Luperosuchus: an erythrosuchid, not a rauisuchian

Luperosuchus fractus (Romer 1972, PULR 04) was considered a indistinct pseudosuchian originally and later a rauisuchian by Desojo and Arcucci (2009). The large reptile tree recovers it as an erythrosuchid and a sister to Shansisuchus, which had an even larger subnarial fenestra. Earlier we looked at the two Shansisuchus specimens, noting that the referred specimen was much larger than the holotype with a distinct morphology, more like Luperosuchus.

Figure 1. Luperosuchus restored based on Romer 1971. Above: original drawing by Romer. Below tracing based on photo in Romer 1971, specimen PULR 04. Extension of the qj and a deeper max gives it more of a erythrosuchid look. At right is referred specimen PULR 057. Although related, the referred specimen strikes me as generically different with the low placement of the naris and large postorbital. Analysis on PULR 057 has not been done.

The reconstruction by Desojo and Arcucci (2009, Fig. 1, above) assumes a short quadratojugal, but a longer qj (Fig. 1, below) matches sister taxa.

This one is probably a rauisuchid
Another much smaller specimen (PULR 057, Fig. 1) was referred to Luperosuchus. That seems doubtful based on the lower placement of the naris, the straighter rostral profile, the larger antorbital fenestra, the deeper pmx/mx notch and the more robust postorbital. These traits appear to lead to Ticinosuchus and the aetosaurs as other archosauriformes retain a high naris. A second possibility leads toward the euparkeriid Osmolskina. A phylogenetic analysis was not attempted due to the small number of traits shown.

References
Desojo JB and Arcucci AB 2009. New material of Luperosuchus fractus (Archosauria: Crurotarsi) from the Middle Triassic of Argentina: the earliest known South American ‘Rauisuchian’. Journal of Vertebrate Paleontology 29(4): 1311-1315. 
Romer AS 1971. The Chañares (Argentina) Triassic reptile fauna. VIII. A fragmentary skull of a large thecodont, Luperosuchus fractus. Breviora 373:1-8.

Short note: ReptileEvolution.com has just passed a million hits for this year. Between 4.2 and 5.7 thousand unique visitors access the site every month.

Brachycheirotherium – probably not produced by Typothorax

A recent paper on complete Typothorax specimens (Heckert et al. 2010, Fig. 1) also raised the possibility that Brachycheirotherium tracks (Fig. 1) could be made by aetosaurs like Typothorax.

Figure 1. Typothorax from Heckert et al. 2010 along with reconstructed pes, and Stagonolepis manus and pes. Brachycheirotherium track is too narrow for this tank-like taxon and digit 4 is too long in Brachycheirotherium compared to Typothorax. The Brachycheirotherium pedes line up on digit 2. The manus track is wider, but not as wide as reconstructed in Typothorax.

Close but no cigar.
The Typothorax pes is not the best match for Brachycheirotherium tracks. Such a narrow-gauge (pedal 2 is on the midline) would seem unlikely for such a wide tank-like reptile. The Stagonolepis pes is a better match by virtue of a relatively longer pedal digit 4. Ticinosuchus (Fig. 2) is also better by virtue of its more slender body, capable of producing such a narrow-gauge trackway in which pedal digits 2 were aligned along the midline during the step cycle, one of the traits of the Brachycheirotherium track (Fig. 1).

Figure 2. Ticinosuchus overall, hand, foot and skull. Ticinosuchus nests as the ancestor taxon to aetosaurs.

References
Heckert AB, Lucas SG, Rinehart LF, Celesky MD, Spielmann JA and Hunt AP 2010. Articulated skeletons of the aetosaur Typothorax coccinarum Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA. Journal of Vertebrate Paleontology 30 (3): 619–642.

New Pterosaur Tree – Andres and Myers 2013

Pterosaur worker, Brian Andres, has produced a larger pterosaur tree than prior efforts by combining those prior efforts (Fig. 1). This was part of his 2010 PhD dissertation. The Wiki version (easier to read) can be seen here. I understand this tree will appear in a forthcoming (2014) volume entitled, “The Pterosauria.” Hope it avoids all past pitfalls, but judging by this tree (Fig. 1) and the description of the volume (“important new finds such as Darwinopterus“) we are due for another few years in the “dark ages.”

