Go back far enough in dinosaur ancestry and you come to: Heleosaurus

With our never-ending fascination with dinosaurs
it’s interesting to list some of the taxa in their deep, deep!, deep!! ancestry. One such ancestor is Heleosaurus (Fig. 1; Broom 1907; Middle Permian ~270 mya, ~30 cm snout to vent length), the first known basal prodiapsid, the clade the includes diapsids (sans lepidosaurs, which are unrelated but share the same skull topology by convergence).

Figure 1. Heleosaurus is closer to the main lineage of dinosaurs. It retained canine fangs.

Figure 1. Heleosaurus is close to the main lineage of dinosaurs. It retained canine fangs. Note the squamosal distinct from the quadratojugal, as in Nikkasaurus. Also note the continuing lacrimal contact with the naris, as in Protorothyris.

But first
I want to discuss a derived Heleosaurus cousin, Nikkasaurus (Ivahnenko 2000; Fig. 2), also one of the most basal prodiapsids.

It is only by coincidence
that Ivahnenko labeled Nikkasaurus one of his ‘Dinomorpha,’ a clade name ignored by other authors. Wikipedia considers Nikkasaurus one of the Therapsida and possibly a relic of a more ancient stage of therapsid development. Like Heleosaurus, Nikkasaurus had a single synapsid-like lateral temporal fenestra. Only their nesting outside of that clade and basal to the clade Diapsida in the LRT tell us what they really are. Most of the time, as you know, we can tell what a taxon is simply by looking at it. In this case, as in only a few others, we cannot do so readily.

Figure 1. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Figure 2. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Nikkasaurus tatarinovi (Ivahnenko 2000) Middle Permian was a tiny basal prodiapsid with a large orbit. It retained a large quadratojugal. The fossil is missing the squamosal. Others mistakenly considered the quadratojugal the squamosal, as in therapsids. That’s an easy mistake to make. Compare this bone to the QJ in Heleosaurus (Fig. 1), another prodiapsid. Nikkasaurus has small sharp teeth and no canine fang. Nikkasaurus is a sister to Mycterosaurus. They both share a large orbit and fairly long snout. What appears to be a retroarticular process may be something else awaiting inspection in the actual fossil. Based on all other data points, I don’t trust that post-dentary data. It doesn’t match the in situ figure.

Distinct from other prodiapsids,
the Nikkasaurus, Mycerosaurus and Mesenosaurus maxilla extended dorsally, overlapping the lacrimal and contacting the nasal, as it does in Dimetrodon and basal therapsids like Hipposaurus and Stenocybus. This trait tends to be homoplastic / convergent in all derived taxa, but the timing differs in separate clades.

Figure 1. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it.

Figure 2. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it. Note the posterior mandible mismatch in the purported retroarticular process. I suspect the process is not there.

And finally we come back to Heleosaurus.
Slightly closer to the lineage of dinosaurs is the slightly more basal prodiapsid, Heleosaurus (Fig. 2), which retained canine fangs, had a more typical posterior mandible and retained a lacrimal / naris contact. This naris trait was retained by Petrolacosaurus, Eudibamus, Spinoaequalis and other basal diapsids (archosauromorpha with both upper and lateral temporal fenestra ). The maxilla did not rise again to cut off lacrimal contact with the naris in the ancestry of dinosaurs until the small Youngina specimens huddled together, SAM K 7710 and every more derived taxon thereafter, up to and including dinos.

References
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Ivahnenko MF 2000. 
Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe. Paleontological Journal 42(9):859-995. DOI: 10.1134/S0031030108090013

Lagerpeton: not the first of its kind, but the last of its kind

Quick note
I updated the reconstruction and nesting of Colobomycter, which you can see here.

Traditional paleontologists
consider Lagerpeton (Fig. 1, Romer 1971) a basal dinosauromorph, thus the first of its kind (ancestral to dinosaurs).

In contrast
Lagerpeton nests as a terminal taxon in the large reptile tree, leaving no known descendants. Here (Fig. 1) convergent evolution has created a bipedal chanaresuchid, derived from Tropidosuchus that has similar pedal proportions to the second specimen attributed to Tropidosuchus.

Figure 3. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

Figure 1. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

According to
Wikipedia, seven fossil specimens have so far been attributed to L. chanarensis. They don’t add up to much more than a hind limb and pelvic girdle.

