Arizonasaurus vs Spinosaurus

Two unrelated reptiles
evolved similar morphologies, Arizonasaurus and Spinosaurus (Fig. 1), a long rostrum filled with sharp teeth, a bipedal configuration and enormous neural spine arising from the dorsal vertebrae. One was a giant. The other about waist high. Seen here together for the first time…

Figure 1. Spinosaurus and Arizonsaurus, together for the first time. The similarities are obvious and intriguing.

Figure 1. Spinosaurus and Arizonsaurus, together for the first time. The similarities are obvious and intriguing. Spinosaurus courtesy of Scott Hartmann.

Spinosaurus is a famous giant theropod dinosaur. Arizonasaurus is none of these things. It’s a member of a clade that has no name, but arose from basal rauisuchids, like Venjukovia. It was a sister to Ticinosuchus + Aetosaurs and Yarasuchus + Qianosuchus, none of which have much of a sail back. I thought comparing these two might provide clues to their convergent looks.

Arizonasaurus comes from the Middle Triassic Moenkoepi Formation, which included fresh water and a diverse fauna. Earlier we looked at the possibility that this predator was bipedal, based on the very small pectoral girdle and very deep (for its time) pelvic girdle, almost like that of T-rex, but more gracile. Relatives include fish eaters, like long-necked Yarasuchus and plant eaters, like aetosaurs. So this is already a diverse clade that no doubt will provide many surprising morphologies in the future. Originally described as a prestosuchid rauisuchian, Brusatte et al. (2010) nested it with poposaurs. In the large reptile tree poposaurs nest a little closer to dinosaurs and basal crocs.

Spinosaurus comes from the Middle Cretaceous of northern Africa, which, at the time included tidal flats, mangrove forests and several other giant theropods. Only a few other dinosaurs had such long neural spines. The question is, where they more like sails, and aid in thermoregulation? Or did they support a buffalo-like hump of fat? Spinosaur relatives, all smaller, did not sport much of a sail back. So whatever its utility was, it was unique.

Sail backs seem to spring up occasionally and quickly around the reptile family tree. They never seem to last.

Moving on
to those long jaws, Spinosaurus was considered a quick-strike artist, feeding on everything from fish to small dinosaurs, but with that size it could have taken on any prey. No such claims have been made for Arizonasaurus, perhaps because not much of the skull is known. But the teeth were sharp

My take
I have no expertise and no stake in the hump vs. sail argument. Since these sails seem to come and go rather quickly, my opinion is they are literally a flash in the pan, thus they have no real utility and are only for show… secondary sexual traits. Popular one day, not so popular the next. The blessing probably becomes a curse over time, as the sail gets bigger, so the trait and the animal disappears. The neural spines are broad because they have “roots” that are broad, unlike Dimetrodon and like Sphenacodon.

Bailey JB 1997. Neural spine elongation in dinosaurs: sailbacks or buffalo-backs?. Journal of Paleontology 71 (6): 1124–1146.
Butler RJ, Brusatte SL, Reich M, Nesbitt SJ, Schoch RR, et al. 2011. The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation. PLoS ONE 6(10): e25693. doi:10.1371/journal.pone.0025693 Plos One paper
Nesbitt SJ 2003. Arizonasaurus and its implications for archosaur divergence. Proceedings of the Royal Society, London B (Suppl.) 270, S234–S237. DOI 10.1098/rsbl.2003.0066
Nesbitt SJ, Liu J and Li C 2010. A sail-backed suchian from the Heshanggou Formation (Early Triassic: Olenekian) of China. Transactions of the Royal Society of Edinburgh 101 (Special Issue 3-4):271-284.
Welles SP 1947 Vertebrates from the Upper Moenkopi Formation of the Northern Arizona. Univ. California Publ. Geol. Sci. 27, 241–294.
Wu X-C 1981. The discovery of a new thecodont from north east Shanxi. Vertebrata PalAsiatica 19: 122–132.