Sorry about that, kids. l’m really trying to fix things here by pointing out obvious errors.

According to Wikipedia
Andres’ phylogenetic analysis combines data mainly from three different matrixes: Kellner’s original analysis (2003) and its updates (Kellner (2004), Wang et al. (2005) and Wang et al. (2009)), Unwin’s original analysis (2003) and its updates (Unwin (2002), Unwin (2004), Lu et al. (2008) and Lu et al. (2009)) and previous analyses by Andres et al. (2005), Andres and Ji (2008) and Andres et al. (2010). Additional characters are taken from DallaVecchia (2009), Bennett’ analyses (1993-1994) and various older, non-phylogenetic, papers.

Figure 1. Click to enlarge. Pterosaur family tree produced by Andres 2010 and in press. Color added to show clades according to the large pterosaur tree (that included several specimens of certain genera and included tiny pterosaurs, lacking here). The indigo bar by Eosipterus indicates one specimen is indeed a ctenochasmatid, but the other is a germanodactylid, as we covered earlier. So, specimens are needed here, not just genera.

It’s big but still incomplete
Andres’ taxon list excludes distinct variations within certain genera, like Dorygnathus and Scaphognathus, that proved important in the large pterosaur tree. Andres’ taxon list also excludes all tiny pterosaurs. Those likewise proved important in the large pterosaur tree.

Strange bedfellows
The Andres’ tree nested several taxa that also nest together in the large pterosaur tree. However, the Andres tree also produces several nesting partners that don’t look alike. They don’t share many traits. For instance, Andres tree nests the basal anurognathid Dendrorhynchoides with the germanodactylids (Fig. 2) at the base of the “pterodactyloidea.” Few pterosaurs are so different from one another. It seems improbable that one would evolve from the other. In the large pterosaur tree this transition actually was several convergent transitions — and all sister taxa document a gradual accumulation of derived traits — not the skull-jarring leap shown here (Fig. 2).

Figure 2. According to Andres these two pterosaurs are sisters. This, obviously, is erroneous.

Andres tree also nests the dorygnathid, Parapsicephalus (= Dorygnathus) purdoni with Dimorphodon (Fig. 3). Again, these two share very few traits. Click on the links above to see and read about pterosaurs that are more similar to these two, and you’ll see what I mean.

Figure 3. According to Andres, these two pterosaurs, Dorygnathus and Dimorphoson, are close sister taxa. According to the large pterosaur tree, they nest far apart.

There are several other misfits in the Andres tree, as shown by the various color codes (Fig. 1) that represent clades in the large pterosaur tree. For instance, Andres tree doesn’t recognize that azhdarchids and chaoyangopterids evolved from protoazhdarchids, like Beipiaopterus and Huanhepterus.

Some highlights
Andres tree does not put darwinopterids at the base of the “pterodactyloidea” but between Sordes and Changchengopterus and close to Scaphognathus. This is very close to results of the large pterosaur tree (Fig. 4).

Figure 4. The evolutionary lines that gave rise to Germanodactylus and Darwinopterus according to the large online pterosaur tree by yours truly. Small changes, gradual accumulations of derived traits are shown here. No strange bedfellows.

Andres nests the Triassic Raeticodactylus and Preondactylus at the base of the Pterosauria. While MPUM6009 is ancestral to both, these nestings are not bad (Fig. 5).

Figure 5. The origin of the Pterosauria from basal Fenestrasauria. This image is not current, but will update when enlarged. Note: Euparkeria is not involved here.

Some lowlights
Andres tree nests most ornithocheirids following the pteranodontids (Fig. 6). This means large teeth reappeared on a broad rostrum evolving from a toothless sword-like rostrum. Sharp rostrum germanodactylids make better ancestors for pteranodontids and scaphognathids make better ancestors for ornithocheirids via Yixianopterus (Fig. 6). The warp in the humerus deltopectoral crest is distinct in ornithocheirids and pteranodontids and evolved convergently.

Figure 6. Click to enlarge. Left – evolutionary lineage according to Andres in which toothless pteranodontids give rise to toothy ornithocheirids. Right –  corrected lineages for each clade.

