Jucaraseps – tiny sister to snake ancestors living later than the earliest known snakes

This post was originally posted December 6, 2014 and removed as a courtesy to Sebastian Apesteguia, who mentioned their team came to a similar conclusion with regard to snakes and is awaiting publication December 29. This is a case of convergent discovery and attests to the validity of methods used here. 

A few years ago
Bolet and Evans (2012) introduced us to one of the smallest fossil lizards ever found, Jucaraseps grandpes (Fig. 1).

Figure 1. Jucaraseps in situ. Click to enlarge. This tiny long lizard is in the lineage of terrestrial snakes. Note the actual size of this lizard is about a third of that seen here at 72 dpi.

from the Bolet and Evans 2012 conclusion:
“Jucaraseps lies at the lower end of the modern lizard size range and demonstrates that very small-bodied lizards existed in the Early Cretaceous. Its slender, elongated body is reminiscent of many small living scincid lizards and some gymnophthalmids, and Jucaraseps is likely to have had a similar lifestyle. Phylogenetic analysis places Jucaraseps on the stem of a traditional monophyletic Scleroglossa (sensu Estes et al. 1988 and Conrad 2008; contra Townsend et al. 2004 and Vidal and Hedges 2005), which would be consistent with the frequency of body elongation ⁄ limb reduction within this group. However, further material (especially of the skull) may lead to a refinement of this position, as may more global molecular ⁄morphological analyses of Squamata as a whole.”

That basal position in the Conrad 2008 tree nests Jucaraseps with Eichstaettisaurus (Fig. 2) in the Bolet and Evans (2012) tree. The large reptile tree (still not updated)  nests Jucaraseps and Eichstaettisaurus with Adriosaurus and snakes. Note the similarity between Adriosaurus and Jucaraseps, especially in the rib shape This shape is carried forward in the basal snake, Pachyrhachis.

Figure 2. Snake ancestor sisters to two scales. Above Jucaraseps with Eichstaettisaurus. Below both with Adriosaurus.

An important abstract on snake origins: Caldwell, et al. 2014.
“Previous reports identifying the oldest known fossil snake specimens were based on isolated vertebrae from sediments in North Africa and North America (Albian to Cenomanian: ~105-100 Ma). Bathonian (Middle Jurassic; 167 Ma) to Barremian (Lower Cretaceous, 143 Ma) squamate fossils from Colorado, Portugal and England are here recognized as the geologically oldest known fossil snakes, extending the fossil record of snakes by approximately 70 million years. The cranial, dental and postcranial anatomy of these well-preserved but fragmentary fossil snakes indicates that the basic architecture of the modern snake skull and dentition had evolved as early as 167 million years ago.

“These oldest fossil snakes show unmistakable features of the snake skull (e.g., recurved
teeth with labial and lingual carinae, teeth attached to margins of distinct alveoli with interdental plates, long toothed suborbital ramus of maxillae, laterally emarginated dentary, well developed descensus frontalis with suboptic shelves). There are also several lizard-like features (e.g., pronounced subdental shelf/gutter, multiple mental foramina on the dentary) as would be expected in early snakes not long after their divergence from a lizard ancestor. These vertebrae show critical similarities to much younger Mesozoic snakes such as Coniophis, Dinilysia, Najash, Pachyrhachis, Simoliophis, etc., and to all extant snakes (e.g., wide neural arch, low neural spines, prominent arcual ridges well developed zygosphenes, zygosphenal tectum with festooned anterior margin, condyles and cotyles are circular and offset from centrum ventral margins that are strongly rectangular in outline, with a squared margin immediately anterior to the condyle). The paleobiogeography (e.g., islands in epicontinental seas and continental Laurasia) and paleoecology (e.g., coal swamps, lacustrine and fluvial systems) of these earliest snakes is diverse and complex, and suggests that snakes had already undergone significant habitat differentiation and geographic radiation by the mid-Jurassic. Phylogenetic analysis recovers these early snakes as basal to all other snakes (fossil and modern). The snake origins debate, both in terms of ancestral forms and environments, is strongly impacted as these most ancient snakes show unexpected anatomies and paleoecologies.”

That will be a very interesting paper! 
Jucaraseps lived in the Early Cretaceous, but sister taxa must have lived in the Jurassic, prior to the origin of the most primitive snakes. Jucaraseps contemporaries include many protosquamates. Jucaraseps is thus a rare representative of the squamates in the Early Cretaceous.

