Phylogenetic miniaturization in Theropoda: two hypotheses

Lee, Cau, Naish and Dyke 2014
described the ‘sustained miniaturization…in the dinosaurian ancestors of birds’ starting with an unspecified basal tetanuran (Earliest Jurassic, 190mya) at 163kg, then evolving to a basal neotetanuran at 46kg, followed by a basal coelurosaurian at 27kg, a basal maniraptoran at 10kg, a basal parades at 3.3kg, and a basal bird at 1kg. We first looked at that theropod-bird evolution paper earlier here.

A distinctly different theropod tree topology
is found in the large reptile tree (LRT, 1315 taxa). More theropods have been added since 2014 (subset Fig. 1). Here small (goose-sized) theropods (in hot pink) arise from late (Early Cretaceous) survivors of Late Triassic radiations (based on the presence of Coelophysis as a derived taxon in an early clade, derived from a sister to Sinocalliopteryx, which was fully feathered with filaments in the fossil). So feather filaments were present in theropods from at least this point forward, with scales/placodes replacing filaments in very large taxa.

Figure 1. Subset of the LRT focusing on basal theropods. Pink area are more or less goose-sized and smaller taxa.

Figure 1. Subset of the LRT focusing on basal theropods. Pink areas are more or less goose-sized and smaller taxa leading to birds and following Tawa and Sinocalliopteryx. Smaller theropods evolve to become large and giant theropods several times by convergence here.

We’ve seen phylogenetic miniaturization before
attending the genesis of new clades in turtle origins, pterosaur origins, derived pterosaur origins, dinosaur origins, archosauriform origins, reptile origins, mammal origins, snake origins, squamate origins and I’m forgetting a few.

Given that the basalmost archosaur,
PVL 4597, is also goose-sized or smaller, it is possible that small undiscovered theropods were present throughout the Mesozoic, giving rise to larger taxa in the Herrersaurus, Tawa, Zuolong and the Coelophysis clades in the early part of the Age of Dinosaurs. 

Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomicial innovation in the dinosaurian ancestors of birds. Science 345(6196):562–566.

Anchiornis or not? And what about Pedopenna?

Xu et al. 2009
described a new genus, Anchiornis huxleyi IVPP V14378 (the holotype), along with LPM-B00169A, BMNHC PH828 as referred specimens), from the Late Jurassic of China. Two of these (Fig. 1) were added to the large reptile tree (LRT, 1315 taxa, subset Fig. 2). They nest in the LRT in the clade traditionally considered Troodontidae, between Velociraptor and Archaeopteryx. (Note other traditional troodontids, like Sinornithoides and Sauronithoides, do not nest in this pre-bird clade, but within the Haplocheirus clade.

Last year
a paper by Pei et al. 2017 described “new specimens of Anchiornis huxleyi. Two of these (Fig. 1) were also added to the LRT (subset in Fig. 2).

Figure 1. Four specimens attributed to Anchiornis. Two of these nest apart from two others (see figure 2).

Figure 1. Four specimens attributed to Anchiornis along with two others related to Anchiornis, but given different names. Two of these Anchiornis specimen nest apart from two others (see figure 2).

In the LRT
only two of the four tested Anchiornis specimens nested together (one was the holotype). That means the two other specimens were originally mislabeled. Moreover, a specimen attributed to a separate genus, Jinfengopteryx, nests with the holotype of Anchiornis and a referred specimen.

So do a few of the referred specimens need to be renamed? Perhaps so. Beyond the distinctly different skulls (Fig. 1), various aspects of the post-crania are also divergent.

Figure 2. Cladogram of taxa surrounding four specimens attributed to Anchiornis, which do not nest together in the LRT.

Figure 2. Cladogram of taxa surrounding four specimens attributed to Anchiornis, which do not nest together in the LRT. The holotype is the IVPP specimen in a darker tone and white arrowhead.

Pedopenna daohugouensis
(Xu and Zhang 2005; IVPP V 12721, Fig. 3) is a fossil theropod foot with long stiff feathers from the Middle or Late Jurassic, 164mya.

According to Wikipedeia
“Pedopenna was originally classified as a paravian, the group of maniraptoran dinosaurs that includes both deinonychosaurs and avialans (the lineage including modern birds), but some scientists have classified it as a true avialan more closely related to modern birds than to deinonychosaurs.”

Figure 1. Pedopenna in situ. Very little is known of this specimen.

Figure 3. Pedopenna in situ. The large alphanumerics are original. The color is added here. Very little is known of this specimen, but clearly long feathers arise from the metatarsus.

The first step
in figuring out what Pedopenna is, is to create a clear reconstruction (Fig. 4). Only then will we be able to score the pedal elements in the LRT.

Figure 2. Pedopenna in situ and reconstructed using DGS techniques.