  1. UPLR 06 (holotype) – articulated right hindlimb
  2. PVL 4619 – articulated pelvis with sacrum, partial right and complete left hindlimbs
  3. PVL 4625 – left pelvis with left femur and articulated vertebral column (dorsal, sacral and anterior caudal vertebrae
  4. PVL 5000 – proximal end of left femur
  5. MCZ 4121 – complete left, and partial right, femur.

Brusatte et al.
found Early Triassic footprints they attributed to lagosuchids. In reality the ichnites were closer to Rotodactylus tracks, which match the feet of fenestrasaurs, like Cosesaurus through pterosaurs.

In the large reptile tree
archosauriformes split at their origin, shortly after Youngina (AMNH 5561) and Youngoides (UC 1528) into two clades. The larger specimens start with Proterosuchus and radiate into choristoderes, parasuchians, doswellians and chanaresuchians terminating with Lagerpeton and its sister, Tropidosuchus (Fig. 1). The other branch starts with Euparkeria and extends to crocs, dinos and birds.

So,
Lagerpeton is not a close relative of dinosaurs, but convergent in several regards. The odd feet and pelves give them away as distinctly different from dinosaurs. Even so paleontologists continue clinging to this hypothesis. Better dino ancestors can be found here.

References
Arcucci A 1986. New materials and reinterpretation of Lagerpeton chanarensis Romer (Thecodontia, Lagerpetonidae nov.) from the Middle Triassic of La Rioja, Argentina. Ameghiniana 23(3-4):233-242. online pdf
Brusatte SL, Niedźwiedzki G, Butler RJ 2011. “Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic.”Proceedings of the Royal Society B: Biological Sciences 278 (1708): 1107–1113. doi:10.1098/rspb.2010.1746PMC 3049033PMID 20926435.
Romer AS 1971 The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria, n. 378, p. 1-10.
Sereno PC and Arcucci AB 1993. Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology, 13, 385–399.

wiki/Lagerpeton

News on the Origin of Dinosaurs: Megapnosaurus and Zupaysaurus

Coelophysis bauri and the former Syntarsus rhodesiensis
(= Coelophysis rhodesiensis, Coelophysis kayentakatae, Megapnosaurus kayentakatae)
 have been considered congeneric. Unfortunately, the large reptile tree (subset Fig. 1) did not recover that relationship based on data below.

Figure 6. Proto-dinosaurs including Zupaysaurus and Megapnosaurus.

Figure 1. Proto-dinosaurs including Zupaysaurus and Megapnosaurus. They do not nest with the theropod dinosaur Coelophysis.

According to Wikipedia
on the topic of its name, “It was formerly called Syntarsus, but that name was already taken by a beetle, and was subsequently given the name Megapnosaurus by Ivie, Ślipiński & Węgrzynowicz, in 2001, though many subsequent studies have classified it in the genus Coelophysis.”

According to Wikipedia
on the topic of Coelophysis rhodesiensis, “Syntarsus” rhodesiensis was first described by Raath (1969) and assigned to Podokesauridae.[6] The taxon “Podokesauridae”, was abandoned since its type specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

Figure 1. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here. 

Figure 2. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here.

Colbert (1989) illustrated the two palates together
of Coelophysis and Megapnosaurus (Fig. 2) and they sure do not look alike. I wondered about this discrepancy in two supposedly close sister taxa. I finally found a solution when I added Megapnosaurus and Zupaysaurus to the large reptile tree. They both nested between the basal proto-dinosaur Lewisuchus and the clade of basal pro to-dinosaurs that includes Pseudhesperosuchus.

Note in the palates gradual evolution in
the coming together and fusion of the vomers, the development of the anterior and posterior embayments of the palatine, the enlargement of the cheek articulation of the ectopterygoid and the anterior angle of the transverse process of the pterygoid, among other evolutionary advances here, along with individual variations.

Megapnosaurus_skull588

Figure 3. Megapnosaurus kayentakatae does not nest with Coelophysis, but with Zupaysaurus, between Lewisuchus and the proto-dinosaurs in the Pseudhesperosuchus clade.

Megapnosaurus rhodesiensis (kayentakatae) (Raath 1969, renamed by Ivie, Ślipiński & Węgrzynowicz, in 2001, Fig. 3, early Jurassic, 3m length) 30 individuals were found in a fossil bed. According to Wikipedia, “Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

This is what happens with taxon exclusion.

