The Flat-Head Pterosaur

Part of a Private Collection Made Public
There is a tiny little anurognathid pterosaur in a private collection that Dr. S. Chris Bennett (2007) described as a small Anurognathus ammoni (Döderlain 1923). Here that specimen was found to nest next to Anurognathus, but it was neither conspecific nor congeneric with Anurognathus. It was distinct in morphology from flat head to tiny toe. It actually shares more characters with it phylogenetic predecessor,  Dendrorhynchoides (Ji and Ji 1998 ), including a very wide sternal complex and torso. While most pterosaurs had long pointed jaws and most anurognathid pterosaurs had a round, bubble-like skull, this particular anurognathid had the flattest, widest, most pancake-like skull of all. The eyeballs would have popped up above the skull outline, like a frog’s eyeballs. Figure 1 portrays both anurognathids to scale. The many differences are easy to see. Let’s run through them.

Figure 1. The flat-head pterosaur, a private specimen (on the left) attributed by Bennett (2007) to Anurognathus ammoni (on the right).

Figure 1. The flat-head pterosaur, a private specimen (on the left) attributed by Bennett (2007) to Anurognathus ammoni (on the right).

The Skull
The skull was described by Bennett (2007) as having an enormous orbit in the anterior half of the skull, little to no antorbital fenestra, and a broad set of parietals with widely spaced upper temporal fenestra among several other autapomorphies. (You can view those illusory interpretations here). No sister taxa have these traits. Nevertheless, this false and frankly, goofy to monstrous reconstruction has become widely accepted. Such a reconstruction replaces the large air-filled antorbital fenestra of all other pterosaurs with gel-filled eyeballs. Such a reconstruction moves the eyeballs into the anterior half of the skull, the opposite of all other pterosaurs. Bennett (2007) mistook the curved and dentally subdivided maxilla for a giant sclerotic ring preserved on edge, which no other crushed specimen of any tetrapod ever does. Bennett (2007) was unable to segregate the layers of bones so reconstructed a wide, flat parietal, the opposite of all other pterosaurs. Here DGS (digital graphic segregation) was able to delineate all the skull bones recovering identical left and right elements that resemble those of sister taxa and produce a reconstruction in line with sisters, rather than completely different as in the Bennett (2007) reconstruction (see both here).

At left the traditional Bennett (2007) interpretation. On the right, interpretation based on finding and tracing paired bones.

Figure 2. At left the traditional Bennett (2007) interpretation. On the right, interpretation based on finding and tracing paired bones.

The Post-Crania
The rest of the skeleton was much more typical of anurognathid pterosaurs. The cervical series was relatively longer than in Anurognathus. The torso was not as wide as in Dendrorhynchoides and the dorsal ribs were more gracile. The caudals were greatly reduced. The sternal complex was not quite as wide. The pteroid was smaller. Bennett (2007) determined that manual phalanx 4.4 was missing, but it is largely buried. The distal portion reappears at the pelvis and all sister taxa have four long wing phalanges. Pedal digit 2 is not the longest. The proximal pedal phalanges had more typical proportions than the short ones in Dendrorhynchoides.

The Flathead Anurognathid

Figure 3. The SMNS anurognathus as reconstructed in various views. Black circle is hypothetical egg.

Why The Wide Face?
Obviously the wide flat skull gave the private specimen some sort of competitive advantage. Certainly the wider gape captured more tiny insects. The disc-like shape, like a flying saucer, may have been raised and lowered in the airstream to affect the flightpath and such a shape reduced aerodynamic drag while streamlined in the neutral position.

Think About the Size of the Egg!
With such a tiny pelvic opening, the egg of the private specimen would have been very tiny, on the order of 3-4 mm in diameter. The hatchling would have stood one-eighth as tall as the 6 cm adult or less than 8 mm in height (possibly taller if the egg was elongated).  Such a fly-sized pterosaur risked desiccation if it flew in dry air, so it may have scurried about in damp leaf litter snatching insects on the ground as a juvenile.

