Dinosaurs!

   O.K., now we get into the fun stuff.   How this is going to work is I will have a sort of "cladogram" (it's not a real cladogram, wherein the characters pertaining to each taxon are run through computer models to test taxon relationships, but more a sort of composite of well-regarded trees), with various clades, some of which I'll expand on on different pages.  Any dinosaurs falling out at a given clade will be profiled below the cladogram (you'll be able to tell the dinosaurs from the classification level names because the dinosaur names will be in italics).  Further navigation can be done with the table at the bottom of the page.
    I'd like to take a moment and briefly (for now; I hope to expand later) place dinosaurs among the reptiles.  Dinosaurs, including birds, are members of Archosauria, a group of reptiles also including the crocodilians.  This group in turn is part of Diapsida, which also includes the lepidosauromorphs, which include the lizards, snakes, and mosasaurs, a group of marine reptiles from the upper Cretaceous usually allied with the monitor lizards, but sometimes considered closer to snakes.  Diapsids are known by their two temporal fenestrae, among other things.  Diapsids together with Anapsida [turtles] are the main members of Reptilia.  
    Other famous prehistoric reptiles (and "reptiles") include the pterosaurs, ichthyosaurs, plesiosaurs and allies, "thecodonts," and things like Dimetrodon.  Pterosaurs were recently thought to be very close to Dinosauria, but new study indicates that they are farther away, possibly not even archosaurians.  Ichthyosaurs, famous for their close resemblance to dolphins, have an uncertain placement, due to the general lack of good remains from very basal members.  In the past, they were sometimes classified with the plesiosaurs and their allies in Euryapsida (again due to temporal fenestrae), a group which turned out to be unnatural.  The plesiosaurs and their allies, including the nothosaurs (which looked broadly like less marine plesiosaurs) and placodonts (unusual, heavily-armored forms with robust teeth for crushing shells) are classified in the Sauropterygia, which likely is a subgroup of Diapsida.  "Thecodontia" is an old name for an assemblage of of archosaurs that were linked by their not being anything else; the ancestors of the derived archosaurian lineages were often considered simply archosaurs, for example the "lagosuchids."  Animals like Dimetrodon (which greatly predated the dinosaurs, but still finds its way into "dinosaur" toy sets; it must be in its contract) are not true reptiles, but closer to mammals, hence the old phrase "mammal-like reptiles."  
    Eventually, I'd like to take the information in the preceding two paragraphs and put it on a new page in a much expanded form, for one because it's kind of unfair to just throw the dinosaurs out without giving a sense of their background, and second because the other animals (I find pterosaurs and plesiosaurs particularly interesting) are worthy of it in their own right.  I hope I haven't been too confusing in this short introduction.
    Meanwhile...  

<--Dinosauromorpha
      |--"Lagerpetonidae"
      |    |--Dromomeron
      |    `--Lagerpeton
      `--Dinosauriformes
           |--Marasuchus
           `--+--"Silesaurs"
                |     |--Eucoelophysis
                |     `--Silesaurus
                `-- Dinosauria
                      |--Saurischia
                      |   |--Herrerasauridae
                      |   |   |--Herrerasaurus
                      |   |   `--Staurikosaurus
                      |   `--+--Eoraptor
                      |        `--Eusaurischia                  
                      |             |-->Theropoda
                      |             `-->Sauropodomorpha
                      `-->Ornithischia (aka Predentata)
                
Poor Remains That May Come From Dinosaurs\Dinosauromorphs, But For Which The Evidence Is Equivocal At This Time: You may remember the spurt of "Triassic tooth ornithischians" in the 1990s and early 2000s.  The flagship genus was Revueltosaurus, which ungratefully turned out to be a pseudosuchian after the skull was found (and a rather odd one to boot, related to the armored, herbivorous aetosaurs).  A pseudosuchian, despite its name ("false croc"), is actually related to the base of the croc lineage.  Since then, Crosbysaurus, Lucianosaurus, Protecovasaurus, and Tecovasaurus have been shown to be probably valid, but also only classifiable to Archosauriformes i.s. (so pretty far from dinosaurs), and probably not dinosaurs (they lack a cingulum on the teeth, a sort of ridge around the base of the crown), while Pekinosaurus and Galtonia are regarded as Revueltosaurus sp., and the newly-minted Krzyzanowskisaurus hunti, a former species of Revueltosaurus with an imitation cingulum, now regarded as possibly pertaining to Revueltosaurus (again).  Azendohsaurus, first thought to be a "fabrosaurid", then a prosauropod, is also probably not dinosaurian.