Figure 3. Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Figure 4 Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Zupaysaurus rougirez (Arcucci and Coria 2003, Latest Triassic to Earliest Jurassic,  PULR-076, up to 4m long, 45cm skull length, Fig. 4) nests as a late-surviving pre-dinosaur (dinosauromorph) with Megapnosaurus between Lewisuchus and the Pseudhesperosuchus clade. Everyone else considers it a theropod, but my guess is they have not tested against these candidate taxa. Traditional paleontologists are still stuck on Lagerpeton. Here are the verified dinosaur precursors to scale (Fig. 5).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 5. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

And here’s the skull of Coelophysis for comparison (Fig. 6).

Figure 4. Coelophysis skull for comparison.

Figure 6. Coelophysis skull for comparison.

Just found this, so this was added to the post less than 24 hours later.
Down 2000 compared Coelophysis bauri with Syntarsus rhodesiensis and concluded that both were “remarkably similar morphologically.” He reported, “Although I have not been able to personally study fossils of Syntarsus rhodesiensis, I am confidant that, except for possible mis-articulation of the skull roof, the drawings prepared for Raath’s dissertation accurately represent this taxon. Raath’s illustrations, in almost every case, depict the Ghost Ranch theropod Coelophysis bauri more exactly than do the drawings and descriptions in Colbert’s (1989) monograph (which I glean data from). C. bauri and S. rhodesiensis…differ only in minor details such as neck length, proximal and distal hind limb proportions and proximal caudal anatomy. These differences do not justify a generic separation. The genus Coelophysis has priority over the genus Syntarsus.”

Take a look for yourself
at figures 3 and 6. They appear to be distinct to my eye. What do you think? Note that Downs does not mention the distinct palate. I’d like to see more postcranial data on Megapnosaurus. It’s out there. If you have it, send it. A shorter neck than in Coelophysis is to be expected if Megapnosaurus is a sister to Lewisuchus. There is no indication that Downs compared his taxa to Lewisuchus or the Pseudhesperosuchus clade, nor is there any indication of phylogenetic analysis.

References
Arcucci AB and Rodolfo AC 2003. A new Triassic carnivorous dinosaur from Argentina. Ameghiniana 40(2):217-228.
Cope ED 1889. On a new genus of Triassic Dinosauria. American Naturalist 23: 626
Late Triassic Norian.
Colbert E 1989. The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57: 160.
Downs A 2000. Coelophysis bauri and Syntarsus rhodesiensis compared, with comments on the preparation and preservation of fossils from the Ghost Ranch Coelophysis quarry. In:Lucas, S.G.; Heckert, A.B. (eds.). “Dinosaurs of New Mexico”. New Mexico Museum of Natural History Bulletin 17: 33–37.
Raath  MA 1969. A new Coelurosaurian dinosaur from the Forest Sandstone of Rhodesia. Arnoldia Rhodesia. 4 (28): 1-25.
Raath MA 1977. The Anatomy of the Triassic Theropod Syntarsus rhodesiensis (Saurischia: Podokesauridae) and a Consideration of Its Biology. Department of Zoology and Entomology, Rhodes University, Salisbury, Rhodesia 1-233.
Rowe T 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology. 9, 125-136.

wiki/Zupaysaurus
wiki/Coelophysis_rhodesiensis

SVP 23 – a new archosaur close to Junggarsuchus

Sullivan et al. 2015
describe a sister of Junggarsuchus sloani (Fig. 1) which is a basal archosaur and a basal dinosauriform, not far from crocs and a sister to Pseudhesperosuchus.

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

From the abstract
“The Middle-Late Jurassic Daohugou (or Yanliao) Biota and Early Cretaceous Jehol Biota are successive assemblages from northeast China. Nno crocodylomorph has been reported from either assemblage. However, a basal (i.e., non-crocodyliform) crocodylomorph was recently recovered from the Daohugou Biota site of Mutoudeng, Hebei Province. The meter-long skeleton shows crocodylomorph synapomorphies, including elongated carpals, and lacks crocodyliform traits such as an expanded coracoid process. The specimen shares key features with the Chinese basal crocodylomorph Junggarsuchus sloani, including vertebral hypapophyses and a slender, rotated metacarpal I. However, the Mutoudeng specimen differs from J. sloani in having shorter distal forelimb segments, and in lacking enlarged anterior maxillary teeth. Phylogenetic analysis recovers this specimen and J. sloani as sister taxa slightly outside Crocodyliformes.

