What makes a bird a bird? Everyone knows, it’s not feathers any more…)

The line between birds and theropod dinosaurs
has become increasingly fuzzy now that so many non-birds have feathers and other former bird-only traits.

This is a good sign
that evolutionary theory embraces: small changes and a gradual accumulation of traits in derived taxa.

Ultimately
it may come down to a single defining trait (like mammary glands in mammals, or alternatively a squamosal/dentary jaw joint when soft tissue is missing) when you have lots of taxa near the base of a new major clade. So what is that trait? Or what are those traits as recovered by the large reptile tree?

The basal bird and its proximal outgroup
At present the last common ancestor of all extant birds, scansoriopterygids and enantiornithes in the large reptile tree. is the Thermopolis specimen of Archaeopteryx (Fig. 1). The original authors (Mayr et al. 2007; Rauhut 2013) did not employ a phylogenetic analysis, so perhaps did not realize what they had.

For now
the pre-bird theropod, Eosinopteryx (Fig.1) nests just basal to the basal bird theropod, Archaeopteryx. You might find it interesting to see which traits differentiate the latter from the former in the large reptile tree. This list, short as it is, is by no means complete. It simply reflects the general characters used for all reptiles in the large reptile tree.

Figure 1. Eosinopteryx, a pre-bird, compared to Archaeopteryx, a basal bird to scale. Click to enlarge.

Figure 1. Eosinopteryx, a pre-bird, compared to Archaeopteryx, a basal bird to scale. Click to enlarge.

Archaeopteryx (Thermopolis) novelties vs. Eosinopteryx

  1. Frontal/parietal suture straight and > than frontal/nasal suture
  2. Metacarpals 2-3 subequal
  3. Pubis and ischium oriented posteriorly (convergent with some deinonychosaurs)
  4. Pedal 4 subequal to metatarsal 4  (convergent with some deinonychosaurs)
  5. Pedal 2.1 not > p2.2
  6. Metatarsal 5 shorter than pedal digit 5 (all vestigial, of course)
Figure 2. The coracoid of the Thermopolis specimen is not as elongate as in the more derived taxa. It is just barely not a disc. Thus, this basal taxon was not quite the flapper as the other Solnhofen birds.

Figure 2. The coracoid of the Thermopolis specimen is not as elongate as in the more derived taxa. It is just barely not a disc. Thus, this basal taxon was not quite the flapper as the other Solnhofen birds.

Unfortunately
none of these traits are unique to the bird clade.

I thought, perhaps
that an elongate and locked down coracoid (the key to the origin of flapping) would prove to be present in all basal birds. Such a coracoid is indeed present in other specimens of Solnhofen birds, but not in the Thermopolis specimen (Fig. 2), the basalmost example. 

So what we are seeing
in these six Solnhofen birds are discrete steps in the evolution of the flapping behavior, necessary for creating thrust and ultimately flight, as in many living birds. Just as in Late Jurassic pterosaurs, the island/lagoon environment of Solnhofen was as powerful an agent as the Galapagos islands at splitting basal birds into various clades.

From the Mayr et al. abstract on the Thermopolis specimen:
“We describe the tenth skeletal specimen of the Upper Jurassic Archaeopterygidae. The almost complete and well-preserved skeleton is assigned to  Archaeopteryx siemensii
 Dames, 1897 and provides significant new information on the osteology of the Archaeopterygidae. As is evident from the new specimen, the palatine of Archaeopteryx
 was tetra-radiate as in non-avian theropods, and not triradiate as in other avians. Also with respect to the position of the ectopterygoid, the data obtained from the new specimen lead to a revision of a previous reconstruction of the palate of Archaeopteryx. The morphology of the coracoid and that of the proximal tarsals is, for the first time, clearly visible in the new specimen. The new specimen demonstrates the presence of a hyperextendible second toe in Archaeopteryx*.  This feature is otherwise known only from the basal avian Rahonavis and deinonychosaurs (Dromaeosauridae and Troodontidae), and its presence in Archaeopteryx provides additional evidence for a close relationship between deinonychosaurs and avians**. The new specimen also shows that the first toe of Archaeopteryx was not fully reversed but spread medially, supporting previous  assumptions that Archaeopteryx was only facultatively arboreal*. Finally,we comment on the taxonomic composition of the Archaeopterygidae and conclude that Archaeopteryx bavarica Wellnhofer, 1993 is likely to be a junior synonym of  A. siemensii****, and Wellnhoferia grandis Elzanowski, 2001 a junior synonym of  A. lithographica***** von Meyer, 1861.”