Bird stem evolved faster than other theropods

Figure 1. The evolution of birds as a consequence of miniaturization. Artist: Davide-Bonnadonna

Figure 1. The evolution of birds as a consequence of miniaturization. Artist: Davide-Bonnadonna. I think the last taxon on the right has been enlarged somewhat for clarity. See Figure 2. 

A new paper by Lee et al. (2014)
reported on their phylogenetic analysis of theropods employing 1549 characters. They found two drivers underlying the dinosaur-bird transition: 1) sustained miniaturization; and 2) the evolution of skeletal adaptations 4x faster than other dinosaurs.

They found that miniaturization facilitated the evolution of morphological novelties associated with small size: reorientation of the body mass; 2) increased aerial ability; and 3) paedomorphic skulls with enlarged eyes and brains along with a reduced snout and smaller teeth without serrations.

I heartily endorse this work.
It supports a paradigm that miniaturization that produces new clades. Cope’s Rule generally does not. We see similar miniaturization at the genesis of reptiles, amphibians, therapsids, mammals, dinosaurs, crocodylomorphs (together the archosaurs), pterosaurs, diapsids and several clades within each of these, like bats and pterodactyloid-grade pterosaurs. We knew for a long time that sustained miniaturization also produced birds. So that’s not news. It just has never been so well laid out before.

Figure 2. Sinocalliopteryx along with Aurornis and Archaeopteryx to scale. This illustration produced over a year ago, tells the same tale as the new Lee et al. paper, but without the great supporting details.

Figure 2. Sinocalliopteryx along with Aurornis and Archaeopteryx to scale. This illustration produced over a year ago, tells the same tale as the new Lee et al. paper, but without the great supporting details.

Lee et al. conclude: 
“Because size reduction, feather elaboration, paedomorphism, and other anatomical novelties permitted by small size all evolved in concert along the bird stem, identifying the primary driver of this sustained trend is probably impossible. It is likely that all traits influenced and provided the context for the evolution of others.” 

Actually not so impossible.
The authors make no mention of the fact that smaller taxa generally mature more quickly, breed more often and die sooner. In other words, generational turnover happens more quickly in smaller taxa, as everyone know. The rate of evolution over time is accelerated by the rate of reproduction over time (all other things being equal). That’s the primary driver. 

In the family tree of amniotes,
you see this all the time, not just in bird origins. Good to see this get the press it deserves.

Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds.


Czerkas and Feduccia disconnect birds and dinos

Figure 1. Reconstruction of Scansoriopteryx with possible feather extent by Stephen Czerkas. Good thing that second branch or telephone wire is available for balance!

Figure 1. Reconstruction of Scansoriopteryx with possible feather extent by Stephen Czerkas. Good thing that second branch or telephone wire is available for balance!

A new paper by Czerkas and Feduccia
attempts to unlink birds with dinosaurs and to link birds with some unspecified archosaur by their reexamination of Scansoriopteryx, a tiny Chinese fossil of the Jurassic. Much has already been said about this paper — all negative.

Czerkas and Feduccia report the “absence of fundamental dinosaurian characteristics,” but do not do so with phylogenetic analysis, which would have nested their study subject somewhere else that they could support, but can’t. They seem stuck in a trees-down vs. ground up battle when plenty of ground-dwelling dinosaurs seem fully capable of climbing a tree by grappling or simply by running up a vertical trunk bipedally, as some modern birds do (any Dial reference below). Their illustration (Fig. 1) seems to say that whether bird or dinosaur or non-dinosaur, Scansoriopteryx was not capable of standing balanced on its (apparently splayed?) hind limbs, despite the fact that it’s forelimbs appear poorly designed for walking. They’ve been accused of LarryMartinizing and it seems they have indeed been doing so. For those interested, Larry Martin preferred to discuss individual characters rather than suites of characters of a sort used in phylogenetic analysis.

I can’t buy into their particular heresy.
There’s no support for it. We need to see details and analyses. And they need to present their best alternative candidate among the non-dinosaurian archosaurs out there as a sister to Scansoriopteryx. 