Click here for more information and images.

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.

References
Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Bennett SC 2008. Morphological evolution of the wing of pterosaurs: myology and function. Zitteliana B28: 127-141.
Döderlain L 1923Anurognathus ammoni, ein neuer Flugsaurier. Sitzungsberichte der Königlich Bayerischen Akademie der Wissenschaten, zu München, Mathematischen-physikalischen Klasse: 117-164.
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica doi: 10.4202/app.2009.0145 online pdf
Ji S-A and Ji Q 1998. A New Fossil Pterosaur (Rhamphorhynchoidea) from Liaoning. Jiangsu Geology 4: 199-206.
Peters D 2001. A New Model for the Evolution of the Pterosaur Wing – with a twist. Historical Biology 15:277–301.
wiki/Anurognathus

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The Origin and Evolution of Tanystropheus

The King of the Bizarre Triassic Lizards
The Triassic is known for its many bizarre “experiments” in reptile morphology. It’s not commonly realized, but many of these oddballs nest together in one lizard clade, the Tritosauria. The largest of these Dr. Seussian reptiles was Tanystropheus, famous for its extremely long neck. You may wonder, as many have, how does an animal get such a long neck? The answer is: blame the parents. Here we’ll look at Tanystropheus and its ancestry, a subject that has never been accurately documented (that is, following a large phylogenetic analysis).

Key note: Tanystropheids were NOT prolacertiforms. Prolacerta and Protorosaurus nested with archosauriforms.

Tanystropheus and kin going back to Huehuecuetzpalli.

Figure 1. Tanystropheus and kin going back to the basal tritosaur lizard, Huehuecuetzpalli.

The Long Neck Has a Long History
The basal tritosaurian lizard, Huehuecuetzpalli, does not have a long neck. That honor goes to it sister taxon, Macrocnemus. There is a gradual size increase in various specimens of Macrocnemus that traditionally has been ascribed to levels of maturity. Here the morphological differences indicate phylogenetic variation. The smallest Macrocnemus is the most primitive, the one closest to Huehuecuetzpalli. The next largest one, T4822, gave rise to Tanystropheus and its kin, Tanytrachelos and Langobardisaurus. The largest Macrocnemus, T2472, gives rise to the underwater sit-and-wait predator, Dinocephalosaurus, which turns out to be not as closely related to Tanystropheus as everyone thought.

Next Step Langobardisaurus
The longer, more gracile neck vertebrae found in Tanystropheus are first seen in Langobardisaurus, a small, terrestrial tritosaur that likely experimented with a bipedal configuration. Note that Dinocephalosaurus had a longer neck made longer by adding vertebrae, a pattern unlike that of Tanystropheus and Langobardisaurus, which elongated its cervical vertebrae without increasing their number.

Teeth
The teeth of Langobardisaurus were unusual. The front teeth were rake-like. The posterior teeth were flattened and multi-cusped, like molars. The smaller Tanystropheus, wrongly considered a juvenile (MSNM BES SC 1018, Exemplar A) also had multi-cusped posterior teeth, though not quite the same shape. The large Tanystropheus had only simple conical teeth with a single sharp point. The anterior teeth were also longer and more rake-like. The change probably reflected a change in diet, perhaps from small insects to large vertebrates like fish.

Where Did Tanystropheus Live and Eat?
A diet of fish might involve a move from a terrestrial environment to an aquatic one, at least to the sea shore where a long neck could extend out over the surf to dip into deeper waters. Fish bones and squid jaws were found in the stomach area. There is not much else about the Tanystropheus skeleton that indicates any marine adaptations. However, such adaptations can be seen in Dinocephalosaurus. I also wonder if Tanystropheus raided tree limbs for the various climbing, gliding and flying animals that could be found there. That’s how the incremental changes employed by evolution could make a tiny, long-necked lizard reaching into bushes and trees into a very large long-necked creature reaching into taller bushes and tree — or capable of dipping into surf.