References
Bolet A and Evans SE 2012. A tiny lizard (Lepidosauria, Squamata) from the lower Cretaceous of Spain. Palaeontology 55:491-500.
Caldwell M, Nydam R, Palci A and Apesteguía S 2014. The oldest known fossil snakes: a tempera range extension of 70 million years. Journal of Vertebrate Paleontology abstracts.

Miscellaneous year end stuff…

Just a few odd and ends as we near the the end of the year.

Here’s something from our guru:
Dr. Bob Bakker’s studies on Placerias locomotion (Fig. 1) at the Houston Museum of Natural History blog site.

Figure 1. Flexibility study of Placerias by Dr. Bob Bakker at the HMNS blog site. Click to view site.

These were produced to “add life” to a skeletal mount and a fantastic illustration by Julius T. Csotonyi of Placerias chased from its watering hole by Smilosuchus.

On YouTube
Apple chief evangelist Guy Kawasaki talks about what he learned working for Steve Jobs.

Figure 2. Guy Kawasaki gives a TED talk, what he learned working for Steve Jobs. Here he lists: “Don’t let the bozos grind you down.” Click to view.

Kawasaki’s TOP 12

1. “Experts” are clueless. Listen to your heart. When you encounter naysay, go against the naysay. (Remember inspirational talks like this usually take on this David vs. Goliath attitude. My advice: pick and choose by testing. Much of what experts say is right on the mark. But look for red flags and strange bedfellows. And don’t let the bozos grind you down.)
2. Customers cannot tell you what they want. (innovation leapfrogs the desires and paradigms of the masses, in this case reliance on textbooks, rather than testing)
3. The action is on the next curve. That’s why reptile evolution.com tested (on this date 12/29/14) 476 taxa vs. 228 characters, far more than any prior study. Yes, the experiment.com taxon goal has been reached (although the funding goal will not be reached at the present rate, 27 days to go, 99% unfunded).
4. Biggest challenges beget the best work. In our case, more taxa (=the biggest challenge) reduce morphological distance between taxa, provide more nesting opportunities for enigmas and previous ‘by default’ nestings. The large reptile tree has been a big challenge, and I’ve learned  a lot along the way over the last four years. Other than my kids, the large reptile tree represents my best work and my purpose in life.
5. Design counts. (Shiny, thin aluminum is more appealing than clunky black plastic, according to Kawasaki). Hopefully the accuracy (whenever possible) and design of the website and skeletal graphics have attracted a certain amount of interest.
6. Use big graphics and big fonts. (a suggestion for PowerPoint presentations.)
7. Changing your mind is a sign of intelligence. Getting it wrong and making it right is okay! That’s good Science. I do this (get it wrong and make it right) all the time.
8. Value is not equal to price. (more for marketing).
9. A players hire A+ players. (for employee acquisition).
10. Real CEOs demo. They put their neck on the block and their ass on the line.
11. Real entrepreneurs ship. They ship, then they test. Kawasaki says, “Don’t worry, be crappy.” Put out the innovation, the revolution, then fix the minor problems. I wish this wasn’t true in my case, but sometimes it is. Finding a problem is disheartening. Fixing a problem is a relief.
12. Marketing = unique value. Be in the upper right hand corner of the chart: uniqueness and value, not just valuable, not just unique.
13. Bonus: some things need to be believed to be seen. Foster the belief in what you dream in order to make it a reality. The opposite is also true, some things cannot be seen unless you open your mind to the possibility. In Science testing lots of taxa that everyone ‘knows’ are not related ultimately provides a family tree in which every taxon is related.

I recently had a paper rejected.
Among the objections were “the results are unconventional” and “I don’t believe you can get complete resolution for 360 taxa with only 228 characters.” Well, now the total is 476 taxa with only 228 characters and the matrix shows no signs of slowing down.  In theory, theory and practice should produce identical results. In practice they often do not. “Unconventional” is not a bad thing. You can’t shed new light on a subject without changing conventional thinking. A discovery, by definition, breaks convention. And, of course, ‘belief’ is the prevue of religion and politics. Even though the referee tested the work and confirmed the results, he didn’t let Science trump Belief. And he played his ‘Belief’ card, so now we all know which side of the brain trumped the other.

In the new year
the Pterosaur Heresies and ReptileEvolution.com will continue to promote new discoveries and, when necessary, shed new light on old discoveries. Thank you for your interest and support.

There’s more to come.