The irony here
is that the same sort and style of argumentation is being used to support a pterosaur/archosaur connection by the same set of paleontologists who support the dino/bird connection. By that I mean, they present no archosaurian candidates that more closely match pterosaurs than our own favorites: the lepidosaur, tritosaur, fenestrasaurs.

So, if you’re a finger pointing paleontologist, be careful. Don’t fall into  that same trap.

Czerkas SA and Feduccia A 2014. Jurassic archosaur is a non-dinosaurian bird, Journal of OrnithologyDOI: 10.1007/s10336-014-1098-9
Dial KP, Jackson BE and Segre P 2008.  A fundamental avian wing-stroke provides a new perspective on the evolution of flight. Nature (online 23 Jan 08)
Padian K and Dial KP 2005. Could the “Four Winged” Dinosaurs Fly?  Nature: 438:E3-5.
Dial KP, Randall R and Dial TR 2006. What use is half a wing in the evolution of flapping flight? BioScience 56: 437-445.
Tobalske BW and Dial KP 2007. Aerodynamics of wing-assisted incline running. J. Exp. Biol. 210:1742-1751.
Bundle MW and Dial KP  2003. Mechanics of wing-assisted incline running.  J. Exp. Biol., 206:4553-4564.
Dial KP 2003.  Evolution of avian locomotion: Correlates of body size, reproductive biology, flight style, development and the origin of flapping flight. Auk 120:941-952.
Dial KP 2003. Wing-assisted incline running and the evolution of flight.  Science 299:402-404.
Read more at:


New Farlow et al. (2014) Poposaurus foot paper

Farlow et al. (2014) has a new paper on the foot of the poposaurid, Poposaurus.

Figure 1. Revised skull reconstruction for the PEFO specimen. Here the anterior is considered a premaxilla. Those teeth are shaped like triangles, but they are very deeply rooted and exposed very little, which casts doubts on its hypercarnivory.

Figure 1. Poposaurus in lateral view. This dinosaur like reptile really is a dinosaur with a calcaneal heel.

From their abstract:
“The crocodile-line basal suchian Poposaurus gracilis had body proportions suggesting that it was an erect, bipedal form like many dinosaurs, prompting questions of whether its pedal proportions, and the shape of its footprint, would likewise “mimic” those of bipedal dinosaurs.

Bivariate and multivariate analyses of phalangeal and digital dimensions showed numerous instances of convergence in pedal morphology among disparate archosaurian clades.

Overall, the foot of Poposaurus is indeed more like that of bipedal dinosaurs than other archosaur groups, but is not exactly like the foot of any particular bipedal dinosaur clade.” 

Included is a comparison with other archosaur taxa, (Fig. 1). Note Terrestrisuchus has no calcaneal heel. It develops in the derived Protosuchus and also poposaurid dinosaurs, according to the large reptile tree.

Figure 1. Archosaur feet divided into traditional croc-line and bird-line clades

Figure 1. Archosaur feet divided into traditional croc-line and bird-line clades

These feet can be reordered according to the large reptile tree (Fig. 2). Though many taxa are missing that would fill in morphological gaps, the general trends are more clear here.

Figure 2. Same feet, reordered according to the large reptile tree. Only Terrestrisuchus and Protosuchus are croc-like archosaurs here. Poposaurs are basal dinosaurs.

Figure 2. Same feet, reordered according to the large reptile tree. Only Terrestrisuchus and Protosuchus are croc-like archosaurs here. Poposaurs are basal dinosaurs. Silesaurus converged with theropod dinos, as did Brachylophosaurus. Note the lack of a calcaneal heel on Terrestrisuchus, a basal croc and the development of one on Protosuchus. In similar fashion poposaurid dinosaurs developed a calcaneal heel. 

Farlow et al. noted several instances of convergence (homoplasy). Indeed homoplasy is present here, even in this small sample.

On a separate note, 
Farlow et al. was kind enough to publish a radiograph of an Alligator. I added PILs and they are quite precise in this living reptile.