At the base of the pterosaurs Andres uses the derived erythrosuchid, Euparkeria, which shares no traits with pterosaurs. He would have been better off using a tritosaur or fenestrasaur, but chose to ignore the literature on pterosaur origins (Peters 2000, 2002, 2007).

Andres nests anurognathids with darwinopterids in a false clade, “monofenestrata” in which the naris and antorbital fenestra are supposed to be confluent, evidently based on the bogus reconstruction of Anurognathus by Bennett 2007, which we dismantled earlier here and here.

Some thoughts
Since so many pterosaurs are preserved in a crushed fashion it is imperative that they be reconstructed in order to ascertain the identification of bony elements. Few other workers attempt this and Andres is not known for doing so.

It is also imperative that pterosaur workers know what a pterosaur is. Phylogenetic analysis (Peters 2000, 2007 and online studies) and character analysis (Peters 2013) demonstrate pterosaurs are not related to archosaurs, like Euparkeria, but to tritosaur fenestrasaurs.

Finally it is imperative that pterosaur workers employ the tiny pterosaurs in their analyses. The family tree will never make sense and will never produce gradual accumulations of derived characters unless the tiny pterosaurs are included.

References
Andres BB 2010. Systematics of the Pterosauria PhD dissertation. Yale University, 2010, 366 pages; 3440534
Andres B and Myers TS 2013. Lone Star Pterosaurs. Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 103: Issue 3-4, p 383-398.
Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2013. A gradual accumulation of pterosaurian traits within a series of Lepidosauriformes. Rio Ptero Symposium 2013.

wiki/Phylogeny_of_pterosaurs 

Isochirotherium = Erythrosuchid tracks

Haubold 1983 described Isochirotherium tracks (Fig. 1) from the Early Triassic Bundsandstein Formation as “thecodonts with partial affinities to dinosaurs or crocodiles.”

Figure 1. Isochirotherium tracks matched to erythrosuchid pedes based on the longest digits from the heel are 2 and 3, unique to this clade.

Isochirotherium tracks have sub equal digits 2 and 3. The only reptiles with such a pes are derived erythrosuchids. If digit 3 is longer than 2 (X’d out in figure 1), then the track is more likely a rauisuchid or a more derived taxon. Shansisuchus and Erythrosuchus have a pes matching Isochirotherium. Primitive Garjainia does not. It has a longer pedal digit 4, like Proterosuchus and, to a less extent, Ornithosuchus.

References
Haubold H 1983. Archosaur evidence in the Buntsandstein (Lower Triassic). Second Symposium on Mesozoic Terrestrial Ecosystems, Jadwisin 1981. Acta Palaeontologica Polonica 28 (1-2):123-132.

New big Shansisuchus – close to Garjainia

Shansisuchus is a small erythrosuchid (Fig. 1) immediately identified by its very large subnarial fenestra between the naris and the antorbital fenestra. It looks like it has two antorbital fenestra, one before the other. The holotype (IVPP V2503,Young 1964) is a little less than 2 m in length.

Figure 1. Erythrosuchids. Click to enlarge. Long-legged Vjushkovia was basal to rauischids and the line that led to archosaurs.

A new specimen (Wang et al. 2013) from the Late Middle Triassic is quite a bit larger and more like Garjainia, it’s sister taxon.

Figure 2. The holotype Shansisuchus (below) and the new one (above) both to scale. The 9 cervicals are green. The estimated 19 dorsals are red. The big intercentra are yellow. Those are pretty big cervical ribs. This reptile was big overall, seemingly a new genus nesting close to Shansisuchus.

Wang et al. (2013) note the premaxilla bears 6 teeth, unique among erythrosuchids. The naris is close to the midline and rather small. The skull is very narrow, as in other erythrosuchids.

In my opinion, the two specimens are not conspecific and perhaps not congeneric.

Wang’s phylogenetic analysis includes several taxa that just don’t belong including, Mesosuchus and Dimorphodon (lepidosaurs), Vancleavea (thalattosaur) and Chanaresuchus (pararchosauriform). Otherwise the tree echoes the the large reptile tree (and most every other study) in nesting Shansisuchus with erythrosuchids (Garjainia not included and close to Fugusuchus, Osmolskina and Euparkeria.