A purported “juvenile” Tanystropheus (Li 2007) was slightly larger than the A specimen and had fish bones in its gut area. The skull is unknown. Since juveniles were identical to adults, without similar specimens nearby, or eggshells, there is no way to tell if this specimen was indeed a juvenile or just small. The skull is unknown. The lack of ossification in the carpus is not a sign of immaturity, but a trait shared by other clade members, large and small, beginning with Huehuecuetzpalli.

The Pedal Digit 5 Problem
Careful observers will note that pedal phalanx 5.1 is elongated in most basal members of this clade and this extends into fenestrasaurs, like Cosesaurus and Sharovipteryx and basal pterosaurs. Three phylogenetic successors to Macrocnemus (Langobardisaurus, Tantrachelos and Tanystropheus) also had such a toe. Unfortunately no Macrocnemus, large or small, has this trait. Dinocephalosaurus, a fourth Macrocnemus successor, did not have an elongated p5.1 either. If Macrocnemus had a short p5.1 while its predecessors and successors had an elongated p5.1, this is a phylogenetic problem. Two solutions: 1) Pedal 5.1 re-elongated by convergence after Macrocnemus, and 2) there is yet another to-be-found Macrocnemus that retained an elongated p5.1.  Note that both Tantrachelos and Langobardisaurus are smaller than the BES SC specimen of Macrocnemus (Fig 1). If these taxa matured faster and smaller while retaining embryonic traits, they could have reverted to the elongated p5.1 of the ancestors of Macrocnemus, following solution #1. Let’s keep looking at the toes.

Post-Cloacal Bones – Traditional Hypothesis
Postcloacal (aka heterotopic bones) beneath the tail in some specimens is indicative of sexual dimorphism. Wild (1973) wondered if these were male copulatory organs, but they were too large and too complex. Another theory suggests they supported a brood pouch. They were not chevrons.

Post-Cloacal Bones – Heretical Hypothesis
Note that where postcloacal bones are present, chevrons are not. Smaller postcloacal bones on more primitive taxa more closely resemble typical chevrons. Therefore postcloacal bones were transformed chevrons. Dr. Silvio Renesto (2005) reported a great mass of flesh at the base of the tail, which would have helped balance (or in the heretical hypothesis, provide a base) for Tanystropheus. Postcloacal bones could have helped support this mass. The postcloacal bones doubled as tail skids (Fig. 1) wheneverTanystropheus was vertical, like the double-beam chevrons of the rearing sauropod Diplodocus.

Juvenile Tanystropheus from China

Figure 2. Juvenile Tanystropheus from China (Li 2007). It was slightly larger than the A exemplar of Wild (1973) and had slightly different neck vertebrae and limb proportions. The ribs were more robust. The head and teeth are unknown.

What Would a Tanystropheus Egg Look Like?
As a tritosaur, Tanystropheus hatchlings greatly resembled their parents, as in pterosaurs and Huehuecuetzpalli. The purported juvenile (Li 2007, Fig. 2) had a long neck that would have been flexed within the eggshell bringing the head down to the level of the groin or tail. That produces a very long egg. The examples of pterosaurs indicate that other tritosaurs may have retained eggs within the cloaca until just prior to hatching. If Tanystropheus was like the pterosaur Darwinopterus, only one egg at a time was produced. The giant chevrons/postcloacal bones of only some Tanystropheus may indicate an extension of the cloaca further beneath the tail, beyond the hips to carry hyperelongated eggs.

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.

References
Bassani F 1886. Sui Fossili e sull’ età degli schisti bituminosi triasici di Besano in Lombardia. Atti della Società Italiana di Scienze Naturali 19:15–72.
Li C 2007. A juvenile Tanystropheus sp.(Protoro sauria: Tanystropheidae) from the Middle Triassic of Guizhou, China. Vertebrata PalAsiatica 45(1): 37-42.
Meyer H von 1847–55. Die saurier des Muschelkalkes mit rücksicht auf die saurier aus Buntem Sanstein und Keuper; pp. 1-167 in Zur fauna der Vorwelt, zweite Abteilung. Frankfurt.
Nosotti S 2007. Tanystropheus longobardicus (Reptilia, Protorosauria: Reinterpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, Vol. XXXV – Fascicolo III, pp. 1-88
Peyer B 1931. Tanystropheus longobardicus Bass sp. Die Triasfauna der Tessiner Kalkalpen. Abhandlungen Schweizerische Paläontologie Gesellschaft 50:5-110.
Renesto S 2005. A new specimen of Tanystropheus (Reptilia Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia vol. 111, no. 3, 377–394. online pdf
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus (Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-16.
wiki/Tanystropheus