Figure 3. Radiograph of Alligator foot with PILs (parallel interphalangeal lines) added. Hone and Bennett tried to argue against the presence of PILs but did not have the nerve to show a foot with more than three toes.

Figure 3. Radiograph of Alligator foot with PILs (parallel interphalangeal lines) added. Seems rather clear that such lines representing phalanges working in sets is indeed present here.

Poposaur footprints have not bee found yet. Farlow et al. (2014) reported, “With a digit III length of about 16 cm, Poposaurus gracilis may have been comparable to a small to midrange theropod in overall body size (somewhere between the makers of Anchisauripus sillimani and A. minusculus, in the terminology of Lull [1953]). The dinosaur-like pedal proportions of Poposaurus, and the similarity of its reconstructed footprint to those of some dinosauromorphs, suggest that some grallatorid forms could well have been made by Poposaurus and its close relatives. However, mistaking Poposaurus tracks for dinosaur (particularly theropod) tracks would be less likely to occur if digit I of Poposaurus routinely touched the ground. Furthermore, trackways made by Poposaurus would probably have a shorter stride/footprint length ratio than grallatorid trackways.”

Personal thought
Seems to me that on Poposaurus pedal digit one is going to impress creating a four-toed ichnite.

Farlow JO, Schachner ER, Sarrazin JC, Klein H and Currie PJ 2014. Pedal Proportions of Poposaurus gracilis: Convergence and Divergence in the Feet of Archosaurs. The Anatomical Record. DOI 10.1002/ar.22863

Walking with Dinosaurs – The Movie

Well, it’s getting bad reviews, mostly and only for the sound track. Otherwise it looks spectacular, except for the disfigured pterosaurs. Here’s the poster with editorial comments added, followed by a selection of reviewer comments from

Figure 1. Walking with Dinosaurs poster.

Figure 1. Walking with Dinosaurs poster. Again, disfigured pterosaurs. Reviews:
“The photo-realistic look is striking, but the dialogue is occasionally wince-inducing. Think a sub-par Flintstones episode.”

“The beauty and majesty of the great creatures is marred a bit by too much focus on poop and barf jokes, silly winks at the audience,” and distracting anthropomorphism.”

“If you’re a fan of the 1999 BBC documentary series Walking with Dinosaurs and are hoping for more of the same, get ready for an Apatosaurus-sized level of disappointment from Walking with Dinosaurs 3D.”

“Features animation stunning and accurate enough to make up for its simple story and unnecessary voice-over dialogue.”

“Walking With Dinosaurs: The Movie boasts some impressive special effects but is ultimately let down by a terrible script, a dull story and a poorly conceived American voice dub that is extremely grating.”

YouTube previews London Gala Screening here and here.

Standard trailer here.

Fantastic New Paleoartist: Andrey Atuchin

Once again, I’m going to send you to another website to see the dino-wonders wrought by a new paleoartist: Andrey Atuchin and to read his interview. His talent for light, form and detail are well worth the visit.

Atuchin also has his own website:

And his own Deviant Art page:

Figure 1 Demandasaurus and company by Andrey Atuchin. Click to go to his website.

Figure 1 Demandasaurus and company by Andrey Atuchin. Click to go to his website.

Highest marks for this very talented young artist.

Marasuchus: In or out of the Dinosauria?

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy.

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared from an earlier blog post. Is Marasuchus in or out of the Dinosauria? Either way it was probably close to the origin of dinosaurs. Oddly the scapulocoracoid was quite robust in Marasuchus.

Here’s the problem:
Earlier we looked at the difference between Lagosuchus and Marasuchus, too small dino-like bipeds of the Late Triassic. The Nesbitt (2011) tree nests Marasuchus (Fig. 1) outside the Dinosauria. The large reptile tree nests Marasuchus as a small theropod. Let’s look at both sides of this situation. It appears to come down to micro-traits vs. macro-traits, a theme we’ll return to over the next few blog posts.