These erythrosuchids appear to have been like hippos of the Triassic, but carnivorous. It would be interesting to know if the new specimen bore large running legs or short wading ones. Recently we looked at the resemblance of the basal erythrosuchid Garjainia to Youngina, the phylogenetic ancestor. Youngina belongs at the base of archosauriform analyses, not Mesosuchus.

References
Wang R-F, Xu S-C, Wu X-C, Li C and Wang S-Z 2013. A New Specimen of Shansisuchus shansisuchus Young, 1964 (Diapsida: Archosauriformes) from the Triassic of Shanxi, China. Acta Geologica Sinica (English Ed.) 87(5):1185-1197.
Young C-C 1964. The pseudosuchians in China: Palaeontologia Sinica 151, new series C., 9:1-205.

wiki/Shansisuchus

What about those really BIG Rotodactylus tracks?

While virtually all Rotodactylus (Peabody 1948) tracks (digitigrade, proximal phalanges elevated, long stride, narrow gauge manus / wider pes, occasionally bipedal, first digit impresses at tip only, fifth digit impresses far behind the others, extremely variable speed) are the right shape to fit the Cosesaurus  (Ellenberger and Villalta 1974) pes, as we learned earlier here, some Rotodactylus tracks are BIG (4-5 cm length)! That’s way too big for Cosesaurus to fill. So the search is on for something like Cosesaurus, but far bigger and wide ranging (Fig. 1). Rotodactylus tracks have been found across Europe and the western USA and they range across the Early to Middle Triassic.

Figure 1. Click to enlarge. Scaling a quadrupedal Cosesaurus to the larger Rotodactylus tracks from Haubold 1983. Quadrant represents center of balance in the closeup foot showing how pedal digit 5 made those posterior impressions with a claw mark (Peters 2000). Graphic representation of a butt joint is nearby. The actual Cosesaurus is much smaller than these trackmakers. I enlarged the coracoid on the larger hypothetical trackmakers because they were not bipedal flappers. This configuration of pedal digit 5 is often preserved in basal pterosaurs.

So, after touting the perfect match of Cosesaurus to Rotodactylus tracks (Peters 2000), this is the first time I’ve conformed Cosesaurus to a quadrupedal pose to match these much larger tracks from the Early (=Lower) Triassic (Solling and Röt formations. Scythian/Anisian) of Germany. Haubold (1983) likened Lagosuchus (Maraschus), but  that’s not as good a match as Cosesaurus and Langobardisaurus, which were not so well known or described in the early ’80s.

So, Rotodactylus tracks are not archosaurian, but proto-pterosaurian, fenestrasaurian.

Figuure2. Cosesaurus matched to Rotodactylus from Peters 2000.

Haubold listed 4 points that were significant in the development of archosaurs:

1. “Reduction of the manus as [a] function of bipedalism;
2. Stride length in relation to width of trackway and pace angulation (small trackway pattern) as a function of semierect to erect gait;
3. Reduction of pes digits 1 and 5 as a function of tridactylism (this point is unique in Rotodactylus, which impresses  digit 5 far behind the others).
4. The cross axis of the pes and the outward orientation of the pes axis to the direction of movement. A more rectangular cross axis may demand a mesotarsal joint.”

Figure 3. Click to enlarge. Cosesaurus and Rotodactylus, a perfect match. Elevate the proximal phalanges along with the metatarsus, bend back digit 5 and Cosesaurus (left) fits perfectly into Rotodactylus (right).

Rotodactylus tracks show extreme speed variation, which is rare for reptiles, but compliments the higher metabolic niche of fenestrasaurs.

By assigning Rotodactylus tracks to basal bipedal archosaurs, Haubold made the same hopeful mistake that Brusatte et al. (2011) and Niedzwiedzki et al. (2013) made assigning Rotodactylus tracks to  Lagerpeton. These workers hoped it was transitional to dinosaurs, but the match was poor, both phylogenetically and morphologically. The better match is between Cosesaurus and Rotodactylus (Peters 2000, Fig. 3).

So, what about those really BIG Rotodactylus tracks? They were made my really big mostly quadrupedal cosesaurs, evidently. And evidently, only the little cosesaurus were better bipeds, capable of flapping.