Another Use for Pterosaur Tale Vanes

Where are Pterosaur Tail Vanes Found?
Basal pterosaurs are often illustrated with tail vanes, but they are not found on many basal pterosaurs. Soft tissue preservation is rare. The Campylognathoides/Rhamphorhynchus clade had the most prominent tail vanes. Various Dorygnathus may have had something like a vane. It’s never clear. Sordes had some sort of tail expansion and Pterorhynchus did not have a single vane, per se, but several very short ones along the length. The tail vane seems to have coalesced from several smaller vanes, which, in turn, may have developed from specialized ptero-hairs seen on the tail of Cosesaurus.

Tail vane animation on the C5 specimen of Campylognathoides.

Figure 1. Click to animate. Tail vane animation on the C5 specimen of Campylognathoides zitteli.

Tail Vane Usage
The tail vane has typically been considered a steering mechanism, but airplanes don’t steer with their tail (vertical stabilizer). That just produces a skid and lots of drag. To initiate a turn airplanes, birds and bats roll into a bank.  Presumably pterosaurs did likewise. The tail vane would have worked like feathers on an arrow shaft, keeping the back of the tail in line with the line of least drag, in line with the body at all times, and all without effort.

Correlations
You might note that the most prominent tail vanes are also found in the clade with the longest wings in relation to their body size. In Campylognathoides and Rhamphorhynchus the wing tips extend far above their head. The tail itself was stiff, able to rise and fall tilting at its base. The same was true of the wings. They were stiff and able to fold and unfold at the metatarsophalangeal joint. Those are similar actions as seen from the side. That got me thinking.

The Metronome Hypothesis
What if the tail rose and fell like a metronome? The wings could open and fold in counterpoint. Together the three elements might have produced a secondary sexual behavior that attracted mates… or was just a way to relax.

Pure speculation.  Enjoy the animation.

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.

Aurorazhdarcho – Unfortunately, Not Related to Azhdarcho.

A recent paper by Frey, Meyer and Tischlinger (2011) reported on a pterosaur that has been known for several years, but only now published and given a name, Aurorazhdarcho. The authors erected a new family, the Protazhdarchidae and attributed the specimen to the Azhdarchoidea. Here, along with Wellnhofer’s no. 13,  Eopteranodon and Eoazhdarcho, Auroazhdarcho nests at the base of Pteranodon + Nyctosaurus, far from any azhdarchids despite overall appearances. The details tell another story.

Convergence Again
Below we can see members of the Azhdarchidae (Fig. 1) and the Eopteranodon clade (Fig. 2) that includes Aurorazhdarcho. Keys to making this nesting were the distinct proportions of the manual and pedal elements. The prepubes were virtually fused at the midline in Auroazhdarcho, as in sister taxa.

The Azhdarchidae.

Figure 1. The Azhdarchidae. Click to enlarge. Note the resemblances to Auroazhdarcho, all by convergence.

Golden Flakes in UV
Apparently without a skull and cervicals, Aurorazhdarcho preserves a faint skull-like patch of golden flakes seen in ultraviolet light. In Figure 2, the abstract tracing above the reconstruction of Aurorazhdarcho was taken from this amorphous area. The gray skull and cervicals in Figure 2 are a best guess reconstruction. Apparent rows of no. 13-like teeth are barely visible, but let’s not put too much stock in those. I don’t know what to make of all the curved lines, including a highly curved mandible. Did the fossil soften up before burial? The rest of it did not. Did they get washed away before burial, leaving only drifting impressions? Good question. No answer. The rest of the fossil was swirled a bit.

