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Archosauria Classification Essay

Diapsida : Archosauromorpha : Archosauriformes

The "ruling reptiles"

The archosaur skull includes two new fenestra (skull openings). The first of these is the mandibular fenestra, an opening through the jawbone. The other is the antorbital fenestra, a term that means "the opening in front of the eye."
Archosauria (the "ruling reptiles") is a major group of diapsids, differentiated from the other diapsids by the presence of single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics. This continues the tetrapod trend of the reduction of skull bones by the fusion of multiple bones and the opening of fenestrae in the skull. This helps to lighten the skull, provides more room for muscles and other tissues, and allows more skull flexibility (kinesis) when eating. Other typical archosaurian characteristics include another opening in the lower jaw (the mandibular fenestra), a high narrow skull with a pointed snout, teeth set in sockets (called thecodont tooth implantation), and a modified ankle joint.

The ancestral archosaurs probably originated some 250 million years or so ago, in the late Permian Period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and crocodilians exist to provide a glimpse into the past glory of archosaurs.

Archosauria is defined as the group that includes the common ancestor of crocodiles and birds and all of its descendants. Because the group is defined this way, all archosaurs fall into one of two big clades: Ornithosuchia, or archosaurs on the evolutionary line to birds, and Pseudosuchia, or archosaurs on the evolutionary line to crocs. Ornithosuchia includes dinosaurs, dinosaur relatives such as Marasuchus, pterosaurs, and a few oddballs like Scleromochlus, whose relationships are a bit uncertain. In addition to crocs, Pseudosuchia includes the croc-like phytosaurs, armadillo-like aetosaurs, carnivorous ornithosuchids, and rauisuchians, which themselves included both large carnivores and toothless, beaked animals that probably ate plants.

The archosaurs are characterized by numerous synapomorphies that lend strong support to the hypothesis that they form a monophyletic group (clade) exclusive of other Reptilia. First of all, the "stem archosaurs" (properly termed Archosauromorpha), including Champsosauridae and Euparkeria, have a calcaneal tuber. This is a bony process projecting posteriorly from the ankle joint that serves as an attachment point for some of the lower leg flexor muscles. If you feel your heel, the bone that forms it is your calcaneal tuber. This is an example of convergent evolution; synapsids and archosaurs evolved these features independently.

The two major groups of archosaurs are distinguished by differences in the joint of their ankle. The crocodiles and their relatives (Pseudosuchia) have a crurotarsal ankle joint (left), while dinosaurs and pterosaurs (Ornithosuchia) have a mesotarsal ankle joint (right). The red line in each image shows the plane of the ankle hinge. (T=tibia, F=fibula, A=astragalus, C=calcaneum)
Later archosauromorphs, like Euparkeria and the true archosaurs, evolved several key diagnostic features. The graphic above shows the two new skull openings, the mandibular fenestra and the antorbital fenestra, that appeared in archosauromorphs close to the Archosauria. Another new feature that appeared around this point was the presence of serrated (saw-edged) teeth set in sockets (a condition called thecodonty). Also, a large process on the shaft of the femur, the fourth trochanter, served as the attachment point for major tail muscles, the caudofemoralis group of thigh retracting muscles. Finally, ancestral archosaurs had a double row of bony plates (called scutes, or osteoderms) running along the backbone.

True archosaurs (the Archosauria proper), including the major stem groups Pseudosuchia (the crocodilians and their relatives) and the Ornithosuchia (the birds and their relatives) evolved yet more new features. The fifth toe in the foot, homologous with your "pinky toe" was reduced in size. In earlier vertebrates, the palate (roof of the mouth) bore at least one row of accessory teeth, but the archosaurs appear to have lost this feature, as did many other lineages of tetrapods. Finally, the two main lineages of archosaurs each have a specialized ankle joint that changed differently in each group, as shown below.

The Pseudosuchia have what is called a crurotarsal, or "crocodile-normal" ankle. This is a very flexible arrangement in which the astragalus (medial proximal ankle bone) bears a peg that fits into a socket in the calcaneum (lateral proximal ankle bone). Ankle rotation then occurs between these two bones, permitting both a somewhat erect stance (like the crocodilian "high walk") where the hindlimb is held closer to the midline of the body, and a more sprawling stance like that of earlier tetrapods. The result of this is that the pseudosuchians can move in two different ways; walking with an erect or sprawling posture.