* Actually not as prominent as in deinonychosaurs. Such a toe works just as well at climbing tree trunks as climbing dinosaur flanks.

**This may be a convergence as the two clades are separated by taxa without a hyper extensible pedal 2.

*** Perhaps facultatively able to perch, but arboreality would have been a precursor behavior.

**** These two are sisters in the large reptile tree.

***** These two are not sisters.

Other traits in the Theromopolis specimen 
visible in Figure 1 not present in the large reptile tree include the following:

  1. Smaller antorbital fenestra
  2. Longer attenuate tail
  3. Slightly narrower coracoids
  4. Slightly larger forelimb
  5. Bowed gap between ulna and radius
  6. More gracile pubis, posteriorly oriented
Figure 3. Archaeopteryx Thermopolis pedal digit 2 (in pink). Pedal 2.2 was capable of hyperextension (see figure 4).

Figure 3. Archaeopteryx Thermopolis pedal digit 2 (in pink). Pedal 2.2 was capable of hyperextension (see figure 4).

Mayr et al. looked at pedal digit 2
and noticed it was capable of hyperextension (Fig. 3). They likened it to pedal digit 2 in deinonychosaurs (Fig. 4) which is famous for its ability to elevate the ‘killer claw’.

Figure 4. Deinonychus with elevated pedal digit 2 demonstrating hyperextension.

Figure 4. Deinonychus with elevated pedal digit 2 demonstrating hyperextension.

The large reptile tree
does not nest birds with deinonychosaurs. Rather Xiaotingia and Eosinopteryx nest between these clades. And Xiaotingia also has a similar pedal 2.1 (Fig. 5).

Figure 5. Pedal digit 2 in Xiaotiniga shows the ability to hyperextend pedal 2.2.

Figure 5. Pedal digit 2 in Xiaotiniga also shows the ability to hyperextend pedal 2.2.

On a final note:
Mayr et al. (2007) report four premaxillary teeth in the Thermopolis specimen. I think they might have missed counting the anteriormost premaxillary tooth (Fig. 6) bringing the total to five.

Figure 6. Archaeopteryx, Thermopolis specimen, premaxilla with five teeth, not four, identified here.

Figure 6. Archaeopteryx, Thermopolis specimen, premaxilla with five teeth, not four, identified here.

References
Rauhut OWM 2013. New observations on the skull of Archaeopteryx. Paläontologische Zeitschrift 88(2)211-221.
Mayr G, Pohl, B, Hartmann S and Peters DS 2007. The tenth skeletal specimen of Archaeopteryx. Zoological Journal of the Linnean Society 149:97-116.

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.

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.

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.

 

 

Reconstructing Cathayornis using DGS methodology

Updated October 23, 2015 with modifications to the ectopterygoids from data beneath the mandibles.. 

Cathayornis yandica (Zhou et al. 1992, Figs. 1-3, IVPP V9769) was an Early Cretaceous enantiornithine bird known from a virtually complete skeleton on plate and counter plate. It is crushed flat.

The best published tracings
of this specimen are shown here (Fig. 1). I wonder if you’ll agree there is too much left to the imagination in both of these professional tracings. The easy parts are correctly labeled, but I sense confusion in the more difficult details. Some of these were labeled originally with a “?”.

Figure 1. Above, Tracing of Cathayornis from Zhou and Zhang 1992. Below tracing of Cathayornis skull by O'Connor and Dyke 2010 traced using camera lucida. Some element labels are guesses. A few are mistakes.

Figure 1. Previous best efforts at tracing Cathayornis. Above, Tracing of Cathayornis from Zhou et al.  1992. Below tracing of Cathayornis skull by O’Connor and Dyke 2010 traced using camera lucida. Some element labels are guesses (See “?”). A few are mistakes.

Try DGS just once to see if it works for you.
Applying color overlays to digital images of Cathayornis (Fig. 2, 3) recovers more bones more accurately than prior efforts (Fig. 1). And these can be used in reconstructions (Fig. 3). Note the postorbital and squamosal both drifted over the right frontal. That was a surprise. Yes, a tiny postfrontal is present, not fused to the frontal. Broken bones can be identified and repaired. Even the palatal bones can be identified.

Figure 2. Cathayornis skull animated GIF. Each frame lasts 5 seconds. Here virtually all skull elements are identified and applied to the reconstruction in three views (below). Compare the results of this technique to figure 1. Note how the upper and lower jaws match curves.

Figure 2. Cathayornis skull animated GIF. Each frame lasts 5 seconds. Here virtually all skull elements are identified and applied to the reconstruction in three views (below). Compare the results of this technique to figure 1. Note how the upper and lower jaws match curves.