Figure 4. Pedopenna in situ and reconstructed using DGS techniques.

and despite the relatively few pedal traits, the LRT is able to nest Pedopenna between and among the several Anchiornis specimens (Fig. 5). Specifically it nests between the holotype IVPP specimen and the LPM specimen. So is Pedopenna really Anchiornis? Or do all these taxa, other than the holotype, need their own generic names?

Figure 3. Where feathers on the foot are preserved on the LRT.

Figure 5. Where feathers on the foot are preserved on the LRT.

Earlier we looked at the development of foot feathers to aid in stability in pre-birds and other bird-like taxa just learning to flap and fly, convergent with uropatagia in pre-volant pterosaur ancestors.

A note to Anchiornis workers:
Try to test all your specimens in a phylogenetic analysis for confirmation, refutation or modification of the above recovery. Pei et al. considered all the specimens conspecific. They are not conspecific, as one look at their skulls alone (Fig. 1) will tell the casual observer.

Pei R, Li Q-G, Meng Q-J, Norell MA and Gao K-Q 2017. New specimens of Anchiornis huxleyi (Theropoda: Paraves) from the Late Jurassic of Northeastern China. Bulletin of the American Museum of Natural History 411:66pp.
Xu X, Zhao Q, Norell M, Sullivan C, Hone D, Erickson G, Wang X, Han F and Guo Y 2009. A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin. Chinese Science Bulletin 54 (3): 430–435. doi:10.1007/s11434-009-0009-6
Xu X and Zhang F 2005. A new maniraptoran dinosaur from China with long feathers on the metatarsus. Naturwissenschaften. 92 (4): 173–177. doi:10.1007/s00114-004-0604-y.



Triassic origin of scales, scutes, hair, etc. as biting fly barriers?

During the Middle to Late Triassic

  1. Mammals developed fur/hair.
  2. Aetosaurs developed plates and horns beyond the earlier paired dorsal scutes.
  3. Crocodylomorphs developed large scales beyond the earlier paired dorsal scutes.
  4. Dinosaurs lost those paired scutes and developed placodes and quills. Ultimately these became scales and feathers.
  5. Turtles developed hard scales over a carapace and plastron.
  6. Lepidosaurs developed small scales.
  7. Pterosaurs and their Late Triassic sisters developed pycnofibers

All of these developed on the soft, naked skin
(think of a plucked chicken) that was a universal covering for Carboniferous and Permian tetrapods (Early forms retained large ventral scales inherited from finned ancestors, but these were lost by the Permian).

All of these extradermal structures have one thing in common.
They separated and/or protected the animal’s naked skin from the environment, one way or another. They developed by convergence. Dhouailly 2009 and other workers discussed the chemical similarities of the keratin found in these dermal structures. 

The question is:
What was different about the Triassic environment that was not present in earlier Carboniferous and Permian environments? We can 
eliminate heat, cold, UV rays, rain, aridity, etc. as possible reasons for the development of insulator structures because those factors had always been present. So what was new in the Triassic that affected all terrestrial tetrapods?

Flies and their biting, piercing kin.
“The earliest definitive flies known from the mid-Triassic of France, approximately 230 Ma (Krzemiski and Krzeminska, 2003)” according to Blagoderov, Grimaldi and Fraser 2007. The order Diptera (flies, mosquitos and kin) tend to land on large tetrapods for food, blood, etc. Scales, scutes, hair, feathers, etc. all separate flying insects from the naked skin of Triassic terrestrial tetrapods. Williams et al. 2006 even found mosquito repellents in frog skin. It is notable that, except for armored placodonts and mosasaurus (derived varanid lepidosaurs), aquatic and marine tetrapods also had naked skin with the thalattosaur, Vancleavea, a notable sermi-terrestrial exception. Is that because they had aquatic antecedents in the Triassic that were never affected by flying insects?

It’s not just the insect bite that drives this evolution,
it’s the appearance of new vectors for the rapid spread of disease that drives this evolution.

Interesting coincidence.
If this is not the case, this will take further study.

Figure 1. Lacertulus, a basal squamate from the Late Permian

Figure 1. Lacertulus, a basal squamate from the Late Permian

Carroll and Thompson 1982 report
on the Late Permian lepidosaur, Lacertulus (Fig. 1), “No scales dermal or epidermal are evident in the specimen.”

From the Dhouailly 2009 abstract:
“I suggest that the alpha-keratinized hairs from living synapsids may have evolved from the hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians.

Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.” Not realized by Dhouailly, the purported clade ‘Sauropsida’ is paraphyletic and a junior synonym for Amniota and Reptilia in the LRT.

Earlier we looked at the first appearances
of hair, quills, pycnofibers and hard scales in a three-part series here, here and here

Exceptionally, humans are terrestrial tetrapods
that have lost most of their hair, more or less returning to the primitive naked state. And yes, flies and mosquitos do bother humans. It is the price we pay for the benefits of naked skin. Clothing helps provide a barrier.