J. sloani is known from one individual lacking the hindquarters, but the Mutoudeng specimen preserves two surprising features in this region: the iliac preacetabular process is very long, and the middle and posterior parts of the tail are entirely sheathed in osteoderms. Such extensive caudal armor is common in basal crocodyliforms but rare in more basal crocodylomorphs. The elongated preacetabular process likely gave parts of the iliotibialis and puboischiofemoralis internus musculature a near-horizontal orientation, allowing them to act as strong femoral protractors. This arguably supports prevailing views of basal crocodylomorphs as terrestrial cursors, but soft tissues preserved with the skeleton unexpectedly challenge the conventional wisdom. Scaly skin is associated with the superimposed feet, and with the right hand, forming sheets of small (< 1 mm) polygons defined by dark lines. In the right hand, patches of skin are visible between digits IV and V, extending distally to the second phalanges of the two digits. The presence of skin in this area implies the hand was webbed, which in turn suggests the Mutoudeng crocodylomorph frequented wet environments and was probably at least semi-aquatic.”

Just a few clues here
as to what will be presented, but more taxa in the ancestry of dinosaurs, or crocs, even if they are not recognized as such by Sullivan et al., are always welcome.

References
Sullivan C et al. 2015. A new basal crocodylomorph with unexpected skeletal and soft-tisse features from the Middle Late Jurassic Daohugou biota of Northeast China. Journal of Vertebrate Paleontology abstracts.

Barberenasuchus… now the basalmost phytodinosaur

Updated Feb 8, 2016 with a new nesting for this taxon, away from Herrerasaurus and towards a newly added taxon, Eodromaeus.

According to Wikipedia
“Barberenasuchus brasiliensis (Mattar 1987; Fig. 1) is an extinct genus of an archosauriform. Fossils (poorly preserved skull and axis vertebra) have been found from the Santa Maria Formation of southern Brazil of Late Triassic age. Its phylogenetic position within Archosauriformes is uncertain; the author of its description classified it as a sphenosuchid crocodylomorph, while Kischlat (2000) considered it to be a rauisuchian. Irmis, Nesbitt and Sues (2013) stated that they “could not find any crocodylomorph character states preserved in the holotype specimen”. Based on the presence of an antorbital fenestra the [latter] authors assigned Barberenasuchus to Archosauriformes, but stated that without further preparation and study it is not possible to assign it to any specific archosauriform group.”

Figure 1. Barberenasuchus, the only data I was able to find, a small image in Riff et al. 2012.

Figure 1. Barberenasuchus, the only data I was able to find, a small image in Riff et al. 2012.

In phylogenetic analysis
the line drawing (from Riff et al. 2012)  nests Barberenasuchus with the protodinosaurs Junggarsuchus, Pseudhesperosuchus and Carnufex in the large reptile tree. Tradtiional paleontologists consider these sphenosuchid basal crocs. So did Riff et al.

It’s pretty obvious at first glance, isn’t it? The odd sort of postorbital that rises in a dorsal arc lateral to a crested parietal is a dead giveaway for this clade. No other reptiles have it. Compare this drawing of Barberenasuchus to Pseudhesperosuchus here.

Unfortunately, 
the line drawing does not do justice to a jpeg of the fossil itself (Fig.2, courtesy of D. Riff). That ‘odd sort of’ postorbital turns out to be not so odd. There is no parietal crest.

Figure 1. Barberenasuchus traced using DGS color over the bones, then transferred and shifted to recreate the in vivo skull in two views. Click to enlarge. Note the parietal does not produce a crust, but is flat. The maxilla ventral rim is convex. The postorbital does not have an acute curve, but standard  right angle shape. Both the lacrimal and postorbital were split longitudinally during taphonomy. All of the bones are cracked, making sutures hard to distinguish. This specimen is a sister to Herrerasaurus in the large reptile tree, based on this skull material, but note the lack of a lateral flange of the nasal.

Figure 1. Barberenasuchus traced using DGS color over the bones, then transferred and shifted to recreate the in vivo skull in two views. Click to enlarge. Note the parietal does not produce a crust, but is flat. The maxilla ventral rim is convex. The postorbital does not have an acute curve, but standard  right angle shape. Both the lacrimal and postorbital were split longitudinally during taphonomy. All of the bones are cracked, making sutures hard to distinguish. This specimen is a sister to Herrerasaurus in the large reptile tree, based on this skull material, but note the lack of a lateral flange of the nasal.