Figure 2. Left to right, Eopteranodon, Wellnhofer's No. 13 and Aurorazhdarcho, sisters that nest far from the Azhdarchidae.

Figure 2. Left to right, Eopteranodon, Wellnhofer’s No. 13 and Aurorazhdarcho, sisters that nest far from the Azhdarchidae.

Missing Pedal Phalanges?
The authors did not make mention of the missing disc-like pedal phalanges (p3.2, p4.2, p4.3) typically found in pterosaurs of this grade, nor did they illustrate them. If the disc-like phalanges were fused to the larger phalanges, or had just disappeared (as in higher cynodonts), that would be news that was apparently overlooked.

The Skinniest Pterosaur?
Aurorazhdarcho may have had the skinniest legs of all pterosaurs (but see Rhaeticodactylus for more gracile wings).  Compared to its sisters (Fig. 2) Aurorazhdarcho was more gracile in the wings as well.

The Sternal Complex
The sternal complex is quite large and broad in Aurorazhdarcho and its sisters. Not so in Quetzalcoatlus and its azhdarchid sisters back to the Jurassic taxa where the non-azhdarchid sisters to the azhdarchidsHuanhepterus, no. 44, no. 42 and no. 57 have a large sternal complex.

Male or Female? Juvenile or Mature?
Frey, Meyer and Tischlinger (2011) reported, “The partial fusion of the glenoideal suture of the scapulocoracoid and the near complete co-ossification of the olecranon process with the basal wing finger phalanx suggests a late juvenile or subadult individual (cf. Bennett 1993; Frey and Martill 1998).”

Figure 3. The faintest impressions of a skull were found by the original authors and colorized here.

Figure 3. The faintest impressions of a skull were found by the original authors and colorized here. I think the tan element is a palatal element. Hard to say.

Unfortunately, these are phylogenetic characters, as we learned earlier. As lizards, pterosaurs don’t follow archosaur bone growth patterns (Maisano 2004).

Frey, Meyer and Tischlinger (2011) also reported, “We suggest here that a ventrally open pelvis lacking a puboischiadic symphysis would be indicative for a female because of the wide pelvic apperture would serve as an egg passage (Unwin 2006). Those pterosauria with a tightly fused puboischiadic symphysis and a narrow pelvic aperture like NMB Sh 110 are likely to have been males.”

Unfortunately the ventrally open/closed pelvis is also a phylogenetic character. The symphysis is a trait shared with sisters. Fusion of the pubis and ischium occurs in Eopteranodon (Fig. 2).

Summary
Aurorazhdarcho was a Jurassic sister to Wellnhofer’s (1970) no. 13 and the Early Cretaceous sisters Eoazhdarcho and Eopteranodon. Jurassic sisters to the Azhdarchidae include Wellnhofer’s no. 42 and no. 44, just as skinny and crane-like.

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.

References
Frey E, Meyer CA and Tischlinger H 2011. The oldest azhdarchoid pterosaur from the Late Jurassic Solnhofen Limestone (Early Tithonian) of Southern Germany. Swiss Journal of Geosciences, (advance online publication) doi:10.1007/s00015-011-0073-1

Maisano JA 2002. Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrate Paleontology 22:268-275.


Youngina, Youngoides and the Younginiformes

There are several skulls and fewer post-crania attributed to Youngina and Youngoides, originally by R. Broom (1914), but CE Gow (1975) and others (see below) also made contributions.

The big question is: are the skulls crushed into a variety of shapes? Or do the variety of shapes reflect important morphologies that separate the various specimens into various clades? If you have any Youngina/Youngoides skull photos, please send them!!

The other question is: do some specimens harbor an antorbital fenestra?

Here’s why I wonder:

Youngina BPI 375. Is this a nascent antorbital fenestra?

Figure 1. Youngina BPI 375. Is this a nascent antorbital fenestra?