The Ornithosuchia have a mesotarsal ankle, which is a simple hinge joint between the lower leg and astragalus and calcaneum, and the distal ankle bones. This restricts the posture to a more erect orientation, so the gait can be called parasagittal — the limbs move parallel to the vertebral column, and are held relatively vertical. Birds and most mammals have this parasagittal gait; birds inherited it from their dinosaurian ancestors, while mammals evolved it independently. The advantage of a parasagittal gait might be that it improves maneuverability/agility. A concordant disadvantage would be that it reduces stability. It's easier to tip over an erect cow than it is to tip over a more sprawling crocodile of similar size; the crocodile has a wider base of support, and thus can be said to be more stable. However, the relationship between stance and stability is still not completely understood, so it's difficult to say exactly what advantages an erect stance has. Another proposed advantage of an erect stance and parasagittal gait was that it was more efficient, but this has not held up to experimental analysis — erect animals move about as efficiently as similarly-sized sprawling ones.

It is difficult to say what sorts of soft tissues extinct archosaurs really had, but the fact that both crocodilians and birds have a four-chambered heart lends support to the notion that this is a trait inherited from a common ancestor (and another convergence with mammals). Birds and crocodiles also share expanded pneumatic sinuses in their skulls — and elsewhere in bird skeletons — which can also be seen in fossil archosaurs, such as the hadrosaurs, so the expansion of these craniofacial air sinuses is another trait shared by archosaurs. The sinuses appear to serve as skull-remodeling agents, reshaping the bones of the skull in response to stresses and other influences.

The fossil record
Students of archosaur evolution are blessed with a wonderful fossil record for many groups of archosaurs, including some very bizarre extinct taxa. The first archosauromorphs (relatives of the true archosaurs) appear in the fossil record in the Early Triassic, about 245 million years ago, just after the great end-Permian extinction. They include weird hippo-size beaked herbivores called rhynchosaurs, long-necked reptiles called prolacertiforms, evil-looking terrestrial predators like the erythrosuchids and proterosuchids, and close relatives of the true archosaurs, including Euparkeria. Many of these early groups are limited to the Triassic Period, not enduring the extinctions in the Upper Triassic that the dinosaurs and other taxa survived.

The Upper Triassic (about 225 million years ago) witnessed the appearance of several new groups of archosaurs, some of which have living descendants today. Most other archosaurs and archosauromorphs did not make it into the Jurassic, but the crocodilian lineage did, as did the dinosaurs and the tenacious champsosaurs, which all appeared at about the same time in the Upper Triassic. Close relatives of the dinosaurs such as the pterosaurs and the possibly dinosaurian Eoraptor and Herrerasaurus also show up in the Upper Triassic.

The survivors of the Upper Triassic-Lower Jurassic extinctions went on to dominate the rest of the Mesozoic Era, but not without hitches. The Upper Cretaceous Period (around 70 million years ago) was a time of global change and "ecological reshuffling," when many taxa did not make the cut. Pterosaurs, many dinosaurs (including some major bird groups), and some crocodilians vanished around this time.

As the Cenozoic Era dawned 65 million years ago, things were different — crocodiles and champsosaurs were still doing well, but the dinosaurs had taken a one-two punch, and only the neornithine birds persevered to continue the massive diversification that began in the Cretaceous Period. Today we are left with only the crocodilians and the birds as extant (living) archosaurs.

Something to keep in mind
The important thing to remember about archosaurs is that the living forms, birds and crocs, are not typical representatives of their respective groups! Many ornithosuchians, like Pteranodon, Apatosaurus, and Triceratops, were not particularly bird-like, although they share important characters with birds. Similarly, most pseudosuchians lived on land, and quite a few ate plants, unlike living crocs. In fact, the direct ancestors of living crocodiles were long-legged running animals the size of chihuahuas that ate insects. So when you think of Archosauria, remember that it includes animals that walked on two legs and on four, fliers and swimmers, tiny hummingbirds the size of your thumb and giant sauropods the size of a house; animals with saber-like teeth and others with toothless beaks, that ate everything from nectar to pine cones and ants to antelopes — and that only a handful of these animals were semi-aquatic fish-eaters like alligators.