Distinct from most of its sisters (Fig. 3) Barberenasuchus has shorter teeth and a larger orbit. The skull is more gracile, and smaller in size. The rostral tip is not reinforced with laminated bone layers, as in Herrerasaurus or a lateral flange of the nasal, as in other basal dinosaurs, so Barberenasuchus is more plesiomorphic and therefore closer to the basalmost dinosaur. By contrast, look at the nasal of Eoraptor (Fig. 3) and Tawa which compares closely to that of Herrerasaurus. So the Barberenasuchus nasal is similar to that of the protodinosaurs while Herrerasaurus is similar to other dinosaurs.

Shifting Barberenasuchus to proximal nodes adds only 3 steps.

Figure 3. Barberenasuchus to scale with sister taxa, Herrerasaurus, Eoraptor, Lewisuchus and Trialestes and Junggarsuchus, but without the autapomorphies of its sister Herrerasaurus. At present Barberenasuchus is the basalmost dinosaur. Note the difference in the nasal between the dinosaurs and protodinosaurs.

Figure 3. Barberenasuchus to scale with sister taxa, Herrerasaurus, Eoraptor, Lewisuchus and Trialestes and Junggarsuchus, but without the autapomorphies of its sister Herrerasaurus. At present Barberenasuchus is the basalmost dinosaur. Note the difference in the nasal between the dinosaurs and protodinosaurs.

Bottom line:
Barberenasuchus does not look like any true crocodylomorph (agreeing with Irmis, Nesbitt and Sues, i.e. no anterior leaning quadrate/quadratojugal contacting the postorbital) because it is a basal dinosaur, a small (juvenile?) sister to Herrerasaurus. Barberenasuchus does not have a nasal that extends to the base of the naris, distinct from all other dinosaurs and similar to the protodinosaurs, like Junggarsuchus.

Figure 2. The Dinosauria subset of the large reptile tree as of February 5, 2016. Here Proceratosaurus nests with several former long-snouted tyrannosaurs now closer to spinosaurs and allosaurs.

Figure 3. The Dinosauria subset of the large reptile tree as of February 5, 2016. Here Proceratosaurus nests with several former long-snouted tyrannosaurs now closer to spinosaurs and allosaurs.

Too bad
this taxon was not known here when building my ‘Origin of Dinosaurs’ video. It would have been a featured taxon.

References
deFranca MAG, Bittencourt JdS and Langer MC 2013.
Reavaliação taxonomica de Barberenasuchus brasiliensis (Archosauriformes), Ladiniado do Rio Grande do Sul (Zona-Assembleia de Dinodontosaurus). Palaenotogia em Destaque Edição Especial Octubro 2013: 230. 
Irmis RB, Nesbitt SJ and Sues H-D 2013. Early Crocodylomorpha. Pp. 275–302 in Nesbitt, Desojo and Irmis (eds). Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. The Geological Society of London. doi:10.1144/SP379.24.
Kischlat EE 2000.
Tecodôncios: a aurora dos arcossáurios no Triássico. Pp. 273–316 in Holz and De Ros (eds.). Paleontologia do Rio Grande do Sul. Porto Alegre: CIGO/UFRGS.
Mattar LCB 1987. Descrição osteólogica do crânio e segunda vértebrata cervical de Barberenasuchus brasiliensis Mattar, 1987 (Reptilia, Thecodontia) do Mesotriássico do Rio Grande do Sul, Brasil. Anais, Academia Brasileira de Ciências, 61: 319–333.
Riff D et al. 2012.  Crocodilomorfos: a maior diversidade de répteis fósseis do Brasil. TERRÆ 9: 12-40, 2012.

The Origin of Dinosaurs as told by The Smithsonian, Wiki, etc.

Many of the biggest dino museums in the world
have produced their version of the origin of dinosaurs. Here’s what they have to say online:

Smithsonian – National Museum of Natural History
“The earliest dinosaurs were probably carnivorous, bipedal animals less than two meters long and weighing about 10 kilograms. From these small beginnings evolved thousands of different dinosaurs species.”

Wikipedia
“Dinosaurs evolved within a single lineage of archosaurs 232-234 Ma (million years ago) in the Ladinian age, the latter part of the middle Triassic. Dinosauria  is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.

