Archosauriformes

From Palaeos

o Sauria
|--Lepidosauromorpha
`--o Archosauromorpha
   |?--Choristodera
   `--+--Rhynchosauria
      `--+--Trilophosauridae
         `--+--Prolacertiformes
            `--o--Proterosuchidae 
               `--+--Erythrosuchidae
                  `--+--Euparkeriidae
                     `--o--Proterochampsidae
                        `--o Archosauria
                           |--o Ornithodira
                           |  |--Pterosauria
                           |  `--Dinosauromorpha
                           `--Crurotarsi

[edit] Introduction

Archosauriformes (Greek for 'ruling lizards', and 'form') are a clade of diapsid reptiles that developed from Archosauromorph ancestors some time in the Late Permian (roughly 250 million years ago). These reptiles, which include members of the family Proterosuchidae and more advanced forms, were superficially crocodile-like predatory semi-aquatic animals about 1.5 meters long, with a sprawling elbows-out stance and long snouts. Unlike the bulk of their therapsid contemporaries, the Proterosuchids happily survived the catastrophe at the end of the Permian, perhaps because they were opportunistic scavengers, perhaps because they could retreat into water to find respite from an overheated climate. Any such scenarios are hypothetical; what is clearer is that these animals were highly successful in their new environment, and evolved quickly. Within a few millions years at the opening of the Triassic, the Proterosuchids had given rise to the Erythrosuchidae (the first sauropsids to totally dominate their environment), who in turn were the ancestors of the small agile Euparkeriidae, from whom a number of successfully more advanced families - the Archosaurs proper - evolved rapidly to fill empty ecological niches in the devastated global system.

Pre-Euparkeria Archosauriformes were in the past included in the suborder Proterosuchia of the order Thecodontia. Under the cladistic methodology, the Proterosuchia are rejected as a paraphyletic assemblage, and the pre-Archosaurian taxa are simply considered as basal Archosauriformes.

[edit] Information

Author: Gauthier

Stratigrapohic range: Lopingian

[edit] Taxonomy

• Class Sauropsida
  • Infraclass Archosauromorpha
    • (unranked) ARCHOSAURIFORMES
      • Family Proterosuchidae
        • Archosaurus
        • Chasmatosuchus
        • Kalisuchus
        • Proterosuchus
        • Tasmaniosaurus
      • Family Erythrosuchidae
        • Fugusuchus
        • Garjainia
        • Erythrosuchus
        • Shanshisuchus
        • Vjushkovia
      • Family Euparkeriidae
        • Euparkeria
      • (unranked) Avesuchia
        • Yonghesuchus
        • Family Proterochampsidae
          • Tropidosuchus
          • Cerritosaurus
          • Chanaresuchus
          • Gualosuchus
          • Proterochampsa
        • (unranked) Archosauria

Wikipedia

[edit] Technical description

Characters: Antorbital fenestra; orbit shaped like inverted triangle; teeth laterally compressed & serrated; widespread bipedalism; presence of 4th trochanter (Crown group only?) as attachment point for major tail muscles, the caudofemoralis group of thigh retracting muscles; astragalar facets for tibia and fibula separated by flat, non-articular surface [S91]; double row of bony scutes running along the line of the backbone; elaboration of keratin integumentary structures (e.g. feathers).

[edit] Archosauriform evolution

The fine details of archosauriform evolution are still uncertain. In particular, Euparkeria and the proterochampsids continue to switch places as the sister group of Archosauria every few years. What is clear is that Euparkeria, Proterochampsidae, and Archosauria are a natural group and differ from other archosauriforms in a number of ways. The items on the list of synapomorphies of this unnamed group vary a bit from one study to another. We may take Paul Sereno's work as representative. Sereno (1991) includes:

1) Dorsal body armor (specifically, at least one row of bony plates flanking the vertebrae down the middle of the back).

2) Interclavicle with reduced, tab-like lateral processes (as with most derived tetrapods, the structural integration of the two arms gradually decreases).

3) S-shaped femoral shaft (this one is a bit tricky, since the degree of bend in the upper leg bone is quite variable).

4) Loss of bony astragalocalcaneal canal (but it is possible that erythrosuchids have this character and proterochampsids do not -- see Chatterjee, 1982).

