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.

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2 thoughts on “Variation in Archaeopteryx and basal bird radiation

    • To your point, the anterior is clear and has been figured before. The posterior is not. It’s more of a stain or disconformity. That’s how buried bones are excavated in Solnhofen limestones, as you know, from surface clues. I will show my tracings in the next few blogs. Thank you for that question, Lars.

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