Hot on the heels of the rediscovery of Nyasasaurus (did it have heels?), a new book by Fastovsky and Weishampel (2012) Dinosaurs: A Concise Natural History (2nd Ed) discusses the origin of dinosaurs and many other dino topics. That publication inspired this post.
Earlier we looked at various competing hypotheses for the origin of the dinosaurs, closer to their departure from other reptiles, and why pterosaurs and phytosaurs have no business in the lineage of dinosaurs. Earlier we also looked at the base of the Archosauria and the Origin of the Dinosauria. Here we’re going to take this story way, way back and cover more bases.
We’re going to take dinosaur origins back to Ichthyostega, an early tetrapod. Why? Because we can! We have the only large reptile tree that documents every evolutionary step it took to create dinosaurs from their ancestral pollywogs in the Devonian. And we’ll met several dozen very interesting taxa along the way. For more details I encourage you to click on the links that interest you. There are more details and images there. And please remember, these are NOT the direct ancestors, but the closest reps we have to that unbroken string of unknown parents, grandparents, great grandparents, etc. etc.
Icthythostega (still pretty fishy), Pederpes (down to only five toes), Proterogyrinus (nostrils move to the snout tip), Eldeceeon (long toes), Seymouria (more terrestrial but convergently off to the side), Utegenia (more aquatic and off to the froggy side), Silvanerpeton (closer to Eldeceeon, with larger eyes, longer fifth toe) and Gephyrostegus (ankle includes fused elements creating an astragalus, a premaxilla/maxilla notch is present, more gracile limbs and a trend toward a much smaller size, which makes amniotic eggs possible) brings us to the base of the Reptilia.
Cephalerpeton (lose of palatal fangs, fusion of the intertemporal, straight posterior squamosal and longer, more gracile limbs) nests at the base of the otherwise diphyletic Reptilia.
Westlothiana (a little too long waisted, but no tabular horns, mandible reduced to three major bones in lateral view, the premaxilla did not descend), nests at the base of the new Archosauromorpha, followed by Paleothyris (now that’s more like it, short-waisted, athletic, enlarged canines, longer rostrum, high coronoid process, ossified scapulocoracoid, more gracile limbs and toes), Brouffia (minor improvements), Coelostegus (maxilla deeper, supratemporals angled down, vertebrae taller), Hylonomus (jugal invades the squamosal, gastralia present), Protorothyris (larger skull and deeper canine) and Aerosaurus (reduced squamosal, first appearance of the lateral temporal fenestra, but getting too robust in the postcrania) take us to the base of the Synapsida.
Heleosaurus (not so robust, smaller skull, longer neck, pelvis larger than scapulocoracoids, longer limbs, reduced to absent posterior ribs), Milleropsis (finally a tail is preserved and it is whip-like, metacarpal 3 is longer than 4, possible biped), Eudibamus (longer neck, upper temporal fenestra gives us a diapsid configuration, definite biped with another attenuated tail, but the toes were way too asymmetrical), Spinoaequalis (taller tail spines link this taxon to water, ulna and radius are longer and straighter, the tibia and fibula have less interossial space) and Petrolacosaurus (shorter rostrum, larger orbit, longer neck, whip-like tail, tibia and fibula bowed apart) take us past the base of the Diapsida.
Acerosodontosaurus (a little too robust, with smaller cervicals and with forelimbs too long and robust,) and Adelosaurus (smaller and more gracile with shorter fingers and toes) lead toward the Enaoliosauria, a large clade of aquatic and marine reptiles.
Tangasaurus (neck getting way too long, but limbs large and subequal) nests at the base of the Tangasauria (the more terrestrial lineage) followed by Thadeosaurus (gracile palatal elements, sterna appear for a short time), Orovenator (large naris, descending premaxilla, gracile posterior skull elements, skull taller than wide, rostrum concave dorsally) and Youngina (BPI3859) (taller lateral temporal fenestra, maxilla taller, pubis and ischium separate elements) take us to the base of the Protorosauria, which diverge at this point.
Youngina (AMNH5661) (skull a little too flat and wide, heading toward pararchosauriformes) and Youngoides (UC1528) (smaller temporal fenestra, frontals without posterior process, pineal foramen tiny, quadrate leans anteriorly, possible antorbital fenestra) take us to the base of the Archosauriformes.
Now we come to more familiar territory, the base of the Euarchosauriformes, with Proterosuchus (first verified appearance of the antorbital fenestra, drooping premaxilla, mandibular fenestra, more robust taller cervicals, lower limb elements not bowed), Fugusuchus (larger more robust skull, fewer larger teeth), Garjainia (convex maxilla, naris rises on nose tip, shorter tail, deeper chest, deeper pelvis directed ventrally, more upright limbs), Euparkeria (smaller, more gracile, longer tail, higher naris, hooked fifth metatarsal), Ornithosuchus (a side branch experimenting with both convergent bipedality (deeper pelvis) and a deeper, narrower skull) and Vjushkovia (not so derived, taking its time evolving evidently, but with a high naris and pedal digit 3 longer than 4) take us to the base of the Rausuchia and a fish-eating side branch that includes the long-necked Yarasuchus and Ticinosuchus, which had lost its premaxillary teeth.
Now we hit the base of the Archosauria represented by Decuriasuchus (super slender forelimbs and pectoral girdle), Turfanosuchus (looks like a reptilian horse with a smaller skull, longer neck and a high carriage) and Gracilisuchus (at the base of the crocodylomorpha). Some early members were bipeds.
Lewisuchus is known from too few pieces, but might be just what we’re looking for in a basalmost dino. It likely had much longer hind limbs than forelimbs, considering the general proportions that are known. And it was much smaller than its closest kin. Such traits could lead toward crocs or dinos, but at this point the back of the skull suggests this taxon leads to a third bipedal lineage without many descendants.
Trialestes (had croc-like elongated carpals, a tridactyl pes, pelvis perforated, femoral head inturned, vertebral centra with excavated lateral surfaces, longer radius than humerus, but late Triassic) and probably Nyasasaurus (large deltopectoral crest and Early Middle Triassic) nest at the base of the Dinosauria, specifically the Theropoda. Panphagia and Pampadromaeus nest at the base of the Phytodinosauria (sauropodomorphs + the other herby dinos, enlargement of premaxillary teeth, longer neck). Pisanosaurus (shorter neck, blunter teeth) nests at the base of the Poposauridae (traditionally considered dinosaur-like rauisuchians due to the extended calcaneum, but here nesting with dinos). Massospondylus (more cervicals, shorter forelimb, two lateral fingers vestigial) nests at the base of the Sauropodomorpha. Daemonosaurus (reduced antorbital fenestra, longer premaxillary fangs, postnarial process of premaxilla longer, shorter jaw) nests at the base of the Ornithischia.
YouTube has a short video on the Origin of Dinosaurs that pretty much follows the traditional route.
Please remember that, despite tradition and textbooks, the Phytosauria, Proterochampsia and Pterosauria have nothing to do with dinosaur origins. The Proterochampsia include Dromomeron and Lagerpeton, as close relatives to Tropidosuchus, another convergent biped. A large reptile tree demonstrates this. And this experiment can be repeated if anyone cares to add a taxa or two.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.