According to Nesbitt 2011
The following traits include Marasuchus, Silesauridae and the Dinosauria.

  1. Pubis longer than ischium (also in several post-Euparkeria taxa)
  2. Proximal portion of the pubis articular surfaces with the ilium and the ischium separated by a groove or gap. (also in Postosuchus)
  3. Ischio-pubis contact present and reduced to a thin proximal contact.
  4. Ischium, proximal articular surfaces articular surfaces with the ilium and the pubis continuous but separated by a fossa.
  5. Ischium length markedly longer than the dorsal margin of iliac blade (minus the anterior process). (doesn’t seem to be true in basal dinos)
  6. Anterior trochanter forms a steep margin with the shaft but is completely connected to the shaft.
  7. Anterior trochanter shelf proximal to the fourth trochanter (insertion site for M. iliofemoralis externus) present.
  8. Proximodistally oriented groove on the lateral side of the distal portion of the tibia.
  9. Anterior ascending flange (anterior process) of the astragalus present and less than the height of the dorsoventral height of the posterior side of the astragalus.

The following traits support the Dinosauria, exclusive of Marasuchus.

  1. Exoccipitals do not meet along the midline on the floor of the endocranial cavity. (Also in Crocodylomorpha and Effigia + Shuvosaurus.)
  2. Epipophyses present in postaxial anterior cervical vertebrae.
  3. Apex of deltopectoral crest situated at a point corresponding to more than 30% down the length of the humerus .
  4. Radius shorter than 0.8 of humerus length.
  5. Proximal articular surfaces of the ischium with the ilium and the pubis separated by a large concave surface.
  6. Fourth trochanter a sharp flange.
  7. Fourth trochanter asymmetrical, with distal margin forming a steeper angle to the shaft. 
  8. Cnemial crest arcs anterolaterally.
  9. Distinct proximodistally oriented ridge present on the posterior face of the distal end of the tibia.
  10. Proximal articular facet for fibula of the astragalus occupies less than 0.3 of the transverse width of the element.
  11. Concave articular surface for the fibula of the calcaneum.

The following traits support Marasuchus outside of the Theropoda:

  1. Parabasisphenoid recess absent
  2. Parabasisphenoid ant tympanic recess absent
  3. Epipophyses absent in past axial ant cerv verts
  4. Epipophyses absent in post cerv verts
  5. Cerv verts pneumatic features absent
  6. First primorial sacral art surf circular, not C-shaped in lat view
  7. Forelimb/hindlimb ratio > 0.55 (actually this ratio IS .55. est for manus)
  8. Ischium articular surfaces continuous but separated by fossa
  9. Fourth trochanter mound-like and rounded (looks sharp and described as aliform-wing-shaped)
  10. Femur surface between lat condyle and crista tibiofibularis on distal surface smooth
  11. Tibia prox portion cenemail crest present and straight
  12. DT4 medial side, without a distinct medial process
  13. Astragalus prox art facet for fibula occupies more than .3 transverse width
  14. Astragalus post groove present
  15. Calcaneal tuber present (but Eoraptor also has a slight tuber)
  16. Calc art surf for fibula convex

The following traits support Marasuchus within the Theropoda

  1. Opisthotic ventral ramus covered by lateralmost edge of exoccipital in post view
  2. Acetab antitrochanter present
  3. [not scored by Nesbitt] Gastralia present

The following traits support Marasuchus inside the Dinosauromorpha, but outside of the Dinosauria:

  1. Exoccipital lateral surfaces with clear crest lying ant to both ext foramina
  2. Axis, dorsal margin of the neural spine arcs dorsally
  3. Glenoid posterovent
  4. Prox pubis art surface separated by a groove or gap
  5. Ischio pubis contact present and reduced to a thin prox contact
  6. ischium markedly longer than iliac blade minus ant process
  7. Tibia > femur (not all dinos)
  8. Femur postmed tuber present and small
  9. Femur ant torchanter shelf present
  10. Femur post lat portion ventrally descended
  11. Tibia prox portion cenemail crest present and straight
  12. Tibai lat side of distal portion proxdistal oriented groove
  13. DT 4 width subequal to dt3
  14. DT4 size of art facet for mt 5 less than half of lat surface
  15. Astragalus ant ascending flange present less than height of dorsventral height
  16. Astragalus ant hollow reduced to a foramen or absent
  17. Astragalus antmed corner acute
  18. Compact metatarsus
  19. Longest metatarsal > .5 tibia
  20. Mt 5 phalanges: none (but note more derived taxa have phalanges.)