5) absence of ossification of distal tarsals 1 & 2

6) digit IV on the foot is markedly shorter than I.

In the 1970's and 1980's it was common to classify archosauriforms on the basis of their ankle morphology. This practice has been soundly criticized -- not least by Sereno himself. However, it is obvious from Sereno's list that there was a core of truth to the "ankle phylogenies" Sereno (1991: 2). Four of Sereno's six synapomorphies concern the structure of the hind limb. What's going on here is that both the front and hind legs are being reorganized. Conventionally, this is expressed in terms of a trend toward dinosaur-style locomotion: upright carriage with the legs held under the body and a simplified, hinge-like ankle joint. However, crocodiles do not walk in that fashion today, and basal archosaurs certainly didn't. A little later on, we will take up the matter of the archosaurian ankle in more detail. The point here is that we remain a bit uncertain exactly how the early archosauriforms, including basal archosaurs, were getting around -- just as we are unsure how the therapsids of about the same period used their legs.

Since Sereno's seminal paper, others have shown that other parts of archosauriform anatomy were also undergoing change. Gareth Dyke has gone so far as to construct archosaur phylogenies deliberately omitting ankle characters. Dyke (1998). These faithfully reproduce the main lines of descent worked out with ankle characters. From this result, Dyke argues that ankle morphology has no special significance in archosaur evolution. It may be too soon to say. Until we have a better understanding of early archosaur locomotion from a mechanical perspective, the significance of the lumps and bumps on the archosaur tarsus are difficult to evaluate.

Perhaps we should leave well enough alone with that agnostic comment. However, it may also be worth pointing out two factors which inform our uncertainty:

1. The first half of the Triassic was a period of strongly seasonal climates. In the broader context of Earth's history, this is unusual and might easily have had rather significant implications for locomotion. Survival in a strongly seasonal environment usually requires some combination of migration, torpor, and adaptability to varying substrates. Migration and broad adaptability have obvious (and different) implications for locomotion. Torpor usually requires the ability to store significant fat reserves, which has yet a third implication for locomotion. Thus, there are some a priori reasons for thinking that locomotor styles may have been under strong selective pressure -- and a type of pressure that could push in several different directions.

2. The idea that locomotor ability "improved" obscures a multitude of different adaptive strategies. Some of these strategies involve anatomical changes which are quite similar, if one simply adds up character changes, but are entirely different and incompatible in overall effect. For example, moving the legs under the body may be an adaptation for the long-distance strider who needs the speed and elevation for efficient hunting. However, the same trait may also signify adaptation for the slow-moving flat-footed walk of extremely heavy armored herbivores. The difference may show up in the foot. The strider will be digitigrade, while the plodder will be plantigrade. But a digitigrade foot may also represent a sprawling, low-slung animal adapted to sloping or uneven surfaces.

The problem with mixing similar characters which reflect incompatible locomotor designs can be seen in Sereno's synapomorphy #6: digit IV on the foot is markedly shorter than I. This wrongly lumps together animals adapted for three-toed, bipedal running with stiff-legged sprawlers [1]. There is no reason to believe that this character is homologous in all archosaurs. To the contrary, the bipedal runner is only known from the Ornithodiran lineage of archosaurs. The stiff-legged sprawler (so far as we are aware) is only known among Crurotarsi. No taxon smaller than Archosauria itself contains both types. Thus, there is no reason to suppose that this character is a true synapomorphy -- or that it even represents a single character, much less a one-time change both unique and common to the archosaur heritage.

Accordingly, we need to understand much more about the Triassic environment and early archosaur locomotion, before we can make much phylogenetic sense out of early archosaur limb characters.

[1] Many primitive sprawlers rotate the foot outward during stance phase. The outer digits (like digit IV) in this case end up bearing significant weight. A more stiff-legged sprawler keeps the foot pointed more or less forward, so that the weight is disproportionately carried on the inner digit (digit I).

ATW040117.

[edit] References

Jones et al. (2000), Parrish (1997) [P97], Sereno (1991) [S91]. ATW040117.

[edit] Links

• Mikko's Phylogeny Archive

[edit] Credits

• ATW040117Palaeos.com (public domain); MAK050228, Dinoguy2 (taxonomy) and others Wikipedia; MAK61023 Palaoes org