibia with transversely expanded subrectangular distal end

-ascending astragalar process on front surface of tibia

- acetabulum extensively perforated

Some other dinosaur characters mentioned for dinosaurs but not now clear that significant

-elongate vomers

-elngate scapula

-symetrical hand

-three or more sacral vertebrae

Note: Above gives a set of characters as used in defining current clagograms. However, they also do not give a good definition if someone asked you "What is a Dinosaur?". Below are some summary definitions/descriptions to help with this gathered from a web search that offer other ways of thinking about this and may be helpful to you.

A true cladists answers:

1.The Dinosauria were defined cladistically by Padian and May in 1993 as the most recent common ancestor of Passer (the sparrow) and Triceratops, together with all its descendants; or, more tersely, as {Passer + Triceratops}. This definition now seems to be commonly accepted'

2. From

Preferred phylogenetic taxonomic definition for Dinosauria, namely: Dinosauria = all descendants of the most recent common ancestor of Iguanodon and Megalosaurus. I prefer this definition for a number of related reasons:

* It employs as the reference (anchor) taxa two of Owen's original three members of Dinosauria (poor Hylaeosaurus is left outů)

* It uses taxa that have been universally regarded as dinosaurs since 1842

* It retains English genera as the anchor forms, honoring the birthplace of the concept of "Dinosauria"

However, this is NOT the formal definition of Dinosauria in the scientific literature. In fact, that definition is:Dinosauria = all descendants of the most recent common ancestor of Triceratops and modern birds (Aves of Gauthier; Neornithes of other authors).Okay, the taxa included by either definition remain the same: both definitions encompass the same part of the Tree of Life.

However, I dislike this definition for a couple of reasons:

* Triceratops was unknown during the lifetime of Owen and the other founders of the "dinosaur" concept (Mantell, Buckland, etc.)

* Birds have not been central to the historical understanding of the concept of "Dinosauria"

* Furthermore, using a definition without birds as an anchor taxon more accurately reflects the accomplishments of Huxley, Ostrom, Gauthier, etc.: we recognize birds as part of Dinosauria because of discovery rather than because of definition

Nevertheless, because of the principle of priority, the "Triceratops + birds" definition is the proper one.

So, students, please forgive me this little deception. For class purposes use the "Iguanodon + Megalosaurus" definition of Dinosauria, but (should you delve into the depths of recent dinosaurian paleontology) understand that the "Triceratops + birds" definition is the proper form.

For more information, read: Padian, K. 1997. Dinosauria: Definition, pp. 175-179 in P.J. Currie & K. Padian (eds.), Encyclopedia of Dinosaurs (Academic Press)

Some more Descriptive Answers:

Dinosaurs are all those animals that arose from a common reptilian ancestor that had developed a hole in the bone of its hip socket (right). This hole could exist because of the upright posture of the animals which transmitted forces up the leg to the top rather than the side of the socket. Paul E. Olson.

Have the Following Features:

1. underslung legs that gave dinosaurs an erect posture

2. a large hole in the bottom of their basin-shaped hip-socket

3. a secondary palate (uncharacteristic of reptiles) that permits dinosaurs to eat and breathe at the same time

4. a fairly straight thigh bone with an in-turned head

5. two pairs of holes in the temporal region of the skull (diapsid skull)

6. backward-pointing knees (or elbows) of the front legs

7. forward-pointing knees of the rear legs (rather than pointing sideways)

8. front legs shorter and lighter than the rear legs (in almost every case)

9. a special bone (predentary) at the chin that capped the front of the bottom jaw in some dinosaurs (the ornithischians)

10. land-dwelling creature, rather than marine or airborne

Author: Paul S. Taylor

Below from:

Different workers have proposed different sets of characters for the Dinosauria, since putative synapomorphies are matters of observation and interpretation rather than of fact. However, the following list, (from The Dinosauria, ed. Weishampel, Dodson & Osmolska, 1990) is fairly representative:

1. enlongate vomers that reach caudally at least to the level of the antorbital fenestra(Gauthier 1986).

2. three or more sacral vertebrae (paralleled in the crocodylotarsan Postosuchus and the Ornitosuchidae; this character is uncertain in basal dinosaurs, and may apply to a higher node in the cladogram; Lagosuchus and Lagerpeton have only two sacrals: Arcucci 1986).

3. scapulocoracoidal glenoid facing fully backward

4. low deltopectoral crest that runs one-third or one-half of the way down the shaft of the humerus.

5. three or fewer phalanges in the fourth digit of the hand (Gauthier 1986)

6. largely to fully open acetabulum

7. fully offset proximal head of femur with a distinct neck and ball

8. greatly reduced fibula

9. well-developed ascending process of astragalus

The following list - more complete, more up to date, but also more incomprehensible (:-) is from Sereno's 1999 article in Science 284:2137-2147:

1. Postfrontal absent

2. Frontal participates in supratemporal fossa

3. Ectopterygoid dorsal to pterygoid

4. Quadrate head exposed laterally

5. Posttemporal opening reduced to small foramen

6. Dorsosacral (3 sacrals)

7. Ossified sternal plates present

8. Deltopectoral crest 35% or more of humeral length

9. Manual digit IV narrower than digits II and III without terminal ungual

10. Brevis fossa present

11. Femoral greater trochanter angular

12. Femoral medial tuberosity small

13. Cnemial crest present

14. Astragalar ascending process present

15. Astragalar anterolateral process lateral to calcaneum

16. Calcaneal medial process rudimentary

17. Distal tarsal 4 heel shallow

18. Metatarsal IV shaft sigmoid in anterior view