Figure 4. Rotodactylus from Haubold 1983 adapted from Peabody 1948. Unfortunately, no reptiles have a rotated and reversed pedal digit 5. But note the resemblance of the conjectural trackmaker to Cosesaurus, unknown in 1948. Note: most reptiles while moving do not have all four limbs on the  ground at one time. The elongated pedal digit 5 shown here is likely a drag mark. Size of these prints: between 4 and 5 cm in length, about the size of the examples in figure 1. Note, no claw marks on pedal digit 5.

So, widespread Rotodactylus tracks demonstrate that cosesaurs were widespread. They also appeared in a variety of sizes. While the large ones remained quadrupedal, like ancestral macrocnemids, the small ones became increasingly bipedal. This radiation of tritosaur lizards preceded the radiation of squamates in the Jurassic and later epochs.

References
Brusatte SL, Niedz´wiedzki G and Butler RJ 2011. Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic. Proceedings of the Royal Society B, 278, 1107–1113.
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Haubold H 1983. Archosaur evidence in the Buntsandstein (Lower Triassic). Second Symposium on Mesozoic Terrestrial Ecosystems, Jadwisin 1981. Acta Palaeontologica Polonica 28 (1-2):123-132.
Niedzwiedzki G, Brusatte SL and Butler RJ 2013. Prorotodactylus and Rotodactylus tracks: an ichnological record of dinosauromorphs from the Early–Middle Triassic of Poland. Geological Society, London, Special Publications, first published April 23, 2013. doi 10.1144/SP379.12
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
wiki/Cosesaurus

The reduction and duplication of the naris in certain pterosaurs

Most pterosaur paleontologists don’t bother with such details.

The reduction (not assumed confluence) and duplication of the naris in derived pterosaurs (Fig. 1) is not due to neotony (contra Unwin and Lü 2013), but due to phylogeny. Along with phylogenetic size reduction the naris became reduced (Fig. 1) in several pterosaur lineages. Evidently, as in certain birds, like gannets, the naris became less important for certain pterosaur lines.

This is at odds with the origin of pterosaurs when the naris became greatly enlarged compared to outgroups like Cosesaurus. Pterosaurs must have spent more time mouth-breathing, or — in the switch from insect-eating to fish-eating — evolved a way to avoid getting water up the nose.

Figure 1. Click to enlarge. The reduction of the naris (red arrow), the appearance of the secondary naris, and the appearance of the secondary ascending process of the maxilla in a line of scaphognathids, all to the same scale. N0. 12 is basal to germanodactylids and then beyond to pteranodontids, dsungaripterids and tapejarids, most of which lose the naris. The GMu specimen is basal to ornithocheirids and cycnorhamphids. Scale bar = 1 cm.

And along with the reduction and duplication of the naris, these pterosaurs develop a secondary ascending process in the maxilla (blue arrow), always small. There’s even an ascending process of the jugal (light blue bone) laminated against the ascending process of the maxilla. Yes, the naris does eventually disappear in certain taxa, but the bones around it give away its former position.

 

In all cases but one, the reduction of the naris involved the overall reduction of the pterosaur, regardless of rostrum length. That one case is the lineage of Jianchangnathus,  Pterorhynchus and darwinopterids, which we last discussed here.

 

The secondary naris that frequently develops is nothing more than a tiny hole that did not contribute to respiration. So don’t freak out about the concept.

When you have a good phylogenetic tree, you can see how details like this evolve over time and taxa. Removes the mystery, which some folks still don’t appreciate.

Herrerasaurus, the killer and its reinforced skull

In today’s post we’ll look at Herrerasaurus, a very early and basal dinosaur with several unique (autapomorphic) traits that made it a very efficient killer.

In many ways, the basal dinosaur Herrerasaurus was unlike anything that came before or since. At the time, Sereno and Novas (1993) sought to compare these traits to Lagerpeton, Marasuchus and Staurikosaurus, but no skull material was known for these taxa. Here Turfanosuchus and Eoraptor (Fig. 3) are compared to Herrerasaurus. With that reinforced rostrum and scissor-like fanged jaws, Herrerasaurus was indeed a unique sort of basal dinosaur and an excellent killer.

