Archosauromorpha

Archosauria

___Dinosauromorpha

______Marasuchus lilloensis

______Dinosauromorpha gen. sp. indet. 1

______Lagerpetonidae

_________Dromomeron romeri

_________Dromomeron gregorii

_________Lagerpeton chanarensis

_________Lagerpetonidae gen. sp. indet 1

______Dinosauriformes

_________Saltopus elginensis

_________Dinosauriformes gen. sp. indet. 1

_________Dinosauriformes gen. sp. indet. 2

_________Lewisuchidae (Silesauridae)

_________Lagosuchia

_________Dinosauria

____________Saurischia

_______________Theropoda

__________________Herrerasauria

_____________________Agnostiphys cromhallensis

_____________________Caseosaurus crosbyensis

_____________________Guaibasauridae

_______________________Guaibasaurus candelariensis

_____________________Herrerasauridae

________________________Staurikosaurinae

________________________Herrerasaurinae

Archosauromorpha Huene, 1946

Caudodorsal process on premaxilla; sagittal crest on parietal; slender tapering cervical ribs at a low angle to the vertebrae; notch on cranial margin of clavicle; dorsal margin of ilium composed of a small cranial process and a large caudal process; medial centrale of carpus absent.

Archosauria Cope, 1869

Gauthier (1986), urging use of the crown-clade concept, defined Archosauria as the clade subtended by living archosaurs (birds and crocodilians), in other words, excluding proterosuchids, erythrosuchids, proterochampsids, and euparkeriids. He argued that this realignment of the name had the advantage that all archosaurs, including fossil forms, would then have predictable soft-part characters (based on extrapolation from living forms). The apomorphy-based Archosauria was then renamed Archosauriformes (Gauthier 1986).
The following are the hard-part synapomorphies of Archosauria sensu Benton 1999:
1. An antorbital fenestra
2. The postfrontal reduced to less than half the dimensions of the postorbital or absent
3. An ossified laterosphenoid
4. The prootic midline contact on the endocranial cavity floor
5. Teeth with serrated margins
6. A lateral mandibular fenestra

Dinosauromorpha Benton& Norman in Benton , 1985

The subrectangular and distinctly offset femoral head; the astragalar craniomedial corner acute; the astragalar ascending process on the cranial face of the tibia; the calcaneal distal articular face less than 35% of that of the astragalus; the articular facet for metatarsal V less than half of the lateral surface of distal tarsal 4; the midshaft diameters of metatarsals I and V less than those of II–IV; and metatarsal V having no “hooked” proximal end, with the articular face for distal tarsal 4 subparallel to the shaft axis.

Lagerpetonidae Arcucci, 1987

Diagnosis: Differentiated from all other archosaurs by the following unambiguous synapomorphies: 1) presence of a hookshaped femoral head, 2) a lateral emargination ventral to the femoral head, 3) an enlarged posteromedial tuber of the proximal end of the femur, 4) an enlarged crista tibiofibularis of the distal end of the femur, 5) an anteromedial corner of the distal (Nesbitt et al., 2009).
Comments: Definition of Lagerpetonidae: Reptiles of small size (one hind limb 25 cm in length), with very marked locomotor specializations. Last presacral vertebrae with anteriorly-oriented neural spines. Sacrum with two vertebrae, with the last presacral in the process of sacralization. Closed acetabulum, vertical ilium with well-defined preacetabular projection; pubis short and wide; ischium longer than pubis and with an extensive ventral lamina. Hollow femur shorter than tibia, as in Lagosuchus. Tibia transversely extended in its distal region, with the ventral process on the anterior border, differing from other thecodonts. Advanced mesotarsal tarsus, similar to Lagosuchus and Trialestes, but with a more developed ascending process of the astragalus and barely distinguishable calcanealmtubercle. Two distal tarsals that could be fused. Elongate metatarsals. Metatarsal IV longer than the others. Metatarsal I short and V very reduced and lacking phalanges (Arcucci, 1986).

Guaibasauridae Bonaparte, Ferigolo, Ribeiro, 1999
(Dinosauria - Theropoda - Herrerasauria)

Diagnosis. Dorsal vertebrae with parapophysial-prezygapophysial lamina and a fossa below it; infrazygapophysial lamina not divided, pneumatic cavities present in the dorsal neural arches, and hyposphene-hypantrum present. Sacral centra with disparity in size; slender scapula expanded distally; iliac part of acetabulum only slightly reduced, and extremely developed crista supracetabularis; elongated ischia and pubes; femora with small anterior trochanter and lacking trochanteric shelf; calcaneum transversely narrow with a pronounced ventromedial process; reduced metatarsal V, lacking phalanges (Bonaparte, Ferigolo, Ribeiro, 1999).
Dorsal vertebrae with diapophysial-prezygapophysial laminae and a fossa below it; infrazygapophysial lamina not divided; pneumatic cavities present in the dorsal neural arches, and hyposphene-hypantrum present. Sacral centra with disparity in size; acetabulum mostly closed medially and supracetabular crista extremely pronounced laterally; fossa brevis confluent with ischium pedicel; acetabulum longer than high; elongated ischium with triangular cross section of the shaft; pubis laminar with pronounced process for the ambiens; slender foot with metatarsal I longer than V; the latter without phalanges, and metatarsal II longer than IV; slender scapula expanded distally; slender, elongated manus without hypertrophy of digit I (Canale, Scanferla, Agnolin, and Novas, 2009).

Comments: Herrerasaurids clearly have more derived characters than guaibasaurids, in particular the reduction of the scapular blade, reduction of the postacetabular portion of the ilium, the non laminar
ventrally twisted pubis, the axially short centra of the dorsal vertebrae, and high neural arches provided with anteroposteriorly reduced and distally expanded neural spines. At the same time herrerasaurids show more primitive characters than guaibasaurids in the narrow distal end of the tibia, the almost absent brevis fossa and the presence of only two sacral vertebrae (Canale, Scanferla, Agnolin, and Novas, 2009).
Saturnalia is here interpreted as part of the Guaibasauridae because of the general affinities with Guaibasaurus in the organization of the dorsal and caudal vertebrae, the three bones of the pelvis, the
whole hind limb except the proximal tarsal bones and what is possible to compare of the fore limbs. The future knowledge of the skull and cervical vertebrae may support or deny this interpretation. The
differences of the sacral centra and those of the proximal tarsals may be consistent with generic differences (Canale, Scanferla, Agnolin, and Novas, 2009).