Archosauria: Rauisuchiformes: Ornithosuchus

Taxa on This Page

  1. Ornithosuchus X

The Carnian Carnosaurs of Moray Firth

In these latter days of dinosaur paleontology, most of the main lines of descent in that part of phylospace have been sketched out. We tend to dismiss papers like Walker 1964) as hopelessly obsolete. Walker argued that Ornithosuchus was ancestral to the "megalosaurs" and carnosaurs of the Jurassic. Now, it is fair to say that he was quite wrong, and that he was misled by similar characteristics which Carnosauria and ornithosuchians evolved independently. However, it would be a mistake to suppose that Walker was foolish or unobservant. The similarities, as we will see, are rather startling. But, more to the point, it would be an even more serious mistake to disregard Walker's wrong ideas about phylogeny as useless antiques.

Synapomorphies -- unique characteristics -- are the keys to phylogeny.Synapomorphies tell who is related to whom. But homoplasies -- "convergent" characteristics -- are the keys to evolution. Homoplasies might tell us why animals evolved as they did. If two closely related animals are alike, we learn nothing except that they are related. If two unrelated animals are alike, we can legitimately suspect that they were subject to similar selective pressures. By sorting out the characters they do share from the characters they don't share, we may be able to determine what selective pressures were most important in shaping the forms we observe in the fossil record.

The wrong ideas about phylogeny in prior decades are thus potentially much more useful than right ideas. No one today needs to know why Walker thought Hallopus was a crocodylomorph. Walker 1970). He was entirely correct; but we have better fossils now, as well as some improved analytical tools; and these can usually be counted on to give us better reasons than Walker had. On the other hand, erroneous phylogenies by experienced and competent observers like Alick Walker are pre-compiled collections of homoplasies ripe for analysis of selective pressures. This data is much harder to collect today, since compiling this kind of information requires that we put aside what we have learned so laboriously in working out the correct phylogeny.

Some of the homoplasies involved are really extraordinary. By way of example, we show the skull of the ornithosuchid Ornithosuchus on the right from the Carnian Late Triassic) of Scotland [1], adapted from Walker (1964). On the left is the allosauroid  Sinraptor, from the Late Jurassic of China, as reconstructed by Currie & Zhao(1993). Their last common ancestor was probably something like Euparkeria from the Middle Triassic of South Africa.

Thus, eighty million years and tens of thousands of miles separate Sinraptor from its last common ancestor with Ornithosuchus. Yet the two share numerous details of skull structure. The nares are enlarged and bordered almost entirely by the premaxilla. The nares are extended by a well-developed posterodorsal fossa. The anterior margin of the antorbital fenestra is strongly developed as an elaborate, rimmed fossa comprised of an ascending maxillary process. The maxilla and lacrimal meet on the dorsal rim of this fenestra in a tight, complex suture. The skull table is long and narrow. It contains a probable transverse hinge (albeit very differently placed and formed). A transverse ridge marks the posterior end of the skull table, and the upper temporal fenestrae are deeply inset and sharply marked off. The lacrimal and jugal form the antorbital bar. The bar is twisted, with part of the upper surface rugose. The orbit is triangular with markedly roughened surfaces dorsal to the midline of the orbit. The postorbital has a rugose lateral projection above the orbit. The maxilla meets the jugal below the orbit. In Ornithosuchus, the maxilla forks and grasps the jugal with multiple tines. In Sinraptor the junction is simple, but the jugal - quadratojugal suture is split. The lower temporal fenestra is large, and the squamosal and quadratojugal meet behind it at an angle. The dentary is more massive anteriorly in Ornithosuchus, in almost every other respect the jaws virtually identical, with the same bones covering essentially the same areas around an enlarged mandibular fenestra, bearing the same large posterior fossa. Both have a short, fairly strong retroarticular process. The tooth row is rather short and ends about level with the preorbital bar.

These are, obviously, not small matters. In particular, the similarity of the structures surrounding the antorbital fenestra is downright odd. In some imaginative dinosaur reconstructions, this is often an area with a colorful, inflated area of skin used as a mating display, as it is in some lizards today. In other reconstructions, the region is simply patched over since the structure has no obvious purpose.   Yet, this extraordinary similarity suggests that it may have been strictly functional, an organ or structure so highly constrained by its purpose that it appears in almost precisely the same form in two lineages with no direct relationship. The similarities in the orbital region, immediately posterior, are not quite so uncanny at first sight. However, in some ways they are even weirder, since the skull tables of Ornithosuchus and Sinraptor are put together quite differently. What possible use is the little pocket and lacrimal awning above the antorbital fenestra?  Why is the preorbital bar twisted and dorsally rugose? Did they have eyestalks like snails? It hardly seems likely. What does seem likely is that these peculiar similarities are likely the only remaining evidence of important selective factors that shaped large terrestrial carnivores consistently over an enormous period of time in the early Mesozoic and beyond.