 

 

 

 

Oedaleops: why the post-cranial traits weaken the synapsid nesting

A recent paper
by Sumida et al. 2014 gives us our first look at the post-crania of Oedaleops (Fig. 1). That’s fantastic as many of Oedaleops’s sisters are also known from skulls only. Here is the abstract. Notes to follow are numbered in parentheses.

From the abstract: The Early Permian amniote Oedaleops is generally considered to be one of the basalmost pelycosaurian-grade synapsids (1). Thus it occupies a key position for understanding the phylogenetic relationships of basal synapsids (1) specifically and basal amniote interrelationships more generally. This assessment has until now been based almost exclusively on the remains of a single skull from the Lower Permian Cutler Formation of north-central New Mexico. The identification of additional cranial as well as numerous postcranial elements of at least three additional individuals now permits a more complete understanding of its anatomy and allows the first attempt at a partial body reconstruction of this basal pelycosaurian-grade synapsid. Oedaleops is confirmed as an extremely basal synapsid taxon, but the addition of postcranial data from Oedaleops to data matrices of earlier phylogenetic analyses unexpectedly weakens, as opposed to strengthens, support for the hypotheses of a monophyletic Eothyrididae (2).

Notes:
(1) In the large reptile tree Oedaleops and the Caseasauria nest as derived from millerettids, far from synapsids.

(2) Adding the post-cranial traits attributed to Oedaleops cements its place within the Diadectes/Casea clade derived from Milleretta. Synapsids were not the only clade to develop a lateral temporal fenestra, as everyone knows.

Figure 1. Oedaleops with newly recovered post-crania to scale. These new traits are also derived from millerettids, not synapsids. The postcrania, to know one’s surprise, is short-legged and bulky.

References
Langston W 1965. Oedaleops campi (Reptilia: Pelycosauria), a new genus and species from the Lower Permian of New Mexico, and the family Eothyrididae. Bulletin of the Texas Memorial Museum 9: 1–47. online pdf
Sumida SS, Pelletier V and Berman DS 2014. New information on the basal pelycosaurian-grade synapsid Oedaleops. Vertebrate Paleobiology and Paleoanthropology 2014:7-23.
wiki/Oedaleops

The antorbital and lateral temporal fenestrae of the frog , Rana

Earlier we looked at the evolution of the frog, Rana. And it continues to be the most popular blog post of the past year.

Today, after adding Rana to the matrix of the large reptile tree (still not updated), I think it’s time we looked at the antorbital fenestra of Rana, and the lateral temporal fenestra as well (Fig. 1).

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

One usually thinks of additional skull fenestrae in the province of reptiles. As we saw earlier, the antorbital fenestra comes and goes in several reptiles. So does the lateral temporal fenestra. Amphibians (non-amniote tetrapods) typically do not have skull fenestrae. Neither to most basal reptiles.

Relative to the body, the skull of Rana is enormous. So are the hind limbs. Frogs leap, as everyone knows, and if the skull is going to be large it also has to be lightweight to enable longer leaps. So the skull bones are reduced to their bare minimum creating fenestrae.

Proximal outgroup taxa, including long-legged Triadobatrachus, likewise have reduced skull bones.

More distant outgroup taxa, including short-legged Gerobatrachaus and Doleserpeton and Utegenia have relatively smaller skulls and shorter hind limbs — and no skull fenestrae.

 

 

Another turtle with teeth, Elginia – part 3

Earlier here and here we looked at the ancestry of turtles and changes to the labels on the turtle skull. In the next few posts we’ll take a deeper look at Elginia, Meiolania, Odontochelys and Proganochelys, basal turtles that illuminate relationships and bone labels. I encourage you to flip back and forth to compare images or drag an image or two to your desktop for ready comparison.

Figure 1. The skull of Elginia in four views. The basipterygoid and basiocciput are missing. While most skull bones are fused, colors are applied here that delineate the bones. Note the squamosal (lavender) in the central cheek is surrounded by other bones, especially the supratemporal that descends to the quadratojugal as in Meiolania (Fig. 2).

Apparently the loss of teeth in turtles occurred at least twice, once in the clade of living turtles and once in this clade of horned turtles.

Figure 2. The skull of Meiolania platyceps as restored by Gaffney 1983 (in black). Color overlays delineate skull bones identified here. Note the squamosal retains its concave posterior rim while the supratemporal descends to articulate with the quadratojugal, increasing the armor coverage of the neck while retaining an eardrum aperture. Distinct from living turtles, the lacrimal contacts the naris (Fig. 3).