There is no guarantee you’re going to get things right the first time.
I don’t get things right the first time. I make changes as the interpretation runs its course. All DGS does is to remove some of the confusion inherent in the roadkill by segregating one bone after another until most – or all – of the bones are accounted for and fit the reconstruction while matching the patterns of sister taxa.

The postcrania
of Cathaysaurus is traced here (Fig. 3) and used to create a reconstruction in several views. The furcula can be traced here. Originally it was overlooked and misidentified.

Figure 3. Cathayornis tracing and reconstruction from tracing. Boxed area are ventral and rib elements originally segregated on a distinct layer and offset here for clarity. Note the green furcula, overlooked originally.

Figure 3. Cathayornis tracing and reconstruction from tracing. Boxed area are ventral and rib elements originally segregated on a distinct layer and offset here for clarity. Note the green furcula, overlooked originally. Those green bones on either side of the sternum are considered part of the sternum in traditional works. Perhaps they are, but the visible one appears to overlay the sternum, rather than be a part of it.

It may just be a matter of applied effort
When you discover something in paleontology, all you have to do is unveil it. The discovery is the big deal. Not much effort is required, but it is always appreciated. Later workers can add details with appropriate levels of support and criticism. If I had access to the specimen or a higher resolution image, perhaps the level of accuracy would increase further.

Now I’ll ask of the bird people 
what I ask of the pterosaur people. Try to build a reconstruction. It helps when you realize there are parts missing and then you can apply more effort to look for that part in the specimen itself.

If I have made any mistakes here, please bring them to my attention. I’m no bird expert, but I’m learning as I go. Here is a new image of enantiornithine birds to scale (Fig. 4) including Sulcavis, which we looked at recently.

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

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

References
O’Connor J-K and Dyke G 2010. A Reassessment of Sinornis santensis and
Cathayornis yandica (Aves: Enantiornithes). Records of the Australian Museum 62: 7-20.
Zhou Z.-H, Fan F-J and Zhang J 1992.
Preliminary report on a Mesozoic bird from Liaoning, China. Chinese Science Bulletin 37: 1365-1368.

Sulcavis – an enantiornithes bird without a sternum

Figure 1. Pre-bird and bird sternae. Note the replacement of the sternum with gastralia in Sulcavis.

Figure 1. Click to enlarge. Pre-bird and bird sternae. Note the replacement of the sternum with gastralia in Sulcavis.

Ever since the advent
of the dual sternae in Velociraptor and kin, and of the single sternum in Archaeopteryx (Fig. 1), most birds had/have an ossified sternum. One exception is the enantiornithine bird, Sulcavis (Fig. 1-4).

Sulcavis geeorum (O’Connor et al. 2013, Early CretaceousBMNH Ph-000805) is a robin-sized enantiornithes with a relatively small skull and, remarkably, no sternum. Teeth with grooved enamel radiating from the tips gave it its name (sulcus = groove). That was seen as the most distinctive feature. A sternum replaced by gastralia was not considered an issue (see below).

Soft tissue
Although the specimen includes some soft tissue, O’Connor et al. report one pubis missing and another present only proximally. The ischium was reported missing. My examination identifies areas were both pubes (green) and ischia (magenta) used to be (Fig. 2).

Figure 1. Sulcavis in situ with GIF animation original tracing from O'Connor et al. in black and white. Colors identify elements originally reported as missing. Pubis (green), ischium (magenta), ilium (cyan).

Figure 2. Sulcavis in situ with GIF animation original tracing from O’Connor et al. in black and white. Colors identify elements originally reported as missing. Pubis (green), ischium (magenta), ilium (cyan). Reconstruction in figure 2. A proximal ischium was mislabeled a sacral rib.

Enantiornithes are like basal birds
except for the following traditional traits listed by O’Connor et al. 2013 :

  1. Pygostyle proximally forked and distally constructed with ventrolateral processes
  2. Furcula Y-shaped and dorsolaterally excavated
  3. Coracoid with convex lateral margin
  4. Proximal humerus rises dorsally and ventrally to centrally on the concave head
  5. Metacarpal 3 longer than mc2
  6. Distal tarsals fused to metatarsals, but metatarsals unfused distally
Figure 2. Sulcavis reconstruction. PILs on foot. Note the lack of a sternum.

Figure 3. Sulcavis reconstruction. PILs on foot. Note the lack of a sternum. The pedal ungual length and curvature indicate an arboreal lifestyle.

