A Paul Sereno YouTube video (2011) on the discovery of Eodromaeus talks about the search for the competitive advantage that basal dinosaurs had that enabled them to eclipse their Triassic competitors and survive into the Jurassic and rise to prominence throughout the rest of the Mesozoic.
that search has not yet produced any distinct and widely agreed upon answers. Dr. Sereno suggests it might have been, “a lucky break” for dinosaurs.
It’s not an upright/bipedal configuration
Basal crocs, some poposaurs, some rauisuchians, Lagerpeton + Tropidosuchus, and maybe Arizonasaurus were all bipedal, but they did not survive into the Jurassic. Pterosaurs were derived from Triassic bipeds, but that’s another story.
It might be feathers
Feathers used to define birds. Now they define dinosaurs with both small theropods and small phytodinosaurs having them. Big derived dinosaurs had scales, but in birds scales on the feet are former feathers. Phylogenetic bracketing indicates that certain types of feathers go back to basalmost dinosaurs, like Herrerasaurus and/or its ancestors among the protodinosaurs, like Lewisuchus and Trialestes. No evidence of soft tissue is known for these taxa.
Our only clue to basal croc widespread dermal protection
comes from a patch of tiny scales on the back of Scleromochlus. Scales like these don’t ultimately spread across the body in scaly crocs, like the extant Caiman. Osteoderms form along the spine in rauisuchians and are retained in certain basal crocs like Erpetosuchus, and Gracilisuchus. These do spread across the body in Simosuchus and extant crocs.
Evidently spinal osteoderms are not found
in protodinosaurs like Pseudhesperosuchus and kin. We don’t know if they were scaly or naked.
Secondary sexual traits
don’t usually provide the keys to survival in terms of weather, viruses and other external factors. So feathers as secondary sexual traits probably do not give dinos the competitive advantage vs. non-conspecific competitors, like basal crocs, rauisuchians, etc.
Feathers as insulation
A dense coat of feathers insulates the naked skin of dinosaurs (including extant birds) from the elements, including air temperature, ultraviolet light, and damaging obstacles in the environment, including, at times, predators.
Pycnofibers as insulation
By convergence, that ‘other story,’ the fenestrasaurs (including pterosaurs) likewise had a coat of insulating fibers (pycnofibers). And pterosaurs, like dinosaurs, survived into the Jurassic and beyond.
Hair/Fur as insulation
By convergence, hairy/furry mammals survived into the Jurassic and beyond.
Does this tell us something?
It’s just a clue, but a 3x winner is a pretty good clue IMHO.
On the flip side
uninsulated lepidosaurs, turtles, crocs, choristoderes and enaliosaurs also survived into the Jurassic and beyond. Am I forgetting any other large clades? Some of these were aquatic. Others had a protective carapace. Still others could have found refuge in leaf litter. Was there something in the air, like radiation, that killed uninsulated mid-sized terrestrial reptiles leaving smaller and aquatic taxa alone? Or was it just a ‘lucky break’? More on this topic follows:
On the same note, a recent paper
by Benton, Forth and Langer (2014) notes that dinosaurs arose in the Middle Triassic (see PVL 4597, Herrerasaurus and Lewisuchus). They mention Nyasaurus and Asilisaurus, but those are poposaurs, not as close relatives to dinosaurs as are the crocs in the large reptile tree.
Benton, Forth and Langer report, “Tracking the forebears of crocodiles and birds back in time points to a common ancestor in the Early Triassic, and close relatives in the latest Permian, represented by Archosaurus from Russia.” That’s not true according to the large reptile tree which puts the last common ancestor near the Gracilsuchus/PVL 4597 split in the Middle Triassic.
More untested paradigms from Benton, Forth and Langer
who also report, “Within Archosauria, the bird line, Avemetatarsalia (Box 1), includes two subclades, Pterosauria (the flying reptiles) and Dinosauromorpha.” Pterosaurs are lepidosaurs in the large reptile tree, which tests all these relationships.
Then they dig themselves deeper
“All avemetatarsalians have elongate hindlimbs (suggesting bipedal posture), elongate tibiae (suggesting adaptations to fast running), and three or four slender, elon- gate metatarsals in a tightly bound bundle, so these animals all stood high on their tip-toes (digitigrade posture).” So do basal crocs, which they seem to be forgetting.
“Dinosauromorphs had all these characters, as well as further elongation of the metatarsals and reduction of the fifth toe to a short single element.” So do basal crocs, which they seem to be forgetting.
“Dinosauriformes added to these specializations of the hindlimb some further modification of the pelvis and femur for speedy and efficient movement on two legs.” So do basal crocs, which they seem to be forgetting. But I think they are talking about Lagerpeton, which achieved bipedal configuration by convergence in the large reptile tree.
“Among these, the astragalus, the main ankle bone, sends a thin plate of bone up the front of the tibia, so linking the ankle firmly to the shin as a single functional unit. Many of these characters were once seen as exclusive to Dinosauria, but they are now known to exist in larger clades.” Not so fast… the astragalus sends up a thin plate of bone up the -back- of the tibia in Lagerpeton.
“Dinosaurs are characterized by some skull features, an elongate deltopectoral crest on the humerus (a major muscle attachment of the forearm), and an expanded articulation for the tibia on the astragalus.” None of these appear to be key to their survival. The large reptile tree finds another suite of traits for dinosaurs, none of which are exclusive and few of which are retained in all derived forms.
Getting back to competitive advantages
Benton, Forth and Langer (2014) note: “What were the characters that enabled dinosaurs, and indeed archosaurs more widely, to profit from these ecological crises? Two key attributes are their exceptional growth rates and efficient respiration systems. Recent work on dinosaurs shows… rates of growth in line with modern mammals rather than modern reptiles. Dinosaurs almost certainly possessed the unidirectional respiratory system of birds, and apparently crocodiles, which is more efficient than the tidal system in mammals, and this might have characterized all archosaurs.” In the large reptile tree “all archosaurs” includes only crocs and dinos, but high growth rates and efficient respiration systems are both competitive advantages both for quickly producing lots of little dinosaurs and running away from predators without tiring.
To their credit,
Benton, Forth and Langer (2014) note: “Dinosaurian thermoregulation is somewhat speculative, but the majority of evidence now supports a high metabolic rate, especially in the small- and medium-sized feathered dinosaurs. Considering that these include members of both the saurischian  and ornithischian  branches, phyloge- netic bracketing implies that the first dinosaurs might have had a high thermal inertia , given the insulation provided by the coverage of filamentous integumentary structures (Figure 2b), as well as fast growth and avian-like breathing.”
And so we come full circle,
in complete agreement. It’s not upright posture. It’s not great size. It’s not a bipedal configuration. It just might be feathery insulation (on top of upright posture, bipedal configuration and rapid growth) and everything else that comes with that suite of traits.
bigger, more derived, more robust dinos did not need feathers, and so those shrank to become dino scales, distinct from lepidosaur, turtle and croc scales.
Benton MJ, Forth J and Langer MC 2014. Models for the Rise of the Dinosaurs. Current Biology 24, R87–R95, online pdf