Figure 1. Click to enlarge. Herrerasaurus skull in several views. See Figure 2 for color overlays. Note the very robust and reinforced ascending process of the maxilla and the lack of an antorbital fossa (see figure 3 for comparable skulls with a deep fossa).

According to Sereno and Novas 1993:
1. Broad posterolateral process of premaxilla. Sereno and Novas compared this trait to ornithischians, but according to the large reptile tree no intervening taxa, like Eoraptor (Fig. 3), share this trait. So, as described earlier here, this may be a misinterpretation based on anterior extensions and reinforcements of the maxilla that were not noticed by Sereno and Novas. In the new interpretation, no other dinosaur has such a laminated and widened ascending process of the maxilla that practically erases the underlying antorbital fossa found in all sister taxa (Fig. 3), except the closely related Sanjuansaurus.

2. Premaxilla / maxilla fenestra – with the rearrangement of the premaxilla/maxilla border this is now the maxillary fenestra found in a wide variety of theropodss. Considering the long antorbital fenestra and short fossa in the outgroup, Turfanosuchus, the maxillary fenestra may represent the remains of the anterior portion of the antorbital fenestra after a new ascending process or broad ossification divided the antorbital fenestra. Remains to be seen with more discoveries.

Figure 2. Click to enlarge. Herrerasaurus with bones colorized using DGS and with reference to the Digimorph skull. The rostrum was heavily reinforced with extra bone and the narrow jaws were like saw-tooth scissors on flesh.

3. Narrow, u-shaped anterior antorbital fossa – Actually the fossa, though shallow, continues far forward to include the maxillary fenestra. What Sereno and Novas saw was just the part that peeked out beyond the overlying laminations (covered below).

4. Broad supratemporal depression – Better described as a constriction of the original, more open supratemporal fenestra (see Turfanosuchus, Fig. 3). Eoraptor (Fig. 3) shares this trait and was likewise described in 1993. Pratically this addition of bone creates more anchor for stronger jaw muscles, which go along with the reinforced rostrum and extra long teeth.

5. Jugal ridge – unique, but really only visible on one side of the skull. Practically this reinforces the otherwise gracile jugal (Fig. 3) for greater strength.

6. Narrow dorsal portion of laterotemporal fenestra – This is a relic from the crocodylomorpha condition seen in Turfanosuchus and Decuriasuchus and lost in Eoraptor (Fig. 3). In Herrerasaurus the quadratojugal is rotated to align more with the quadrate but the squamosal does not rotate at the same phylogenetic “speed” and does not become so gracile until Eoraptor.

Figure 3. Herrerasaurus close kin, Turfanosuchus (more basal) and Eoraptor (more derived). Note the deep antorbital fossa in each skull. Also note the reduction of the squamosal and quadratojugal. Herrerasaurus represents a transition between these two, yet unique in its own way.

7. Subquadrate ventral process of squamosal with lateral depression – Unique, but transitional as described above.

8. Quadratojugal overlapping posterior side of quadrate shaft – Unique. Again this further strengthens the skull, this time at the jaw joint.

9. Basal tuber and occipital condyle subequal in width – Practically this strengthens the neck / skull joint.

10. Slender, ribbed posterodorsal dentary process and slotted surangular – Unique. Practically this further strengthens the jaw by laminating and interlocking these bones.

And now I’d like to add a few autapomorphies found in this study.
1. Anterior process of maxilla overlaps premaxilla. – Practically this strengthens the rostrum.

2. Robust and long ascending process of the maxilla, infilling the antorbital fossa and reducing the antorbital fenestra to the posterior rostrum only. – Practically this strengthens the rostrum.

3. Nasals extend anteriorly to base of naris. – Practically this strengthens the rostrum.

4. Narrow mandibles in contact for anterior half of their length. – Practically this strengthens the mandible.

5. Tab-like processes of the parietal overlap posterior frontals. – Practically this strengthens the cranium.

6. Dorsal process of jugal extends nearly to squamosal. – Practically this strengthens the postorbital region.

7. Mid nasal depression. – Not sure what’s going on here. It’s not visible on drawings or head-on photos of the skull in Sereno and Novas (1993), but is certainly visible on the rotating Digimorph imagery. This depression adds rugosity to the skull and may serve to redirect impact forces to the sides and rear of the skull, avoiding the midline suture. The bone is so thin in this depression a perforation exists in the fossil.

