However, one of the most compelling evidences of macroevolution is not just the existence of transitional fossils, but the order they appear in the rock record. The basic principles of biostratigraphy, the method paleontologists use to order fossils in time using thier stratigraphic sequence, was developed in the early 19th century by British canal surveyor named William Smith, and more or less independantly by the brilliant French naturalist Georges Cuvier and his collaborator Alexandre Brogniart.
Biostratigraphy as a means of ordering fossils is based on a very simple idea: If you have a pile of sediments, the ones at the bottom (and therefore the fossils in them) are oldest, and the ones at the top (and therefore the fossils in them) are youngest. This is based on the recognition that it is hard to bury something that isn't already there. Makes sense, ja? An added bonus of biostratigraphy is that if you can recognize the same species in piles of sediment from around the world, you can match them up (or "correlate" them) to produce a composite history of life on Earth, like so:
Smith and Cuvier were not evolutionists, and made thier observations that fossils appear in a certain order in a vertical sequence of strata decades before the publication of Origin of Species. Biostratigraphy therefore does not depend on any assumptions about evolution in order to work. As a result, it can be used as an independent check on macroevolution by determining whether or not transitional fossils actually appear in the expected order. The big picture of the origin of life was reconstructed almost entirely using biostratigraphy and biostratigraphic correlation, and gives us a picture of change through time consistent with macroevolution. However, this ordination also supports evolution on smaller macroevolutionary scales.
Archosaurs are better than any other kind of animal, as everyone agrees who is not a complete fool. However, basal archosauriforms and pseudosuchians (crocodile-line archosaurs) are rarely used to demonstrate the reality of evolution, in spite of the fact that both phytosaurs and early crocodylomorphs have beautiful fossil records. The theropod-bird transition is used a lot more commonly, presumably because theropods are dinosaurs and therefore deemed more “sexy” than pseudosuchian archosaurs. However, this is bullshit, because birds are pussies. Just look. Fuck birds.
Phytosaurs were the most common group of archosauriforms in western North America during the Late Triassic. Most phylogenetic analyses have found them to be the most basal members of the pseudosuchian lineage (e.g. Juul, 1994; Benton, 2004), although this opinion may be changing slightly in the near future. Or may not, I don't know. Forget I said anything.
Phytosaurs were the most common aquatic predators, and had a body form, and probably a lifestyle, similar to modern crocodilians (Hunt, 1989). They also got quite large, possibly reaching ten meters in length, although most were in the size range of big modern crocodilians. Phytosaurs are sometimes described as “crocodile-mimics”, although this is crap because they developed the crocodilian body form and lifestyle before crocodilians, and in fact so did earlier and more basal Triassic archosauriforms: the proterosuchids, proterochampsids, and that hideous little freak Vancleavea.
However, the skulls of phyosaurs differ from those of crocodilians, and most other reptiles, in several ways. These include having the external nares pushed back on the snout (whereas they are at the tip of the snout in most reptiles). Moreover most, but not all forms (this is a hint) have features of the back of the skull which are very modified. In most diapsids, the supratemporal fenestrae, the upper of the two diapsid openings at the top of the head, are more or less level with the skull roof. In most phytosaurs however, they are reduced and pushed anteroventrally to varying degrees (another hint). Moreover some phytosaurs (but not all, hint) have a bar forming the lateral border of this opening, which is composed of the postorbital and squamosal bones, which is greatly broadened and sculptured; it is relatively slender and smooth-surfaced bone in some phytosaurs and most reptiles.
In fact, you can roughly line up North American phytosaurs according to how much they differ from the “normal” archosaur condition. Phytosaur alpha taxonomy is notoriously convoluted, but several morphotypes may be identified which fall out in this order in the major phylogenetic analyses of this group (Ballew, 1989; Hungerbeuhler, 2002; Stocker, 2008, in press). Most or all of these may actually be grades, so the genera names are applied loosely. Paleorhinus has nostrils which, although set back on the snout, are anterior to the antorbital fenestra, a slender and smooth postorbitosquamosal bar, and supratemporal fenestrae which are level with the skull roof. Angistorhinus, retains all of these basal features, except that the nostrils are pushed up over the antorbital fenestrae, as they are in all other phytosaurs. Leptosuchus (sometimes called Rutiodon) still has a slender postorbitosquamosal bar, but the supratempotal fenestra are pushed anteroverntrally and more constricted than in Paleorhinus and Angistorhinus. This trend continues in Pseudopalatus and Redondasaurus, and in the latter, the fenestrae are actually completely closed up. Finally, both Pseudopalatus and Redondasaurus have broad and heavily sculptured postorbitosquamosal bars. Click the image to enlarge:
So…how does these different forms fall out stratigraphically? To answer this, we have to look at areas where different forms are known to occur in the same area, so that their stratigraphic relationships can be determined. The Dockum Group of West Texas and eastern New Mexico (see Lehman, 1994; Lucas, 1994; and Martz, 2008 for a lots of really exciting arguments about stratigraphic nomenclature) has all five morphotypes, and the Chinle Formation in the Colorado Plateau region (e.g. Stewart et al., 1972; Dubiel, 1994) has Leptosuchus, Pseudopalatus, and Redondasaurus.