“The process leading up to the Dinosauromorpha and the first true dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae, Erythrosuchidae and Euparkeria which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus 232-236 Ma. Crocodiles are also descendants of mid-Triassic archosaurs.

“Dinosaurs can be defined as the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia) and all the descendants of that ancestor. With that definition, the pterosaurs* and several species of archosaurs narrowly miss out on being classified as dinosaurs. Archosaur genera that also narrowly miss out on being classified as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton* 230-232 Ma and Marasuchus* 230-232 Ma.

“The first known dinosaurs were bipedal predators that were 1-2 metres (3.3-6.5 ft) long. Spondylosoma may or may not be a dinosaur; the fossils (all postcranial) are tentatively dated at 235-242 Ma.

“The earliest confirmed dinosaur fossils include saurischian (‘lizard-hipped’) dinosaurs Nyasasaurus 243 Ma, Saturnalia 225-232 Ma, Herrerasaurus 220-230 Ma, Staurikosaurus possibly 225-230 Ma, Eoraptor 220-230 Ma and Alwalkeria 220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.”

* these are false nestings according to the tree topology of the large reptile tree.

University of Bristol
“Those archosaurs most closely related to the dinosaurs are forms such as Marasuchus. The detailed evolutionary relationships are still debated, but by the late Triassic, several early theropods are known, as the dinosaurs rapidly diversified. These dinosaurs, such as Eoraptor, Coelophysis and Herrerasaurus were all carnivores, and, despite their diversity, were quite rare at this time.”

Natural History Museum of Los Angeles County
“The ancestry of dinosaurs can be traced back some 230 million years ago to the Late Triassic. All dinosaurs belong to a group of reptiles called archosaurs-a group that also includes crocodiles and a variety of Mesozoic reptiles (pterodactyls and others) that are often misinterpreted as dinosaurs. The anatomical characteristics of both the earliest known dinosaurs and their archosaurian relatives suggest that the common ancestor of all dinosaurs was a small bipedal predator, which had forelimbs shorter than hind limbs. This ancestor was probably similar to the 235-million-year-old Lagosuchus from Argentina, pictured below.

“From the most primitive Triassic forms to the most advanced ones of the latest Cretaceous, all dinosaurs share defining traits that distinguish them from their closest archosaurian relatives. Among these innovations, the femur (or upper leg bone) developed a distinct head for a tied attachment into a hollow hip socket. These and other changes resulted in a hind limb that was tucked directly underneath the body, providing upright, pillar-like support of the body and also enhancing locomotive abilities. The changes that led to the erect posture of dinosaurs from the sprawling posture of their reptilian predecessors had a profound effect on the evolutionary success of these animals. These transformations may have also been coupled with the evolution of a higher metabolism (a step towards warm bloodedness) that endowed them with a greater capacity for sustained activities such as running.”

Genesis park genesispark.com
“The Bible states that on the fifth day of creation God created great sea monsters and flying creatures. This would have included the great swimming and flying reptiles (like the plesiosaur and pterosaur creatures mentioned at our Genesis Park website). On the sixth day God created the land animals, which would have included all of the dinosaur kinds (Genesis 1:20-25).”

YouTube
Brief Lecture on the Origin of Dinosaurs – one commenter correctly noted, “This doesnt (sp) explain the actual origin of dinasaurs (sp) like the title states.”

The origin and evolution of dinosaurs Paul Sereno
Annual Review of Earth and Planetary Sciences
Vol. 25: 435-489 (Volume publication date May 1997)

“Phylogenetic studies and new fossil evidence have yielded fundamental insights into the pattern and timing of dinosaur evolution and the emergence of functionally modern birds. The dinosaurian radiation began in the Middle Triassic, significantly predating the global dominance of dinosaurs by the end of the period. The phylogenetic history of ornithischian and saurischian dinosaurs reveals evolutionary trends such as increasing body size. Adaptations to herbivory in dinosaurs were not tightly correlated with marked floral replacements. Dinosaurian biogeography during the era of continental breakup principally involved dispersal and regional extinction.”

American Museum of Natural History
Strangely, they don’t have an online account of dinosaur origins.

ReptileEvolution.com
Meet a long list of the best known taxa preceding dinos, the advent of dinos and see their family tree here and here. More specifics here (Fig. 1).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.