The Stagonolepidae (Aëtosauria) were large herbivorous thecodonts that averaged 3 metres in length, and flourished for a short time during the late Triassic. Examples include Aetosaurus, Stagonolepis and Desmatosuchus. Their fossil remains have been found late Triassic rocks of Scotland, Germany, Argentinia, and the United States, but these creatures most certainly had a world-wide distribution.
These quadrupedal reptiles were well armoured, their bodies protected by bony plates (with large interlocking bony plates over the back) and sometimes spikes as well. They were herbivorous, with bulky bodies, tiny heads, weak leaf-shaped teeth, and strange pig-like snouts. At least in the body and tail somewhat crocodile-like in appearance, but the small heads and weak teeth showed that they were herbivores, probably nipping off leaves and digesting them in their gut with the aid of symbiotic bacteria.
As with other advanced thecodonts, dinosaurs, and Sphenosuchid crocodylomorphs they had a fully erect posture. All were quadrupedal, walking on four legs and probably relying on armour and large size rather than speed to protect them from their enemies.
The Aëtosaurs were the earliest of the reptilian "tanks", they parallel to a remarkable degree the armoured scelidosaur, nodosaur, and ankylosaur dinosaurs that were to appear and diversify in the succeeding Jurassic and Cretaceous periods; and even resembled the armadillos and the giant glyptodontid edentate mammals of the South American Cenozoic era. As is often the case with evolution, we see an example of a particular form, evolving and flourishing for millions of years, then eventually dying out, to be replaced tens of millions of years later by another, unrelated line, which independently evolves the same basic form, and again flourishes before it too dies out, to be replaced again later by yet another evolutionary line.
Aëtosaurs ranged in size from 1 to 4 or 5 meters, the average size being 2.5 to 3 meters.
by Mikko K. Haaramo
`--o Stagonolepididae |?- Stegomus arcuatus |?- Ebrachosaurus singularis |?- Argentinosuchus bonapartei |--o "Aetosaurinae" | `--o Aetosaurus | |-- A. ferratus | `-- A. crassicauda `--o "Stagonolepidinae" |--o "Stagonolepidini" | |-- Stagonolepis scagliai | |-- Coahomasuchus kahleorum | `--o Stagonolepis | |-- S. robertsoni | `-- S. wellesi [Calyptosuschus] `--+-- Neoaetosauroides engaeus ["Neoaetosaurini"] `--o "Typothoraxini" |--+-- Desmatosuchus haploceros | `-- Typothorax coccinarum `--+-- Longosuchus meadei `-- Paratypothorax andressorum
Aëtosaur genus and species list (under construction!)
S. scagliai retains a number of primitive features which distinguish it from other members of the group. The skeleton is very similar to the European species, and the armour lacks the spikes of some of the American species. [From Jose Bonaparte's description:, p.671]:
The genus it very well represented. The type is an incomplete skeleton with most of the skull and jaws (left); other specimens have most of the posteranium. The primitive condition of this genus is seen in its elongated skull, the slender jaws and the small and conical teeth which distinguish it from other Aetosaurs. The postcranium is very similar to Aetosaurus and Stagonolepis. The dorsal armour lacks the spikes found in the North American genera. A good variation in size has been recorded, front 120 cm up to 250cm in total length. It has been found in levels bearing cynodonts, dicynodonts and archosaurs.[Jose Bonaparte, "Annoitated List of the South American Triassic Tetrapods," Second Gondwana Symposium, Proceedings and Papers, 1970, p.671]
The dating of this species is controversial. On the basis of similarities with other global fauna Spencer G. Lucas ("Global Triassic tetrapod biostratigraphy and biochronology" - in Paleogeography, Palaeoclimatology, Palaeoecology, 143, 1998, 347-384) considers this part of the Adamanian faunal zone (latest Carnian). However radiometric dates for the Ischigualasto Formation put it in the early Carnian, which is also the time range Dr Bonaparte gives.
Stagonolepis robertsoni Agassiz
Lossiemouth Beds, Elgin, Scotland
Fragmentary skeletons of about 20 individuals
Length: 2.7 metres
Weight about 200 kg
Duration Latest Carnian
Locality central Laurasia (Pangea)
Stagonolepis wellesi (Long & Ballew)
Lower Blue Mesa member, Petrified Forest Formation
Arizona and New Mexico
Duration Latest Carnian
Locality southwest Laurasia (Pangea)
Neoaetosauroides engaeus Bonaparte 1967
Horizon and locality. Upper beds of the Los Colorados Formation, La Rioja Province, Argentina.
Possibly a relative and descendent of the Carnian Stagonolepis, this relatively primitive aetosaur nevertheless lived right at the very end of the Triassic period, being one of the last of its kind.. The following passage is from Jose Bonaparte's description::
This genus was published with preliminary descriptions. The type includes most of the articulated postcranium, with the jaws, but no trace of the skull. However, in another very incomplete specimen some diagnostic fragments of the skull are present. The jaw is short and rather heavy, with no teeth in site anterior region. The upper jaw bears 4 small "incisors" in the premaxilla, and robust maxillary teeth. It has no spikes or projections in the carapace. The tail is formed by 26 rows of osteotterms. It has been found in levels with melanorosaurids [prosauropod dinosaurs].[Jose Bonaparte, "Annoitated List of the South American Triassic Tetrapods," Second Gondwana Symposium, Proceedings and Papers, 1970, p.671].
Desmatosuchus haplocerus (Cope 1892)
Dockum formation, Texas; Chinle formation,
Arizonia and New Mexico [Parish and Carpenter, p.158]
Length: upto 5 metres
Weight about 1000 kg
Duration middle to late Carnian
Locality Southwest Laurasia (Pangea)
Remarks: This enormous, narrow-bodied aetosaur possessed, in addition to the normal complement of armour plates, a pair of huge, backwardly turned shoulder horns, preceeded by several pairs of smaller spikes on the side of the neck. Despite beinga relatively advanced form, Desmatosuchus appears very early in the history of the group, indicating that the major lineages of aetosaurs evolved in a sudden evolutionary radiation that may have taken no more than one or two million years.,
While most Aetosaurs, such as Stagonolepis and Desmatosuchus, retained a rather narrow crocodile-like body, advanced forms like Typothorax and Paratypothorax had a broad turtle-like midsection, probably expanded to contain a large fermenting gut in the manner of ankylosaurs.
|- Aëtosauridae - Links -||
Aetosauria Translation and Pronunciation Guide - Ben Creisler - lists and describes every Aetosaur genus, includes a sound file (.au) giving the correct pronounciation of each name.
Introduction to the Aetosauria - good non-technical introduction.
Great Triassic Assemblages Pt 1 - The Chinle and Newark- Professor P. E. Olsen. - includes some material on Aetosaurs
The Tiny Triassic - page includes a painting by Doug Henderson of Stagonolepis wellesi
Palaeos Page (incorporates some of this material, plus a lot of additional material)