Bottom line,
Herrerasaurus was one tough dinosaur, reinforced in several ways for a tougher bite, with bigger teeth, stronger jaw muscles, a reinforced mandible and laminated rostrum. That ascending process of the retroarticular process at the back of the mandible helped prevent the jaw from dislocating during head-on impacts.

References
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

wiki/Herrerasaurus

The Lagerpeton pelvis – what it tells us

Figure 1. Chanaresuchids to scale, including Tropidosuchus and Lagerpeton.

Lagerpeton (Fig. 1) is widely and erroneously considered a dinosaur ancestor in various phylogenetic analyses (i.e. Nesbitt 2011). We looked at Lagerpeton relations earlier here and here. Today we’ll compare pelves (Fig. 2).

Figure 2. Click to enlarge. Lagerpeton pelvis compared to its close kin, Tropidosuchus, Chanaresuchus, Parasuchus and Diangongosuchus, in order of increasing distance. These are all pararchosauriformes, unrelated to dinosaurs and their ancestors — until you get to  Youngina, their last common ancestor. These pelves are much more alike than Lagerpeton is to any of the dinosaurs and dino ancestors in figure 3. Dorsal view of Lagerpeton based on other views and descriptions. All these pelves are short and broad. Note from Chanaresuchus on the acetabulum is open posteriorly. 

and the more popular candidates, according to Nesbitt 2011.

Figure 3. The pelvis of Marasuchus, Turfanosuchus, Arizonasaurus, Euparkeria, Silesaurus and Dimorphodon. Among these, Dimorphodon is clearly the most different, because it’s a lepidosaur in reality. Among the rest the posterior acetabulum is always closed. 

Comparisons and Trees
Nesbitt nested Lagerpeton between pterosaurs (like Dimorphodon, Fig. 3) and Marasuchus, but their pelves do not share more traits than the pararachosaurs shown in figure 2. Not sure why recent analyses don’t see this, other than the fact that they fail to include younginids and choristoderes, taxa that nest at the base of the pararchosauriformes in the large reptile tree. And they typically include pterosaurs, which may act like wild cards or jokers in the deck.

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
Bonaparte JF 1994. Dinosaurios de America del Sur. Impreso en Artes Gráficas Sagitario. Buenes Aires. 174pp. ISBN: 9504368581
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
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

Basal Dinos: Fusion of the Postfrontal and Postorbital Documented

Dinosaurs and higher crocs share the trait of a fused postorbital (PO) and postfrontal (POF). Almost universally the postfrontal portion is ignored. T fused bone is simply called the postorbital. But the postfrontal is not absent. The postfrontal doesn’t shrink into nothingness. It fuses to the postorbital, so it’s still there!

Basal archosauriformes, basal crocodylomorphs, like Gracilisuchus, and basal dinosauromorphs, like Turfanosuchus, do not fuse these two bones. But most crocs and most dinos do fuse these bones.

So, the next step up the dino phylogenetic line might show some vestige of the suture, as it appears to do in Herrerasaurus (Figs. 1, 2) and Pampadromaeus (Fig. 3) for which photos are known. Only drawings are published of the pertinent basal crocs, like Terrestrisuchus.

Figure 1. Herrerasaurus has a fused postfrontal/postorbital. Here both sides are shown as if they were both left sides. The postfrontal and postorbital portions are colorized here. Image courtesy of digimorph.org.

And here (Fig. 2) they are without the color overlay.

Figure 2. Herrerasaurus postfrontal/postorbital. Can you see the incomplete fusion?

Figure 3. Postorbital/postfrontal of Pampadromaeus. More fusion here as these two bones remain together after disarticulation. And here also you can see how sutures can be easily misinterpreted as bone cracks and vice versa.

If we look closely and ignore the cracks (they are very confusing here), we can just make out the former outlines of the postfrontal / postorbital suture. Those become less visible on more derived dinos.

The same situation appears in Pampadromaeus, a primitive phytodinosaur. Eoraptor is too broken up in that area to observe sutures.

So, no big discoveries here other than to maybe call the bone what it truly is, the fused postfrontal / postorbital.

References
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

wiki/Herrerasaurus
wiki/Sanjuansaurus