And Here’s Another One:

Youngina AMNH 5561. Is this a nascent antorbital fenestra?

Figure 2. Youngina AMNH 5561. Is this a nascent antorbital fenestra?

At the Base of the Archosauriformes
These two Youngina specimens nest at the base of the Archosauriformes in the midst of several other younginiforms. Do those little skull breaks/indentations represent antorbital fenestra? Good question. The answer is, it really doesn’t matter in phylogenetic analysis because predecessors in the protorosauria do not have an antorbital fenestra and successors in the archosauriformes do. Not all Youngina had or have to have an antorbital fenestra. These things tend to come and go, especially when they first appear.

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.

References
Broom R 1914. A new thecodont reptile. Proceedings of the Zoological Society of London, 1914:1072-1077.
Gardner NM, Holliday CM and O’Keefe FR 2010. The braincase of Youngina capensis(Reptilia, Diapsida): New insights from high-resolution CT scanning of the holotype. Paleonotologica Electronica 13(3):online PDF
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olsen EC 1936. Notes on the skull of Youngina capensis Broom. Journal of Geology, 44 (4): 523-533.
Reisz RR, Modesto SP and Scot DMT 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society, London B
doi:10.1098/rspb.2011.0439

From Whence Arrived the Aetosaurs?

Few paleontologists have ventured to guess, or determine through analysis, from whence arrived the aetosaurs. They don’t look much like any other archosauriforms. They seem to appear as sideshows in various analyses. Notably the latest analyses find no consensus. Nesbitt (2011) nested aetosaurs with Revueltosaurus. Outgroup taxa included Turfanosuchus and Gracilisuchus. Brusatte et al. (2010) nested Aetosauria with Gracilisuchus, Erpetosuchus and Crocodylomorpha. The Phytosauria was the outgroup.

“From whence arrived the praying mantis?” — Ogden Nash

Here aetosaurs nested with Ticinosuchus, a basal rauisuchian with a small head, short rostrum,  a reduced lateral temporal fenestra, a large mandibular fenestra, an upturned toothless dentary tip, a toothless premaxilla, a smaller pectoral girdle and scutes both above and below its tail. The hands and feet are also close matches. Ticinosuchus was also a sister to Qianosuchus and Yarasuchus, the long-necked rauisuchians sharing a dorsal naris with the basal rauisuchian, Vjushkovia and aetosaurs. It helped, of course, to actually reconstruct the skull of Ticinosuchus. It’s more aetosaur-like than previously thought. The size reduction between Ticinosuchus and Aetosaurus, the most primitive aetosaur, parallels other size reductions prior to major morphological changes in basal reptiles, mammals and birds. Chronologically the Late Triassic aetosaurs succeeded the Middle Triassic Ticinosuchus.

Figure 1. Vjushkovia, Ticinosuchus and the base of the Stagonolepidae (aetosaurs)

Figure 1. Vjushkovia, Ticinosuchus and the base of the Stagonolepidae (aetosaurs)

Little Aetosaurus
As we’re finding over and over again, whenever a major clade is introduced, its basal member is small. Aetosaurus is less than a third the size of its phylogenetic predecessor, Ticinosuchus, but the skull length is more than half that of Ticinosuchus. The development of more extensive armor and an herbivorous dentition coincides with this size reduction. The only catch is, Aetosaurus is not the earliest known aetosaur. Perhaps it was a late survivor. All other aetosaurs, including earlier specimens, were larger with a more extensive armor coating and an expanded gut for plant digestion.

 Aetosauroides.

Figure 2. Aetosauroides.

Aetosauroides
Aetosauroides scagliai (Casamiquela 1960) Late Triassic (~210 mya) was a transitional taxon between Aetosaurus and Ticinosuchus. It had unconstricted tooth crowns, postnarial contact between the premaxilla and nasal, and a ventral margin of the dentary without a sharp inflexion. The teeth were primitive. I do not know the size of the skull. It was described as “large.”

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.