According to the large reptile tree,
Marasuchus is a derived theropod nesting with Procompsognathus. Moving Marasuchus out of the Dinosauria (outside Herrerasaurus) adds 15 steps.

The following traits support the inclusion of Marasuchus with Procompsognathus in the Theropoda.

  1. Mid-cervicals shorter than mid-dorsal
  2. Chevrons wider proximally
  3. Pedal 2.1 not > p2.2
  4. Pedal digit 4 not narrower than digit 3
  5. Overall size not > 30 cm tall, 60 cm long

The following traits support the inclusion of Marasuchus with Procompsognathus and Segisaurus in the Theropoda.

  1. Dorsal transv proc shorter than centra
  2. Pubis curving ventral
  3. Pedal 1.1 aligns with mt2, mt3

The following traits support the inclusion of Marasuchus within the Theropoda (represented by the addition of Coelophysis and Tawa in the large reptile tree).

  1. Skull shorter than cervicals (also in Turfanosuchus).
  2. Ant caudal spines shorter than centra.
  3. Caudals 3x longer than tall.
  4. Tibia not shorter than femur
  5. Fibula anterior trochanter low crest
  6. Mt 1 <.5 mt 3
  7. Mt 1 <.5 mt 4
  8. Metatarsals not shorter than half the tibia (also in phytodinosaurs.)
  9. Phalanges pedal digit 5: 0 (also in most crocs)

The following traits support the inclusion of Marasuchus within the Dinosauria.

  1. Interclavicle absent
  2. Forelimb < .55 hindlimb (actually the ratio is right at .55 given the hypothetical size of the manus shown in figure 1).
  3. Ilium anter proces truncated
  4. Acetabulum semiperforate
  5. Femoral head offset and subrectangular
  6. Fourth trochanter sharp
  7. Tibia > 2x ilium length
  8. Advanced mesotarsal ankle
  9. No calcaneal tuber (actually the tuber is vestigial, but poposaurs also have a substantial tuber, a reversal convergent with derived crocs).
  10. Proximal metatarsals 1 and 5 reduced (actually, not so much in Marasuchus, 1 is not reduced in Plateosaurus, 5 is not much reduced in Tawa)
  11. Osteoderms absent

The following traits in the large reptile tree are shared by Marasuchus and other taxa outside the Dinosauria.

  1. Longest metatarsals 3-4

Even a cursory glance through this list demonstrates the Nesbitt characters are often micro-traits representing characters that must be seen in person and in close-up. These include various bumps and holes in various bones. By contrast the Peters characters are largely macro-traits, visible and able to be measured in published images. That means the sister taxa used by the Peters images are more likely to look alike overall. That also means testing the Nesbitt tree is going to take personal inspection of the specimens. What I don’t understand is why the micro-traits and macro-traits diverge so widely, assuming both were scored correctly.

I wonder if the Nesbitt tree included Segisaurus and Procompsognathus if the tree topology would change? Oddly both have advanced theropod feet with 1 and 5 reduced.

Fraser NC, Padian K, Walkden GM and Davis LM 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology. 45(1), 79-95.
Paul GS 1988. Predatory dinosaurs of the world (464). New York: Simon and Schuster.
Romer AS 1971. The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria 378: 1-10.
Romer AS 1972. The Chanares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodonts Lagerpeton and Lagosuchus: Breviora 394: 1-7.
Sereno PC and Arcucci AB 1994. Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis gen. nov. Journal of Vertebrate Paleontology, 14: 53-73