Perhaps the key to understanding turtle skull morphology is the placement of the squamosal, which retains its ancestral shape and placement in horned turtles while the greatly enlarged supratemporal provided a new posterior rim to the skull. I don’t think this has been recognized before. Once that identification is made, the rest of the bones fall into place.

Figure 3. The skull of Meiolania platy ceps in several views from Gaffney 1983. Color overlays identify bones. Note the separation of the postfrontal and postorbital along with the lacrimal naris contact.

Side-by-side comparisons of Elginia and Meiolania (Fig. 4) make the case for homology and close relationship.

Figure 4. Elginia is a toothed turtle, basal to the giant horned toothless turtle, Meiolania. The former has teeth. The latter does not. The former is known from a skull only. The latter is known from several complete skeletons complete with a carapace, plastron and armored, club tail.

Sclerosaurus is another horned reptile, now nesting at the base of the all turtles (Fig. 5). It had a low wide body without a carapace and plastron, but with a pattern of dermal ossicles. Apparently supratemporal horns were primitive and later lost in the clade of living turtles and their prehistoric ancestors.

Figure 5. Traditionally considered a horned procolophonid, Sclerosaurus now nests at the base of all turtles, both horned and hornless. Note the dermal ossicles, unknown in other procolophonids. Evidently horns were lost in the lineage of living turtles.

Figure 6. Bunostegos, Elginia and Meiolania to scale showing the origin of hard shell turtles.

Ironically several ancestors of turtles
have been known for quite some time. They just have not been included in phylogenetic analyses together and in a large enough analysis to eliminate all other possibilities.

References
Gaffney ES 1983. The cranial morphology of the extinct horned turtle, Meiolania platyceps, from the Pleistocene of Lord Howe Island, Australia. Bulletin of the AMNH 175, article 4: 361-480.
Gaffney ES 1985. The cervical and caudal vertebrae of the cryptodiran turtle, Meiolania platyceps, form the Pleistocene of Lord Howe Island, Australia. American Museum Novitates 2805:1-29.
Gaffney ES 1996. The postcranial morphology of Meiolania platyceps and a review of the Meiolaniidae. Bulletin of the AMNH no. 229.
Newton ET 1893. On some new reptiles from the Elgin Sandstone: Philosophical Transactions of the Royal Society of London, series B 184:473-489.
Owen R 1882. Description of some remains of the gigantic land-lizard (Megalania prisca
Owen), from Australia. Part III.Philosophical Transactions of the Royal Society London, series B, 172:547-556.
Owen R 1888. On parts of the skeleton of Meiolania platyceps (Owen). Philosophical Transactions of the Royal Society London, series B, 179: 181-191.

Another turtle with teeth, Elginia – part 2

This post was updated February 17, 2015 with the addition of more turtle taxa to the large reptile tree and these clarified relationships.

Yesterday we opened the door to a new chapter in turtle research by introducing Elginia, known from a horned skull with teeth, as a sister to Meiolania, the horned toothless turtle.

Figure x. Bunostegos, Elginia and Meiolania to scale showing the origin of hard shell turtles.

Earlier the large reptile tree nested the large Early Permian millerettid, Stephanospondylus, as a turtle ancestor. And it remains so, just a little more distantly related now (Fig. 1).

Figure 1. A subset of the large reptile tree showing the ancestry and relationships of basal turtles. Milleretta has costal ribs. Bolosaurs are known only from skulls. Stephanospondylus was considered a diadectid. Sclerosaurus and Scolopharia were considered procolophonids. Elginia was considered a pareiasaur.

Adding Chelonia and Meiolania to the large reptile tree and changing a few character scores with a new understanding of turtle skull morphology changes the tree topology slightly.

As before Stephanospondylus is still basal to pareiasaurs and turtles (Fig. 1).  Now the horned pareiasaurs, Elginia and Sclerosaurus (Fig. 2) nest as a turtle and as a pre-turtle respectively.

One of the problems we’ve had in turtle ancestry research may be the traditional misidentification of several turtle skull bones. With the present family tree, however, the homologies become more clear. Here (Fig. 2) is an alligator snapping turtle (Macrochelys temminckii) skull from the Udo Savalli, Arizona State U. website with traditional bone labels.

Figure 2. Snapping turtle skull with traditional bone labels from the Udo Savalli, Arizona State University website. See figure 3 for revised labels.