Unfortunately, none of theses traits are listed as characters in the large reptile tree, yet Sulcavis nests with Cathayornis sharing the following traits distinct from other birds:

  1. Skull not shorter than cervicals
  2. Posterior quadrate straight
  3. More than 4 premaxillary teeth
  4. Posterior mandible deeper anteriorly
  5. Retroarticular descends
  6. Metatarsals 2-3 aligned with 1
  7. Pedal 2.2 > p2.1

More pertinent taxa would reduce this list.

Figure 3. Sulcavis skull as originally interpreted (above) and traced using the DGS method (middle) to create a reconstruction (below).

Figure 4. Sulcavis skull as originally interpreted (above) and traced using the DGS method (middle) to create a reconstruction (below). Note, several bones here were not originally identified. It looks possible that a substantial mandibular fenestra might have been present.

Due to the contrived problem
of digit identification in birds and bird-like theropods described and falsified here, O’Connor et al. describe the three manual digits as the

  1. alular digit
  2. major digit
  3. minor digit

Such renaming of digits 1-3 is totally unnecessary.

Re: The sternum
O’Connor et al. report, “No direct information regarding the morphology of the sternum is preserved.” That’s because there is no sternum in this taxon (Figs, 1, 2). The gastralia run right up to the coracoids. So, does this taxon appear to demonstrate how the sternum in enatiornithine birds is formed? Yes, by enlarging and fusing the gastralia, not as a new single, complete bone.

Sternae also appear in dromaeosaurs and oviraptors by convergence. Twin sternae in these taxa do not appear to be homologous with the single sternum of birds. A single sternum originates as a small bone, wider than long followed by a long set of gastralia extending to the pubis, distinct from large twin sternae.

References
O’Connor JK, Zhang Y, Chiappe LM, Meng Q, Quanguo L, Di L 2013. A new enantiornithine from the Yixian Formation with the first recognized avian enamel specialization. Journal of Vertebrate Paleontology 33(1):1-12.

Trees of Life: Birds and Pterosaurs

Yale’s Richard Prum recently announced that the Tree of Life of Birds is almost complete. A genomic analysis of 198 species of birds was published in the Oct. 7 edition of the journal Nature. Prum reported, ““In the next five or 10 years, we will have finished the tree of life for birds.” I presume that means fossil taxa will also be included and scored by morphological traits because genes (genomic traits) are not available.

It is not the first time…
Trees of Life for Birds were announced earlier here, here, here and here.

Having been through a similar study, I support all such efforts. AND I will never attempt to add any but a few sample birds to the large reptile tree. Others have better access to specimens and they have a big head start on the process.

Unfortunately,
some workers have ignored the pterosaur tree of life. Recently Mark Witton ignored isometric growth patterns in pterosaurs to agree with Bennett (2013) that the genus Pterodactylus includes tiny short-snouted forms, mid-sized long-snouted forms (including the holotype, of course) and large small-heron-like forms. Witton reports, “Speaking of adulthood, it was also only recently that we’ve obtained a true sense of how large Pterodactylus may have grown. We typically imagine this animal as small bodied – maybe with a 50 cm wingspan – but a newly described skull and lower jaw makes the first unambiguous case for Pterodactylus reaching at least 1 m across the wings (Bennett 2013).”

We looked at Bennett’s paper earlier in a three part series that ended here. The taxon Witton refers to is actually just a wee bit larger than the holotype and is known from a skull, so wingspread can only be guessed. The tiny short-snouted forms are actually derived from the short-snouted scaphognathids as shown here.

The Pterodactylus lineage and mislabeled specimens formerly attributed to this "wastebasket" genus

Figure 1. Click to enlarge. The Pterodactylus lineage and mislabeled specimens formerly attributed to this “wastebasket” genus. Others have split the largest specimens of Pterodactylus from the others without employing a phylogenetic analysis.

You might recall
that one of the largest complete Pterodactylus specimens (Fig. 1) recovered by the large pterosaur tree was mistakenly removed from this genus and lumped with Ardeadactylus, a basal pre-azhdarchid, all without phylogenetic analysis.

Agreeing with Bennett,
Witton deletes some taxa that actually belong to this genus, while accepting others that do not belong, all based on eyeballing specimens without a phylogenetic analysis that includes a large gamut of specimens (that does not delete the tiny forms). Eyeballing taxa is not the way to handle lumping and splitting. Phylogenetic analysis is. We looked at the Pterodactylus wastebasket problem here.

References
Bennett  SC 2012 [2013]. New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift (advance online publication) doi: 10.1007/s12542-012-0159-8
http://link.springer.com/article/10.1007/s12542-012-0159-8