[1]  In fact, on the south shore of Moray Firth -- hence the name of our essay.



Ornithosuchus: Newton 1894. O. longidens (Huxley 1877)

Late Triassic (Latest Carnian) of Scotland

Phylogeny:Ornithosuchidae:Venaticosuchus + Riojasuchus + *.

Characters: length to 4 m, skull to 45 cm [W64]; premaxilla recessed posteriorly for passage of enlarged dentary teeth [W64] [3]; skull table slopes smoothly up to frontals [W64]; nasal with strong medial flange posterior to nares, supported by ascending process of maxilla [W64]; continuous suture between nasal + frontal and lacrimal + prefrontal + postfrontal [W64] [1]; postfrontal confined to skull table [W64]; skull table posterior to frontals slightly raised [W64]; postorbitals & parietals form most of UTF [W64]; parietals with occipital flanges [W64] [2]; maxilla excluded from naris by premaxilla - nasal suture [S91]; maxilla overlies lacrimal with a thin squamous suture and also bears thin process inserting between lacrimal & nasal [W64]; maxilla forks posteriorly, where it meets the jugal [W64] [$S91]; lacrimal with strongly rugose, posteriorly - directed flange on descending process [W64]; jugal, lacrimal process tapers to a point, fitting into a groove in the descending process of the lacrimal [W64]; opposite articulation with postorbital (i.e., groove on the jugal) [W64]; maxilla and jugal with strong lateral ridge [W64] [4]; postorbital dorsally rugose, with strong crest overhanging its own descending process [W64] [$S91]; squamosal - quadratojugal bar with strong anterior projection [W64]; external surface of quadratojugal strongly rugose [W64]; quadrate with small, thin, triangular process extending onto surface of pterygoid [W64]; large quadrate foramen [W64]; occipital area poorly known [W64]; probably a large, rectangular supraoccipital [W64]; laterosphenoid present [S91]; anterior palate [5] not well known & median contact of maxillae is not certain [W64]; palatines arch strongly upwards, forming a vault with the anterior processes of the pterygoids [W64]; pterygoids with long, straight, rod-like anterior bars, meeting antimere along a flat medial surface [W64]; small anterodorsal pterygoid processes may have defined notch for cartilaginous epipterygoid [W64]; pterygoids with short, blunt, ventrally directed transverse flange [W64]; basipterygoid process small, located between parabasal process and quadrate ramus [W64]; dentary symphysis long [W64]; anterior dentary curves medially to form symphysis & accommodate anterior maxillary teeth [W64]; splenials medially flat & meet in short splenial symphysis [W64]; angular with long anterior process passing into jaw and between dentary & splenial, forming floor of Meckelian canal [W64]; angular forming concave lower margin of posterior jaw [$S91] [7]; surangular large, forming much of mandibular fenestra, with marked rugosity posterodorsally [W64];  surangular with foramen near ventral margin, under jaw articulation [$S91]; small coronoid present [W64]; prearticular small, probably with some lateral exposure and short retroarticular process [W64]; articular blocky and complex, with articular surface facing ?medially [W64]; interdental plates alternate with teeth [W64]; teeth laterally compressed, more rounded anteriorly than posteriorly, recurved and finely serrated on posterior & distal anterior edges [W64]; premaxilla with 3 teeth, with 1st or 2nd largest [W64]; <10 maxillary teeth & ~10 dentary teeth [W64]; dentary with small, anterior, procumbent tooth [S91]; neural canal large & "sag" down into centra [W64]; 24 presacral vertebrae, with no clear cervical - dorsal distinction & all of about the same length [W64]; 3 sacrals (plus one partially incorporated) & 45-50 caudals [W64]; cervical centra strongly compressed between articular ends, especially ventrally [5] [W64]; dorsals constricted, but without ventral keel [W64]; parapophyses start low on centrum & move up onto arch, diapophyses begin low on arch, directed ventrally and move dorsally, oriented more laterally, along the dorsal series [W64]; both parapophyses & diapophyses with various supporting ridges, particularly to zygapophyses [W64]; cervicals with narrow neural spines, with transversely expanded apices compare Euscolosuchus) [W64]; ribs short, slender & poorly known [W64]; caudal centra become round & spool-like, ventrally grooved from 3rd caudal & with broadly expanded transverse processes [W64]; proximal caudal neural spines much taller than any others [W64]; caudal centra with anterior vertical median lamina [W64]; clavicle present , interclavicle long & very thin [W64]; scapula slender, with gradual distal expansion [W64] [6]; small, oval muscle attachment area above glenoid [W64]; scapulocoracoid notch minor or absent [W64]; coracoid thin and crescent-shaped, pointed posteriorly, forming most of large, shallow glenoid [W64]; forelimb <2/3 length of hindlimb [W64]; major limb bones probably hollow [W64]; humerus very thin with large deltopectoral crest, small internal tuberosity & deep anterior trochlear recess [W64]; radius very slender with lateral recess distally overlapped by an ulnar shelf [W64]; ulna somewhat stouter, with poorly developed olecranon [W64]; distal ulna curves postaxially (very unusual) [W64]; probably 5 digits on manus, I largest and rest progressively smaller [W64]; manus I may have been opposable (!?) [W64]; ilium tall, peaking above anterior part of acetabulum, curving ventrally & laterally posterior to acetabulum [W64]; ilium with marked medial shelf for sacral ribs [W64]; acetabulum deep, perforate in part, with strong supraacetabular shelf [W64]; pubis with large pubic foramen and long, continuous pubic symphysis, with a sort of bifid pubic boot [W64];  ischium with obturator process interrupting symphysis (medial edges bent anteroventrally to form obturator process) [W64]; femoral head with distinct medial bend, but without well-defined neck [W64]; greater trochanter small, probably finished in cartilage, but lesser trochanter well developed [W64]; 4th trochanter present as elongate ridge on medial face of femur [W64]; femoral shaft with slight anteriorly - directed bow [W64]; tibia with triangular proximal face & well-developed laterally-curving cnemial crest [W64]; crus shorter than femur [W64]; calcaneum with prominent tuber & calcaneal form suggests croc-reversed tarsus, but region is not well-preserved in any specimen [W64]; pes digits symmetrical about III, with none greatly reduced [W64]; skull with irregular ornament of small pits [W64]; paired paramedian scutes sutured across the midline arranged so as to cap neural spines [W64]; caudal scutes fuse across midline after 10th caudal [W64].