With the new understanding of turtle bone homologies here (Fig. 3) are the alternate labels.  The large changes include: the old squamosal is the new supratemporal; the old quadratojugal is the new squamosal, the postfrontals are not fused to the postorbitals; the prefrontals are not fused to the nasals; the frontals are fused and tiny, much smaller than the parietals; and the tabulars are retained on the dorsal surface of the skull.

Figure 3. Alligator snapping turtle skull with new bone labels along with color overlays for bone dimensions. These bone labels are closely comparable and homologous with those of horned turtles, like Elginia, and pareiasaurs. 

Another turtle with teeth, Elginia (Newton 1893)

Several years ago
the world of paleontology was delighted to find a turtle with teeth, Odontochelys. Ironically, we’ve known about a turtle with teeth for over 120 years without realizing it.

Adding
the horned turtle, Meiolania (Owen 1882, 1888), to the large reptile tree (still not updated) was the key to realizing that Elginia (Newton 1893), which is known from a skull likewise festooned with spikes and horns, is an unrecognized turtle with teeth. These two represent a clade separate from the main turtle clade, which includes Odontochelys, Proganochelys and Chelonia, the living green sea turtle.

Figure 1. Elginia is a toothed turtle, basal to the giant horned toothless turtle, Meiolania.

Elginia was long considered an odd sort of pareiasaur, a close outgroup to the turtles. Evidently, like Clark Kent and Superman, these two have never been tested together in phylogenetic analysis. Same old story retold again.

We’ll look at the details over the next few blog posts.

References
Newton ET 1893. On some new reptiles from the Elgin Sandstone: Philosophical Transactions of the Royal Society of London, series B 184:473-489.
Owen R 1882. Description of some remains of the gigantic land-lizard (Megalania prisca
Owen), from Australia. Part III.Philosophical Transactions of the Royal Society London, series B, 172:547-556.
Owen R 1888. On parts of the skeleton of Meiolania platyceps (Owen). Philosophical Transactions of the Royal Society London, series B, 179: 181-191.

The ancestry of the little biped, Scleromochlus

Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus, which appears to be the chubby, short-legged, secondarily quadrupedal one. 

Once miniaturization and bipedal locomotion appeared in a sister to Lewisuchus (Fig. 1), evolution provided a variety of bipeds in basal archosaurs in a variety of sizes. These (Fig. 1) represent one such clade. Another clade derived from a sister to Lewisuchus produced poposaurs. Yet another produced, Litargosuchus, Dibothrosuchus and extant crocodilians. Still another produced Pseudhesperosuchus and basal dinos.

The large reptile tree is now up to 460+ taxa, still working with the same 228 character list.

Synapsid manus and pedes study

A recent online paper by Kümmel and Frey (2014) describes the mobility of the ‘thumb’ and medial toe in non-mammalian Synapsida and one extant mammal.

Figure 1. Manus of Galesaurus, a basal cynodont synapsid. PILs added. This is where grasping first emerged, later dropped by many later mammal clades, but retained by primates and other arboreal forms.

From their abstract
The the reduction of autopodial rotation can be estimated, e.g., from the decrease of lateral rotation and medial abduction of the first phalanx in the metapodiophalangeal joint I. Autopodial rotation was high in Titanophoneus and reduced in derived Cynodontia. In Mammaliamorpha the mobility of the first ray suggests autopodial rolling in an approximately anterior direction. Most non-mammaliamorph Therapsida and probably some Mesozoic Mammaliamorpha had prehensile autopodia with an opposable ray I. In forms with a pronounced relief of the respective joints, ray I could be opposed to 90° against ray III. A strong transverse arch in the row of distalia supported the opposition movement of ray I and resulted in a convergence of the claws of digits II–V just by flexing those digits. A tight articular coherence in the digital joints of digits II–V during strong flexion supported a firm grip capacity.

Figure 2. Pes of Titanophoneus-like synapsid from Kümmel and Frey. PILs added. Approximately middle of the propulsion phase (A), followed by plantar flexion of metatarsalia II–V and distale I (B). C shows the start of the raising phase of the metapodialia and D the start of the raising phase of the digits

Not mentioned, or referenced, but clearly visible
is the presence of PILs (parallel interphalangeal lines) that enable the phalanges to work in sets.

If you ever wondered where your grasping hand first appeared, it is here (Figure 1) in cynodonts.  No matter that grasping later disappeared in many later mammal clades, it was retained by arboreal and carnivorous clades.

The authors discuss the alignment of the phalanges without discussing the 14-year-old paper on PILs (Peters 2000), which might have been appropriate in this study. So, it’s brought up here.

The arching of the metacarpals an metapodials is also shown here. A similar arching was shown to exist in the pes of Pteranodon (Peters 2000). That arching in the human metacarpals produces a fist clearly aligns the knuckles for branch grabbing. Otherwise, when flattened and useful only for clapping and slapping, the knuckles are not clearly aligned.

References
Kümmell SB and Frey 2014. Range of Movement in Ray I of Manus and Pes and the Prehensility of the Autopodia in the Early Permian to Late Cretaceous Non-Anomodont Synapsida. PLoS ONE 9(12): e113911.doi:10.1371/journal.pone.0113911 http://www.plosone.org/article

Peters D 2000. Description and interpretation of interphalangeal iines in tetrapods
Ichnos, 7:11-41.

 

Restoring Pisanosaurus

Updated March 24, 2015 with a newly restored Pisanosaurus pelvis.

Pisanosaurus (Casamiquela 1967) is widely considered the most basal ornithischian dinosaur. Unfortunately Pisanosaurus is only known from a few bones. So we rely on sister taxa to help restore the missing bits and pieces.

But first, a minor problem. 
Only the central portion of the Pisanosaurus pelvis  (Fig. 1) is known, the area surrounding the open acetabulum. So the outline shape of the Pisanosaurus pelvis, a key trait in the identification of ornithischian dinosaurs, is not known.

Figure 1. Pelves of Haya, a typical ornithischian, and what remains of the pelvis of Pisanosaurus. The two pelves do no appear to be completely similar.

xx

I’m not sure about this…
But the placement of the possible obturator foramen and tab-like contribution of the pubis make it appear that the Sereno pelvic elements might have been misidentified — but then the other elements look correct in the Sereno orientation. Contradicting clues make this a difficult problem. Perhaps a photo or firsthand observation would show where the problems are in the drawing. Published data do not indicate where the pelvis was found in relation to the rest of the body, but I can only assume it was correctly oriented, favoring the  Sereno model.

Figure 2. Pisanosaurus pelvis restored the Sereno way (on the right) and like Haya (on the left). Note the possible placement of the obturator foramen, a pubis trait on the alternate orientation. And note the tab-like shape of the pubic contribution of the pubis (compare to figure 1).

Skulls
When the Pisanosaurus skull is restored to resemble the skull of Haya, the resemblance is striking. Not much difference here in the preserved elements.

Figure 3. Skull of Haya and restored skull of Pisanosaurus compared. The resemblance of preserved elements is apparent here. In both cases the mandibular fenestra is filled in. The other holes in the Pisanosaurus mandible are artifacts of taphonomy. Pisanosaurus data from Irmis et al. 2007b.

The pes of Haya and Pisanosaurus
The phalanges are shorter in Haya, but the pedal morphology is otherwise similar to what we know of Pisanosaurus (Fig 4).

Figure 4. The pedes of Haya and Pisanosaurus compared. The patterns are similar despite the shorter phalanges of Haya.

Pisanosaurus is a problem, but not an unsurmountable problem
Perhaps just enough clues are available. All workers nest Pisanosaurus at the base of the Ornithischia.

The origin of the predentary bone preceded Pisanosaurus, and the missing mandible tip probably marks it’s border.  Even so, in the fossil a line of demarcation is present. Not sure what to make of it. It is colored yellow here (Fig. 6).

Figure 6. The restored skull of Pisanosaurus. Note the prominent openings of the posterior mandible are not where the former fenestra was placed, so they are artifacts. The mandibular fenestra is below the second to last tooth, where the (yellow) splenial is showing through. The predentary of ornithischians is a novel and separate bone, and is missing here. A line of demarcation found on the fossil is colored yellow here.

References
Bonaparte JF 1976. Pisanosaurus mertii Casamiquela and the origin of the Ornithischia. Journal of Palaeontology 50(5):808-820.
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007a. 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.
Irmis RB, Parker WG, Nesbitt SJ and Liu J 2007b. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.
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
Casamiquela RM 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina. Ameghiniana 4 (2): 47–64.
Sereno P 1991. Lesothosaurus, “Fabrosaurids,” and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11:168-197.

wiki/Pisanosaurus