References
Desojo JB and Ezcurra M.D 2011. A reappraisal of the taxonomic status of Aetosauroides(Archosauria, Aetosauria) specimens from the Late Triassic of South America and their proposed synonymy with Stagonolepis. Journal of Vertebrate Paleontology 31(3):596-609. doi:10.1080/02724634.2011.572936
Fraas O 1877. Aetosaurus ferratus Fr. Die gepanzerte Vogel-Echse aus dem Stubensandstein bei Stuttgar. Festshrift zur Feier des vierhundertjährigen Jubiläums der Eberhard-Karls-Universät zu Tübingen, Wurttembergische naturwissenschaftliche jahreshefte 33 (3): 1–22.
Krebs B 1965. Ticinosuchus ferox nov. gen. nov. sp. Ein neuer Pseudosuchier aus der Trias des Monte San Giorgio. Schweizerische Palaontologische Abhandlungen 81:1-140.
Lautenschlager S and Desojo JB 2011. Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus. Paläontologische Zeitschrift Online First DOI: 10.1007/s12542-011-0105-1
Schoch R 2007. Osteology of the small archosaur Aetosaurus from the Upper Triassic of Germany. Neues Jahrbuch für Geologie und Paläontologie – Abhandlung. 246/1:.1–35. DOI: 10.1127/0077-7749/2007/0246-0001
Walker AD 1961. Triassic reptiles from the Elgin area: StagonolepisDasygnathus and their allies. Philosophical Transactions of the Royal Society B 244:103-204.

wiki/Aetosaur
wiki/Stagonolepis

Pre-Diapsids. The Opening Act.

The Traditional View 
Diapsids were derived from the Protorothyridae (or Captorhinomorpha), close to Paleothyris (Carroll 1969). Petrolacosaurus is the earliest known diapsid. The diapsid configuration was not preceded by any temporal fenestration. Petrolacosaurus nests at the base of the Neodiapsida, which includes most other reptiles with temporal fenestration other than synapsids and bolosaurids.

The Heretical View
Diapsids were derived from basal synapsids close to Aerosaurus (a synapsid) and Protorothyris (a protosynapsid). Heleosaurus is the most primitive known protodiapsid (it nests outside the Synapsida). Eudibamus and Spinoaequalis are the most primitive known diapsids (two pairs of temporal openings). The diapsid configuration was preceded by lateral temporal fenestration. Click here to see the list of reptiles that succeeded  Eudibamus. Lepidosaurs and their sisters are not included on that list.

The Origin of the Protodiapsida
Petrolacosaurus has been the poster child for the origin of the Diapsida and it’s a good example. However, what evolved before the Diapsida has been largely ignored or overlooked.

Basal Protodiapsida to scale.

Figure 1. Basal Protodiapsida to scale. Diapsids in yellow. The synapsid Aerosaurus is grey.

Basal Archosauromorpha with a focus on the Protodiapsida

Figure 2. Click to enlarge. Basal Archosauromorpha with a focus on the Protodiapsida beginning with Heleosaurus.

Heleosaurus
Heleosaurus was considered an indeterminate diapsid by Broom (1907) and Carroll (1976), but Reisz and Modesto (2007) determined it was a varanopid synapsid. Here, with the benefit of new data on the skull (Botha, Brink and Modesto 2009) Heleosaurus nested just outside of the Synapsida at the base of a previously unrecognized clade, the Protodiapsida. Distinct from its predecessors, the skull was longer and the more cervicals were added. The suborbital jugal was more gracile. The pelvis was relatively larger. The limbs were longer. At 270 million years of age, the sole specimen of Heleosaurus is 40 million years younger than its phylogenetic descendants, indicating a long ghost lineage.

The Reduction of the Lateral Temporal Fenestra
The next two taxa, Archaeovenator (306 mya) and Mesenosaurus (266 mya) spanned that 40 million year gap. They had a smaller lateral temporal fenesatra, reduced by an advancing squamosal. Together with Heleosaurus, these two formed a clade.