Notes:[1] Walker does not mention it, but note that the naso-frontal suture is also continued by the lacrimal - prefrontal suture. It is difficult to explain this unless the skull were highly kinetic in this region. [2] Walker speculates that these supported the most anterior set of armor plates [W64]. [3] Is it also for a sliding joint with the maxilla?  [4] Note the similarity to Erpetosuchus and probably also poposaurs. [5]c.f.Saurosuchus.  [6] The general form of the post-cranial skeleton is illustrated at Ornithosuchidae. [7] however this is also perhaps true of Postosuchus.

Comments: Known from skull and postcrania, Ornithosuchus was the first member of this family to be described, from the late Carnian Lossiemouth Beds of Elgin, Scotland. At 3.3 metres or more in length, this was a large animal, without doubt the top predator of its environment. Originally reconstructed as a biped, and certainly capable of bipedal locomotion, it probably spent most of its time on all fours. The skull is strikingly similar to that of large theropod dinosaurs like Tyrannosaurus, leading to the suggestion at one time that tyrannosaurs originally evolved directly from ornithosuchids (this is now not believed to be the case). Although at one time believed to be a primitive member of the Dinosauria, Ornithosuchus, like other members  of its family, have a number of primitive (plesiomorphic) features, such as a double row of bony plates down its back, a short broad pelvis attached to the backbone by only 3 vertebrae, and five toes on each hind foot (as opposed to three in the case of theropod dinosaurs). These characteristics are shared by most basal archosauromorphs, and show that Ornithosuchus is only distantly related to the dinosaurs. MAK.

References:Sereno 1991) [S91], Walker 1964) [W64]. MAK030730, ATW031130.

checked ATW040109

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