Dinosauromorpha i.s.: This level is just before Dinosauria, and is primitive in several ways.  For example, the animals here still have armor plating down the midline of their backs (Ceratosaurus either retains this or converges [has a similar character evolved from different processes]), sacra with less than three vertebrae contributing, and very underdeveloped pelvic bones compared to true dinosaurs (the pubis of Marasuchus is very short, and the ilium is still small).    

"Lagerpetonidae":  

Dinosauriformes: I'm just throwing this term in for the sake of completeness and accuracy, but it doesn't really add much to what we're doing here, given how few animals are known at the base of the dinosaur lineage.

"Silesaurs":

"Silesaurs" i.s.:

Dinosauria i.s.: Dinosaurians (er, uh, dinosaurs) are set apart ancestrally by a number of characteristics, some the most recognizable including a femur with a fully-offset head and distinct neck and ball, a hip with the acetabulum largely or fully opened (this is later changed in the ankylosaurians, where the acetabulum is secondarily rewalled for strength), a fully backward facing shoulder socket, and a well-developed ascending process of the astragalus (note: dinosaurs, compared to a lot of animals, had very simple ankles and wrists, often reducing the legions of little bones humans find so easy to injure in their wrists and ankles) (these characters taken from The Dinosauria, Weishampel, Dodson, and Osmolska, 1990).  It should be noted that many workers are coming to see some of these characters as convergences among the dinosaurs due to acquisition of bipedal posture.  For example, there is some question about the nature of the ancestral dinosaur acetabulum.
    Many dinosaurs had extensions of the spines of the verts that gave them "fin" backs.  Unlike the fin of the famous non-dinosaur Dimetrodon, dinosaur fins were most likely thick, as seen by the differing forms; dinosaur spines are wide for an aft, unlike the narrow spines of Dimetrodon.  They could have used these "fins" for display, thermoregulation, muscle attachment, fat storage, or other functions. 
    The taxa listed here are mostly very dubious, to the point where they cannot be assigned to a higher level.  Sometimes this is due to poor material, and sometimes this is due to the fact the material does not belong to one kind of dinosaur, but to at least two different types, creating what is called a chimera (or chimaera, if your taste runs to additional "a"s).  Some of the chimerical animals listed, like Sanpasaurus, could become better classified if the remains were to be officially limited to one or the other animals present in the remains, but others, like Sinosaurus, probably are doomed to an eternity in the paleontological wastebasket.
    The earliest known dinosaur remains may belong to a herrerasaurid; a pubic boot from the Anisian-age (MTr) portion of the Moenkopi Formation in Arizona appears to have come from such a beast.  Based on what we know about basal dinosaurs, they probably first radiated in the Middle Triassic, so this wouldn't be surprising.

Saurischia and Ornithischia:  Besides some basal forms, dinosaurs are traditionally divided into two groups based on hip anatomy: the saurischians, which had pelves with the pubis jutting away from the ischium; and the ornithischians, which had pelves with the pubis paralleling the ischium (like in birds).  The saurischians include the theropods and sauropodomorphs, while the ornithischians, also known as predentatans (Predentata is a much better name for Ornithischia, because it describes the predentary at the front of the jaw that "ornithischians" and only "ornithischians" have among dinosaurs, while dinosaurs beside "ornithischians", including dromaeosaurids, therizinosaurians, and, rather anticlimactically, birds, possess an analogue of the "bird-hip" for which Ornithischia is named) include a wide variety of beaked herbivores.  Occasionally, someone has suggested that the sauropodomorphs and the ornithischians go together (Phytodinosauria), but this appealing idea has little evidence for it.

Herrerasauridae: The herrerasaurids have bounced around that space between basal Theropoda and just outside of Dinosauria for years now.  The most stable thing we can say is that they appear to be basal saurischians.  For a while, many early theropods were dumped off here, but the only certain member is its namesake.  One of the unusual aspects of this group is the very birdlike elements combined with such primitive features as a four-fingered hand.  Another odd character is the backshifting of the pubis, similar to, but not to the same extent, as that seen in ornithischians, dromaeosaurids, therizinosaurians, and birds.

Saurischia:

Saurischia i.s.:

Eusaurischia i.s.: