Archaeopteryx: the scattered skull of the London specimen

Updated November 8, 2015 with cranial data from Alonso 2004 that I just became aware of.

Figure 1. Archaeopteryx London specimen skull. The anterior is more clearly presented and has been previously illustrated. Here I colorized matrix discontinuities that could be posterior skull elements. At least they all fit together in a basic Archaeopteryx-type skull that matches other specimens.

Figure 1. GIF animation of the skull of the London specimen of Archaeopteryx. Perhaps other bones are also present. If so I did not identify them. The bones here are clear, less clear and not very clear. Compare these colors to the colors in the reconstruction and you’ll see a close correspondence to the bones of other specimens.

As far as I know,
prior workers did not identify or illustrate the posterior skull bones of the London specimen of Archaeopteryx (Fig. 1, but see below). Bones left only the faintest of impressions (if correct here), but seem to correspond to the same bones of better known specimens (Fig. 4). Higher resolution images should confirm or refute these tracings.

New data (November 8, 2015)
came in the form of Alonso et al. 2004, which extricated and CT scanned the skull of the London Archaeopteryx. The new illustration in figure 2 reflects that data. Apologies that I was not aware of this at the time of this first posting.

Figure 2. A new paper (Alsonso et al. 2004) on the cranium of this specimen has come to my attention. The cranium was buried in the matrix and these new illustrations reflect the more complete data.

Figure 2. A new paper (Alsonso et al. 2004) on the cranium of this specimen has come to my attention. The cranium was buried in the matrix and these new illustrations reflect the more complete data.

Every bone here
appears to fit and not stray too far from morphologies established by better preserved skulls. As noted earlier, the large number of premaxillary teeth in the London specimen, along with other traits, make it distinct from the Eichstaett specimen (Figs. 3, 4).

While we’re on the subject of basal birds,
here are a few to scale (Figs. 3, 4). It is notable that the more primitive ones are the smaller ones in this selection of taxa.

Figure 4. Enanthiornithine birds to scale. Click to enlarge.

Figure 4. Enanthiornithine birds to scale. Click to enlarge. Evidently there are a few other taxa without a sternum in this clade.

Be sure to click on figure 4 to see it at full size.
The stem birds, Xiaotingia and Eosinopteryx form a short-face clade with their own autapomorphies. Rahonavis nests with Velociraptor, not with birds in the large reptile tree.

Figure 4. Archaeopteryx and a few stem birds to scale compared to a chicken (Gallus). Click to enlarge.

Figure 4. Archaeopteryx and a few stem birds to scale compared to a chicken (Gallus). Click to enlarge.

The convergence of Late Jurassic birds and Late Jurassic pterosaurs
Here it is clear that the reduction of the long tail in birds occurred with phylogenetic miniaturization and neotony. Earlier I demonstrated the same tail reduction in four clades of pterosaurs that ultimately developed ‘pterodactyloid’-grade traits. They each had their genesis in tiny pterosaurs experiencing phylogenetic miniaturization and neotony.

The refusal of pterosaur workers
to recognize that embryo and juvenile pterosaurs match their parents, and that tiny Solnhofen pterosaurs are adults the size of living hummingbirds is the reason why their cladograms fail to demonstrate gradual accumulations of traits in derived taxa. Odd that tiny birds get novel generic names, but tiny pterosaurs do not.

It may be
that only tiny birds survived the end of the Jurassic, just like tiny pterosaurs. Later they both developed into larger forms.

Rahonavis
(Forster et al. 1998) survived into the Latest Cretaceous (Maastrichtian). Not sure whether it stayed small or evolved smaller than other velociraptors. At present it nests basal to that clade.

I still think reconstructions bring necessary data to the table. 
Hope you do too.

References
Alonso PD, Milner AC, Ketcham RA Cookson MJ and Rowe TB 2004. The avian nature of the brain and inner ear of Archaeopteryx. Nature 430:666-669.
Forster CA, Scott D, Chiappe LM, Krause DW. 1998. The Theropod Ancestry of Birds: New Evidence from the Late Cretaceous of Madagascar. Science 279 (5358): 1915–1919.

SVP 6 – Aphanizocnemus, a pre-snake or a skink?

Campbell, Caldwell and Dal Sasso 2015 discuss Aphanizocnemus, which nests as a pre-snake in the large reptile tree.

Figure 5. Aphanizocnemus libanensis, previously considered a dolichosaur, here recovered between Jucaraseps and Adriosaurus. Prior snake studies did not include Jucaraseps, a key taxon.

Figure 1. Aphanizocnemus libanensis, previously considered a dolichosaur, here recovered between Jucaraseps and Adriosaurus. Prior snake studies did not include Jucaraseps, a key taxon. Click to enlarge.

From the abstract:
“Aphanizocnemus libanensis is a small monotypic lizard from platy limestones deposited in patch reef lagoons stretching across the Tethyan platform from North Africa to Europe (Cenomanian; Upper Cretaceous). The sole specimen is articulated and nearly complete, though the skull was destroyed during collection. The original description placed the taxon within the Varanoidea as a member of the aquatic Dolichosauridae. Reexamination suggests that characters cited as supporting varanoid-dolichosaur affinities are misinterpreted, i.e., an intramandibular joint, a character diagnostic of pythonomorphs (the group including the Dolichosauridae, Serpentes and the Mosasauria), is actually a break in the dentary associated with the considerable damage to the skull. The single frontal omits this animal from the Varanoidea, which have paired frontals, and the shape of the frontal-nasal suture indicates that the nasals are broad and robust, unlike the splint like condition seen in dolichosaurs. In addition, though we recognize variability in the shape of the parietals of dolichosaurs, the exceptionally large parietal of Aphanizocnemus is far wider and more extended than seen in any dolichosaur, which have posteriorly narrowed parietals far longer than they are wide. The morphology of the scapulocoracoid (rounded, semicircular) and the neural spines (low, posteriorly directed) are common to many squamates, and like many other features of the specimen, i.e., the unfused, simple girdles; the reduced, flattened limbs; the shorter hind limb; and the poorly ossified tarsus, are likely tightly linked to aquatic adaptation. The hallmark feature of the specimen is the strongly regressed tibia, which is short and flat, with unclear articular surfaces. Limb reduction is a characteristic of the Pythonomorpha, but it is also common to numerous families within Squamata, including the Scincomorpha. We hypothesize here that the genus Aphanizocnemus is not a varanoid, nor in fact an anguimorph, but may represent a new form of aquatic scincomorph, a group not previously recognized as having evolved aquatic adaptations.”

Nice to get confirmation here.
Indeed Aphanizocnemus is not a varanoid nor a member of the Pythonomorpha nor, (contra the abstract) a member of the skink/amphisbaenid clade. To shift Aphanizocnemus to the skinks or varanids adds a minimum of 26 steps in the large reptile tree. Despite the fused frontals (an autapomorphy) Aphanizocnemus is a pre-snake nesting between tiny terrestrial Jucaraseps and larger aquatic Adriosaurus in the large reptile tree.

In correspondence with Dr. Caldwell
he does not like the connection of pre-snakes to pre-gekkos recovered in the large reptile tree. preferring varanoid lizards. Unfortunately, when you test them all together, nothing else nests more parsimoniously. Snake/gekko cladograms have been produced and collected here. I’m not saying that snakes arose from geckos, but they did have a common ancestor exclusive of any other squamate clades.

Campbell et al. need to run the phylogenetic analysis and not let minor autapomorphies throw throw them off course.

References
Campbell M, Caldwell M Dal Sasso C 2015. Re-evaluation of Aphanizocnemus libanensis – to be or not to be a dolichosaur.

Variation in Archaeopteryx and basal bird radiation

Updated October 30, 2015 with a new GIF animation that reveals the furcula of this specimen on the newly added counter-plate. 

The basal bird
Archaeopteryx lithographica  (Meyer 1861, Late Jurassic, Solnhofen Formation ~150 mya, 30-50 cm in length) is known from 12 skeletal specimens, 11 of which are published. Two of those are shown here (Fig. 1). Bennett (2008) reports, over the years workers have split these specimens into six generic and ten species names, while others have lumped them all into a single species.

Figure 1. The six tested Solnhofen birds currently named Archaeopteryx, Jurapteryx and Wellnhoferia.

Figure 1. The six tested Solnhofen birds currently named Archaeopteryx, Jurapteryx and Wellnhoferia.

In typical and traditional bird cladograms
only one Archaeopteryx is ever employed. Perhaps the subtle differences between the Solnhofen specimens are considered inconsequential in phylogenetic analyses that attempt to reveal early bird interrelationships. At least that is the tradition.

In like fashion
Solnhofen pterosaurs are also known from hundreds of specimens, but in typical pterosaur analyses only a single specimen from the most common genera, Rhamphorhynchus, Pterodactylus andScaphognathus are ever employed. At least that is the tradition.

As readers know,
I have added dozens of Solnhofen pterosaur specimens to my analysis and found that:

  1. no two tested taxa were identical (except the juvenile/adult pairing in Rhamphorhynchus)
  2. variations in genera are phylogenetic rather than ontogenetic; and
  3. those variations are the overlooked keys to understanding the interrelationships of pterosaurs in general

For instance,
from those results the widely accepted clade, “Pterodactyloidea,” was found to have not one, but four origins, all developing a complete set (rather than a partial set as in wukongopterids) of pterodactyloid-grade traits all by convergence.

So, with the presence of Galapagos-like variation in Solnhofen pterosaurs…
I wondered if there was Galapagos-like variation in the Archaeopteryx specimens. And if so,  what were those variations? Would they be substantial enough to appear in an analysis not focused on birds, like the large reptile tree? (It now includes 594 taxa.)

A little history on Archaeopteryx lumping and splitting
Houck et al (1990) found evidence in scatter plot analysis of immaturity in the six specimens then known and interpreted the specimens as a growth series of a single species.

Elzanowski (2002) rejected the notion that any of the specimens were immature and so recognized the London, Berlin and Munich specimens as three distinct species and the Solnhofen specimen (BSP 1999) as a new genus, Wellnhoferia grandis.

Senter and Robins (2003) repeated the Houck et al analysis with one added and one excluded specimen agreed with Houck et al. on a single species documenting an ontogenic series.

Mayr et al. (2007) described the Thermopolis specimen and lumped all specimens into two species.

Bennett (2008) likewise used statistical analysis in a study of Alligator to document variation a single species, concluding that “lengths of skeletal elements in a sample of a single species can have high correlation coefficients, and that such high correlation coefficients are not indicative of multi-species samples.”

This is all well and good
but where are the phylogenetic analyses? Bennett (1995) lumped all of his Rhamphorhynchus specimens together using statistics, but missed the speciation recovered in phylogenetic analysis. Even the feet show variation! Perhaps the same is true of Archaeopteryx?

I start with just two Archaeopteryx taxa,
the large London and small Eichstaett specimens (Fig.1). I added both to the large reptile tree and was mildly surprised by the unconventional results. The London specimen nested at the base of the few specimens currently tested in the Enantiornithes clade (Fig. 2). The Eichstaett specimen nested at the base of the few specimens tested in the Euornithes clade.

Figure 4. Here I add the Munich specimen of Archaeopteryx to the large reptile tree and recover it basal to the Scansoropterygidae, the clade of basal birds that shares a long finger 3.

Figure 2. Here I add the Munich specimen of Archaeopteryx to the large reptile tree and recover it basal to the Scansoropterygidae, the clade of basal birds that shares a long finger 3.

These are novel nestings
Typically other specimens nest between Archaeopteryx and Enantiornithes. The classic transitional taxa include  RahonavisXiaotingia and Confuciusornis. In the large reptile Rahonavis nests with Velociraptor, Xiaotingia (together with Eosinopteryx) is the proximal outgroup taxon for Archaeopteryx, and Confuciusornis nests as a basal euornithine,

Remember, this is small list of pertinent taxa
with far fewer pre-birds and birds included than are usually found in bird origin cladograms. Likewise, there are also far fewer theropod and bird specific characters employed here.

The key differences
between this study and prior studies are simply the inclusion of one more Archaeopteryx specimen into the matrix, the use of reconstructions, and a set of 228 generic characters that work for reptiles at large, but are not bird or theropod specific.

London specimen enantiornithine traits from the large reptile tree:

  1. snout constricted in dorsal view
  2. nasal shape parallel in dorsal view
  3. premaxilla ascending process not beyond naris
  4. nasals subequal to frontals
  5. maxilla with antorbital fossa
  6. pineal foramen/cranial fontanelle absent
  7. frontal parietal suture not straight
  8. no temporal ledge
  9. quadrate posterior not concave
  10. squamosal + quadratojugal indented, no contact
  11. jaw joint descends
  12. premaxillary teeth: > 4
  13. retroarticular angle: straight
  14. mandible ventrally: straight then convex

Eichstaett specimen euornithine traits from the large reptile tree:

  1. snout not constructed in dorsal view
  2. nasal shape, premaxilla invasion and separation
  3. premaxilla ascending process beyond naris
  4. nasals shorter than frontals
  5. maxilla without antorbital fossa
  6. pineal foramen/cranial fontanelle present
  7. frontal parietal suture straight and wider than n/f suture
  8. squamosal temporal ledge
  9. quadrate posterior concave
  10. only squamosal indented
  11. jaw joint in line with maxilla
  12. premaxillary teeth: 4 or fewer
  13. retroarticular angle: ascends
  14. mandible ventrally: straight then concave

Plus
There are several other traits that are not universal among derived taxa in both clades. These help to lump and split the derived taxa. Request the .nex file here.

Figure 2. London Archaeopteryx pectoral area with a focus on the scapula, coracoid and clavicles.

Figure 2. GIF animation London Archaeopteryx pectoral area with a focus on the scapula, coracoid and clavicles.

And then
there are several enantiornime-euornithine splitting traits not listed as traits in the large reptile tree.

Enantiornithine traits in the London specimen of Archaeopteryx:

  1. coracoid with convex articulation with scapula
  2. coracoid with convex lateral shape
  3. Y-shaped clavicles
  4. metatarsals fused proximally
Figure 2. Pectoral girdle of the Eichstaett specimen of Archaeopteryx.

Figure 2. Pectoral girdle of the Eichstaett specimen of Archaeopteryx. Two frames, each 5 seconds long.

Euornithine traits in the Eichstaett specimen of Archaeopteryx:

  1. coracoid with concave articulation with scapula
  2. coracoid with straight lateral shape
  3. clavicle not preserved
  4. metatarsals: fusion patterns not clear

As mentioned previously
this addition of one more Archaeopteryx to a phylogenetic analysis will not settle any issues. Paleontology rarely settles any issues. But hopefully others will take the time to trace the bones, create the reconstructions and add several Archaeopteryx specimens to future phylogenetic analyses. As has been demonstrated several times now, statistical analyses of Solnhofen taxa don’t reveal what phylogenetic analyses seem to.

References
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species. Journal of Paleontology 69:569–580.
Bennett SC 2008. Ontogeny and ArchaeopteryxJournal of Vertebrate Paleontology 28 (2): 535-542.
Houck MA, Gauthier JA and Strauss RE 1990. Allometric scaling in the earliest fossil bird, Archaeopteryx lithographica. Science 247: 195–198.

SVP 5 – Triassic pterosaur from Utah

Britt et al. (2105) found a Late Triassic pterosaur in Utah, described in the following SVP abstract.

From the abstract:
“We previously reported on a wealth of tetrapods, including multiple individuals each of a coelophysoid, a drepanosauromorph, two sphenosuchian taxa, and two sphenodontian taxa. All are preserved along the shoreline of  a Late Triassic oasis in the Nugget Sandstone at the Saints & Sinner Quarry (SSQ). Recently, we discovered a non-pterodactyloid pterosaur at the quarry, represented by a partial uncrushed, associated/articulated skull imaged via micro CT. The premaxillaries are spoon-shaped rostrally; the maxilla is a simple bar with a needle-like nasal process, the suborbital jugal/quadratojugal blade is high; the nasal is a short, narrow rectangle; and the fused frontals are wide with a moderately high, tripartite sagittal crest. The lower jaws are complete, with a long, slender dentary terminating rostrally in a downward-bend with a ventral expansion, a short postdentary complex and a short retroarticular process. The quadrate-articular joint is well above the tooth row. At least three, widely spaced, conical teeth are in the premaxilla; maxillary teeth are mesiodistally long (3 widely-spaced mesially and 7 close together distally); and on the dentary there are two apicobasally high, widely-spaced mesial teeth and ~20 small, multicusped, low-crowned distal teeth. The frontals and lower jaws are extensively pneumatized. With a 170 mm-long lower jaw, this is two times larger than other Triassic pterosaurs and only the second indisputable Triassic pterosaur from the Western Hemisphere (the other is from Greenland). This is the only record of desert-dwelling nonpterodactyloids and it predates by >60 Ma all known desert pterosaurs. Whereas most pterosaurs are known from fine-grained marine or lacustrine environments, and other Triassic forms are smaller, the SSQ specimen shows that early pterosaurs were widely distributed, attained a large size, and lived in wide range of habitats, including inland deserts far (>800 km) from the sea. Finally, the SSQ pterosaur corroborates the Late Triassic age of the fauna based on drepanosaurs because pterosaurs with multicusped teeth are presently known only from the Upper Triassic.”

The description
sounds like an early dimorphodontid, but withe the deep jugal of Raeticodactylus. The size of the skull is similar to both. Unfortunately, too few clues to go on. I’ll wait for the paper… eagerly!

References
Britt BB, Chure D, Engelmann G and Dalla Vecchia F et al.  2015.
A new, large, non-pterodactyloid pterosaur from a Late Triassic internal desert environment with the eolian nugget sandstone of Northeastern Utah, USA indicates early pterosaurs were ecologically diverse and geographically widespread. Journal of Vertebrate Paleontology abstracts.

 

 

What is Tamaulipasaurus?

Tamaulipasaurus morenoi (Clark and Hernandez 1994, Fig. 1) Early Jurassic ~165mya, was a very early burrowing lizard originally considered “similar to living amphisbaenian and dibamid squamates but represents an entirely new lineage related to squamates but outside the group.” That’s because it has a robust quadratojugal, a bone otherwise unknown in squamates (see figure 2).

Evidently that was confusing back then.
Despite it’s strong resemblance to Bipes (Figs. 2, 3), Clark and Hernandez did not know what Tamaulipasaurus was back in 1994, and they still don’t know what it is, based on Clark’s current website (which also has other issues covered here and here). They put too much importance on the presence of the quadratojugal.

It’s not that important. 
As was demonstrated earlier, the quadratojugal has a way of disappearing and reappearing in various clades. So it’s not a big deal if it’s there or if it is not in a large phylogenetic analysis that includes hundreds of traits. While we’re on the subject, temporal fenestra also appear and disappear on occasion, so don’t apply hard and fast ‘rules’ to their appearance. Just run the analysis. Let parsimony prevail.

Figure 1. Tamaulipasaurus, a burrowing reptile with an autapomorphic quadratojugal.

Figure 1. Tamaulipasaurus, a burrowing reptile with an autapomorphic quadratojugal. The big question is not the novel quadratojugal, but that tiny little fenestra in the rostrum. Is it the orbit, as in Spathorhynchus and as Clark and Herenandez identify it? Or is it simply a rostral feenstra with the orbit confluent with the temporal fenestra, as in Bipes?

Phylogenetic analysis
Clark and Hernandez ran an analysis based on Gauthier (1988) as modified by Laurin (1990). They found 108 MPTs (that’s low resolution). They nested Tamaulipasaurus either between kuehneosaurs and lepidosaurs or between rhynchocephalia and squamata. Pretty broad. No indicated sister taxa.

With more taxa
in the large reptile tree that novel quaratojugal does not affect the nesting of Tamaulipasaurus as a sister to Spathorhynchus. Both are derived from a sister to Crythiosaurus, given the Clark and Hernandez data (i.e. with the labeled orbit considered the actual orbit). I find that problematic given the many other morphological differences (Fig. 2).

Earliest known burrowing lizard
The presence of such a derived lizard as early as the Early Jurassic indicates that predecessors to all these taxa must have first appeared even earlier, likely in the Permian with radiation and diversification throughout the early Mesozoic.

Figure 2. Spathorhynchus, Tamaulipasaurus and Bipes to scale.

Figure 2. Spathorhynchus, Tamaulipasaurus and Bipes to scale. Bones colorized in Tampaulpasaurus as Clark and Hernandez identify them. Note the different ways that the jugal is oriented in Spathorhynchus.

Convergence?
As noted by Clark and Hernandez, Tamaulipasaurus shared several traits and bears a strong general resemblance to the living burrowing lizard with forelimbs, Bipes (Figs. 2,3)

The big question is
not the novel quadratojugal. I’m more interested in that tiny little fenestra in the rostrum. Is it the orbit, as in Spathorhynchus (Fig. 2) and as Clark and Herenandez identify it? Or is it simply a rostral feenstra with the orbit confluent with the temporal fenestra, as in Bipes? In Bipes it appears that the prefrontal and jugal are fused to the maxilla, colorized here (Fig. 3). The only bone missing is the quadratojugal. Even the mandibles look very similar.

Figure 3. Tamaulipasaurus compared to Bipes. There is that tiny foramen in the rostrum in Bipes, exactly where the orbit was identified in Tamaulipasaurus. So, here the eyeballs are added to show where the orbits is and perhaps was in Bipes and Tamaulipasaurus.

Figure 3. Tamaulipasaurus compared to Bipes. There is that tiny foramen in the rostrum in Bipes, exactly where the orbit was identified in Tamaulipasaurus. So, here the eyeballs are added to show where the orbits is and perhaps was in Bipes and Tamaulipasaurus. This is DGS used on a drawing of a 3D specimen, useful here just to readily compare like colored bones.

So which is it? 
In phylogenetic analysis, surprisingly it makes little difference whether the foramen is an orbit or not. All the other traits trump the identification either way. No other of the 592 tested taxa are closer in morphological traits. My money is on the confluent orbit hypothesis. After seeing the photo (Fig. 1) from the Clark website,the opening  looks more like a foramen than an orbit, and it’s right where a foramen should be in a sister. Until higher rez photos become available, I’m working from the drawings of Clark and Hernandez (1994) for ‘circumorbital sutures’, which may include other errors.

References
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, Journal of Vertebrate Paleontoogy 14: 180-195.

Confuciusornis skull reconstructed with DGS

Confuciusornis sanctus 
(Fig. 1) is an early Cretaceous ornithurine bird known from hundreds of specimens (Chiappe et al. 1999) often preserved with full plumage. The skull is of the typical archosauromorph diapsid plan. The premaxilla is very long and toothless, like that of modern birds. The premaxilla extends to the frontal, separates the nasals and greatly reduces the maxilla. This was a bird with large manual claws, a substantial breastbone and a pygostyle instead of a long bony tail.

The problem is 
the skull of Confuciusornis has not been accurately traced (examples in black on Fig. 1). These prior examples do not attempt to capture the detail clearly available from the photographic data. Here DGS (digital graphic segregation) more accurately traces the elements then assembles them back to their in vivo positions.

Figure 1. GIF animation showing DGS tracing and reconstruction (in color) versus prior efforts in black.

Figure 1. GIF animation showing DGS tracing and reconstruction (in color) versus prior efforts in black. Here crushed elements are more accurately traced with colorized elements. These greatly facilitate the reconstruction below.

The resulting skull
greatly resembles that of Struthio and other basal birds and demonstrates the loss of teeth early in the evolution of ornithomorph birds.

Phylogeny
Chiappe et al. 1999 considered Confuciusornis the sister group of a clade composed of the Enantiornithomorpha and the Ornithomorpha. In the large reptile tree (now 592 taxa) it nests one node up, at the base of the Ornithomorpha.

Figure 3. Fossil of Confuciousornis. Note the large manual claws.

Figure 3. Fossil of Confuciousornis. Note the large manual claws.

This is but one more example
of a method that should be used universally for interpreting and reconstructing crushed fossils, DGS. This is also one more example that contradicts the tradition that one has to see the fossil firsthand in order to accurately assess its character traits. Several other examples have been posted previously. Keyword: DGS.

Figure 3. Confuciusornis pedal digit 5. When you look closely, you sometimes find things that are otherwise overlooked.

Figure 3. Confuciusornis pedal digit 5 (3 phalanges)  and metatarsal 5. Short black lines are at the joints. When you look closely, you sometimes find things that are otherwise overlooked.

References
Chiappe LM, Ji S-A, Ji Q, and Norell M 1999. Anatomy and systematics of the Confuciusornithidae (Theropoda: Aves) from the Late Mesozoic of Northeastern China. Bulletin of the American Museum of Natural History 242: 1-89.

 

 

SVP 4 – new data coming in for Scutellosaurus

Figure 1. Scutellosaurus is a small armored ornithischian transitional between Lesothosaurus and Stegosaurus.

Figure 1. Scutellosaurus is a small armored ornithischian transitional between Lesothosaurus and Stegosaurus.

From the abstract
“While rare in Upper Triassic strata, the Ornithischia had achieved a global distribution by the Early Jurassic. The oldest ornithischian fossils from North America have been found in the Silty Facies of the Kayenta Formation in northeastern Arizona. These include the thyreophoran Scutellosaurus lawleri, an unnamed heterodontosaurid, and osteoderms and rib fragments tentatively attributed to the thyreophoran genus Scelidosaurus. I report here new Scutellosaurus material preserving anatomy that is poorly known or not previously reported for the taxon, including the nasal, maxilla, lacrimal, postorbital, quadratojugal, squamosal, opisthotic, scapula, ilium, and metatarsus. Both specimens were mechanically prepared until risk of damaging the fossil bone was deemed too high, at which point the specimens were scanned at The University of Texas High Resolution X-ray Computed Tomography Facility. This approach results in three-dimensional volumetric models of individual bones.”

It is good to see
that more data is coming in for Scutellosaurus. It will be interesting to see if the new material (which I have not seen) fits the best guess offered above (Fig. 1).

In the large reptile tree, Scutellosaurus nests closer to Lesothosaurus not closer to the much larger and more primitive armored Scelidosaurus, which was basal to ankylosaurs and nodosaurs. Earlier the clade Thyreophora was shown to not be monophyletic. Scutellosaurus developed armor independently and was basal to stegosaurs. The long torso and short legs suggest a quadrupedal posture, rather than the traditional bipedal one.

Breeden BT and Rowe TB 2015. New specimens of the threophoran dinosaur Scutellosaurus lawleri from the lower Jurassic Kayenta formation of northern Arizona. Journal of Vertebrate Paleontology Abstracts 2015.