In the Dockum Group, the first Paleorhinus specimens occur in conglomeratic sandstones at the base of the Upper Triassic section (variously called the Santa Rosa Sandstone, Camp Springs conglomerate, and Boren Ranch beds), and continue into the lowest mudstone-dominated units of the Dockum Group overlying these conglomerates (the Tecovas Formation and Garita Creek Formation). These lower mudstones are also where we get the first specimens of Angistorhinus and Leptosuchus. Pseudopalatus appears higher in the section, in sandy units in the middle of the Dockum Group and the upper mudstone-dominated units (in the Trujillo sandstone and Bull Canyon/upper Cooper Canyon Formation), and Redondasaurus appears in the uppermost beds of the Dockum Group (the uppermost Cooper Canyon Formation, and Redonda Formation) (e.g. Hunt and Lucas, 1991; Lucas, 1998; Lehman and Chatterjee, 2005; Martz, 2008).
In the Chinle Formation of northern Arizona, we do not have Angistorhinus, but the other four forms are present. And yes, Paleorhinus and Leptosuchus appear first (from the Placerias Quarry, which may be in either the Bluewater Creek Formation or Blue Mesa Member), Leptosuchus continues higher in the section (in the Blue Mesa and Sonsela Member), Pseudopalatus appears next (in the upper Sonsela Member, Petrified Forest Member, and Owl Rock Member), and Redondasaurus last (in the Rock Point Member at the very top of the Chinle Formation) (e.g. Lucas, 1998; Heckert and Lucas, 2002; Parker, 2006). How about that?
Postscript on lithostratigraphy: In the last strat column figure, "Cooper Canyon Formation" should really be "Bull Canyon Formation." These units have been considered equivalant by most workers, but one of the findings of my dissertation is that the lower part of the type section of the Cooper Canyon Formation is probably correlative with the Tecovas and Trujillo Formations. This works out fine, since the lower parts of the Cooper Canyon Formation in the type area have Paleorhinus and then Leptosuchus showing up before Pseudopalatus as expected, but I didn't want to delve into the nomenclatural mess.
pps. Thanks to Sterling for reminding me of the Placerias Quarry "Paleorhinus" skull.
REFERENCES
Ballew, K.L. 1989. A phylogenetic analysis of Phytosauria from the late Triassic of the western United States. In Lucas, S.G., and Hunt, A.P. (eds.) Dawn of the Age of Dinosaurs in the American Southwest, pp. 309-339. New Mexico Museum of Natural History, Albuquerque, NM.
Benton, M.J. 2004. Origin and relationships of Dinosauria. In D.B. Weishampel, P. Dodson, and H. Osmólska (eds.), The Dinosauria (2nd edition), pp. 7-19. University of California Press, Berkeley, CA.
Dubiel, R.F. 1994. Triassic deposystems, paleogeography, and paleoclimate of the Western Interior. In Caputo, M.V., J.A. Peterson, and K.J. Franczyk (eds.), Mesozoic Systems of the Rocky Mountain Region, USA, pp. 133-168.
Heckert, A.B., and Lucas, S.G. 2002b. Revised Upper Triassic stratigraphy of the Petrified Forest National Park. In A.B. Heckert and S.G. Lucas (eds.), Upper Triassic Stratigraphy and Paleontology. New Mexico Museum of Natural History and Science Bulletin 21, pp. 1-36. Albuquerque, NM.
Hungerbühler, A. 2002b. The late Triassic phytosaur Mystriosuchus westphali, with a revision of the genus. Palaeontology, vol. 45, pt. 2, pp. 377-418.
Hunt, A.P. 1989. Cranial morphology and ecolog y among phytosaurs. In S.G. Lucas, and A.P. Hunt (eds.), Dawn of the Age of Dinosaurs in the American Southwest, pp. 349-354. New Mexico Museum of Natural History, Albuquerque, NM.
Hunt, A.P., and Lucas, S.G. 1991. The Paleorhinus biochron and the correlation of the non-marine Upper Triassic of Pangaea. Palaeontology, vol. 34, pt. 2, pp. 487-501.
Juul, L. 1994. The phylogeny of basal archosaurs. Palaeontologie Afrique, vol. 31, pp. 1-38.
Lehman, T.M. 1994a. The saga of the Dockum Group and the case of the Texas/New Mexico boundary fault. New Mexico Bureau of Mines and Mineral Resources Bulletin, no.150 (April), pp. 37-51.
Lehman, T. and S. Chatterjee. 2005. The depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas. Indian Journal of Earth System Sciences, vol. 114, no. 3, pp. 325-351.
Lucas, S.G. 1998. Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeography, Palaeoclimatology, Palaeoecology vol. 143, pp. 347-384.
Lucas, S.G., Anderson, O.J., and Hunt, A.P. 1994. Triassic stratigraphy and correlations, southern High Plains of New Mexico-Texas. New Mexico Bureau of Mines and Mineral Resources Bulletin, no. 150, pp. 105-126.
Stewart, J.H., Poole, F.G., and Wilson, R.F. 1972b. Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region. Geological Survey Professional Paper, no. 690, 336 pp.
Stocker, M.R. 2008. The relationships of the phytosaur Leptosuchus Cape 1922 with decriptions of new material from Petrified Forest National Park, Arizona. Unpublished master’s thesis, University of Iowa, IA, 220 pp.
The area is also fucking DESERTED. We up at the head of the canyon, pretty far from the shores of Lake Powell, and as a result, nobody wants to camp there. We saw no virtually one the entire time we were there except for some ranchers tending some suicidally depressed cows, and were able to stock up on huge piles of firewood pretty much every day with no problem.
Jim and Andrew were mostly interested in the Monitor Butte Member of the 
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