The Milleropsis Detour
The temporal region of Milleropsis (Gow 1972, 290 mya) deviated from the basic skull pattern of its sisters. The lower temporal bar was absent, convergent with owenettids and squamates. The whip-like tail was incredibly long and the pelvis that anchored it was robust. Not enough is known of this taxon, but it appears able to run bipedally given available data.

Eudibamus
The most primitive diapsid may be Eudibamus (Berman et al. 2000, 290 mya), which was originally considered a sister to BolosaurusThe crushed skull does bear a strong resemblance. The teeth were blunt. The squamosal expanded further anteriorly to reduce the lateral temporal fenestra and the upper temporal fenestra first appeared. As in Milleropsis, the tail was whiplike. With an enlarged hind limb and short torso, Berman et al. (2000) considered Eudibamus an early biped. The proximal fingers and toes were greatly reduced, as in sister taxa, but more so.

Spinoaequalis.

Figure 3. Spinoaequalis. This reconstruction finds tiny upper temporal fenestrae.

Spinoaequalis 
Spinoaequalis (deBraga and Reisz 1995, Fig. 3) had small upper temporal fenestrae and a shorter temporal area. The skull is otherwise a good match for Petrolacosaurus. The tail of Spinoaequalis was distinct from all sisters in having high neural spines and deep chevrons. Spinoaequalis does not nest as a sister to Hovasaurus, a diapsid with a similar deep tail.

Petrolacosaurus
In Petrolacosaurus (Lane 1945, Reisz 1977) we find further reduction in the lateral temporal fenestra and further expansion of the upper temporal fenestra. The fingers and toes were asymmetrical, but less so than in Eudibamus and Spinoaequalis. The neck was further elongated. Interestingly, in Petrolacosaurus (Fig. 1) the two temporal fenestra are visible in lateral view.

Araeoscelis
Another araeoscelid, Araeoscelis (Williston 1910, Reisz, Berman and Scott 1984), completely infilled the lateral temporal fenestra but kept the upper temporal fenestra, which is unlike the vast majority of the other phylogenetic successors of Petrolacosaurus. On the other hand, Mesosaurus infilled the upper temporal fenestra and largely infilled the lower one, leaving only a small opening in some specimens. Restoration is difficult on many others due to crushing and scattering.

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.

References
Botha-Brink J and Modesto SP 2009.Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29(2):389-400.
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.
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.
Carroll R L 1969. A middle Pennsylvanian captorhinomorph, and the interrelationships of primitive reptiles: Journal of Paleontology, 43:1151-170.
Carroll RL 1976. Eosuchians and the origin of archosaurs; pp. 58–79 in C. S. Churcher (ed.), Athlon: Essays on Paleontology in Honour of Loris Shano Russell. Miscellaneous Publications of the Royal Ontario Museum, Toronto.
deBraga M and Reisz RR 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology 38 (1): 199–212. palass-pub.pdf
Gow CE. 1972. The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria). Journal of Zoology, London 167:219-264.
Lane HH 1945. New Mid-Pennsylvanian Reptiles from Kansas. Transactions of the Kansas Academy of Science 47(3):381-390.
Reisz RR 1977. Petrolacosaurus, the Oldest Known Diapsid Reptile. Science, 196:1091-1093. DOI: 10.1126/science.196.4294.1091
Reisz RR and Modesto SP 2007. Heleosaurus scholtzi from the Permian of South Africa: a varanopid synapsid, not a diapsid reptile.
Reisz RR, Berman DS and Scott D 1984. The Anatomy and Relationships of the lower Permian reptile Araeoscelis. Journal of Vertebrate Paleontology 4: 57-67.
Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.
Vaughn PP 1955. The Permian reptile Araeoscelis re-studied. Harvard Museum of Comparative Zoology, Bulletin 113:305-467.

wiki/Araeoscelis
wiki/Archaeovenator
wiki/Eudibamus
wiki/Heleosaurus
wiki/Mesenosaurus
wiki/Milleropsis
wiki/Petrolacosaurus
wiki/Spinoaequalis