Just because an idea is proposed and a hypothesis is advanced doesn’t make it so. In science ideas have to be confirmed or refuted following their first appearance. If anyone has data concerning scales or other dermal structures in Carboniferous or Permian taxa, please make us aware of those.

Blagoderov V, Grimaldi D and Fraser NC 2007. How Time Flies for Flies: Diverse Diptera from the Triassic of Virginia and Early Radiation of the Order. American Museum Novitates 3572:1-39. DOI: 10.1206/0003-0082(2007)509[1:HTFFFD]2.0.CO;2
Carroll RL and Thompson P 1982.
A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.
Dhouailly D 2009.
A new scenario for the evolutionary origin of hair, feather, and avian scales Journal of Anatomy 214(4): 587–606. doi: 10.1111/j.1469-7580.2008.01041.x
Krzeminnski, W., and E. Krzeminska. 2003. Triassic Diptera: descriptions, revisions and phylogenetic relations. Acta Zoologica Cracoviensia (suppl.) 46: 153–184.
Maderson PFA and Alibardi L 2000.
The Development of the Sauropsid Integument: A Contribution to the Problem of the Origin and Evolution of Feathers. American Zoologist 40:513–529.
Rohdendorf BB, Oldroyd H and Ball GE 1974. The Historical Development of Diptera. The University of Alberta Press, Edmonton, Canada. ISBN 0-88864-003-X.
Williams CR, Smith BPC, Best SM and Tyler MJ 2006.
Mosquito repellents in frog skin. Biol Lett. 2006 Jun 22; 2(2): 242–245. doi: 10.1098/rsbl.2006.0448

The first Jurassic feather – SVP abstract 2016

Pittman et al. 2016
describe a new way of looking at fossils, with laser stimulated fluorescence. I can’t show you what attendees saw at SVP as it is awaiting publication, but other examples can be seen here online. This image from Tom Kaye (Fig. 1) was bumped by me with Photoshop to increase contrast and perhaps reveal a wee bit more detail.

Figure 1. Archaeopteryx feather from T. Kaye. Second image is Photoshop contrast bump created here.

Figure 1. Archaeopteryx feather from T. Kaye. Second image is Photoshop contrast bump created here. Pittman et al. laser stimulated fluorescence imagery was shown at SVP and is awaiting publication. 

From the Pittman et al. 2016 abstract
“The single feather initial holotype of Archaeopteryx lithographica is one of the world’s most iconic fossils, but contains a 150 year old mystery. The specimen’s 1862 description by Hermann von Meyer shows that the calamus is 15 mm long and 1 mm wide. However, the calamus is no longer visible on the fossil, and there is no record of when or how it disappeared. The specimen is a rare example of a lone Archaeopteryx feather, giving access to its entire morphology, as opposed to only parts of it in the overlapping feathers of articulated specimens. This makes it an important addition to the anatomical record of Archaeopteryx and basal birds more generally. After 150 years, laser stimulated fluorescence has recovered the calamus as a chemical signature in the matrix and reveals preparation marks where the original surface details have been obliterated. The feather has recently been imaged by others under UV light as well as with X-rays at the Stanford Linear Accelerator Center, with no reports of the existence of the calamus. This demonstrates the capability of laser stimulated fluorescence to visualize important data outside the range of current methodologies. The feather has at different times, been cited as a primary, secondary and covert, and has even been suggested to belong to another taxon. With the new calamus data in hand, the morphology of the feather was examined within the framework of modern feather anatomy. The percentage of calamus length to overall feather length, when plotted against a histogram of 30 phylogenetically and ecologically diverse modern birds, comes out in the middle of the range, placing it in the flight feather regime. The most recent identification of the feather as a primary dorsal covert can be discounted because the rachis is in line with the calamus rather than curving upwind of the calamus centre line. The curvature of the rachis is also too pronounced to function as a primary or tail feather. If the feather is scaled as a secondary in the wing of Archaeopteryx, only five feathers fit the reconstruction along the ulna, rather than the 9-13 that have been estimated for this taxon and the 7-14 that are found in modern birds. These inferences suggest that the isolated feather is fundamentally inconsistent with those of Archaeopteryx and is instead a secondary of another early bird taxon or potentially even a feather of a non-avialan pennaraptoran theropod.”

Kaye’s work with fossil imaging
has revealed many interesting and otherwise invisible traits. Let’s call this one more ‘feather in his cap.’

Pittman M, Kaye TG, Schwarz D, Pei R and Xu X 2016. 150 year old Archaeopteryx mystery solved. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.

Feathered T-rex video: Excellent!*

The best video* I’ve seen on feathered dinosaurs.
*But note: their gliding Anchiornis forgot how to flap. Flapping came first. Then flapping with bipedal climbing. Then flapping with flying. Birds don’t come by gliding except to rest while airborne. Same with bats (if any glide ever). Same with pterosaurs. Let’s take gliding out of the equation for the origin of flight. That’s widespread antiquated thinking not supported by evidence. If you glide you do not flap. If you flap, some of your ancestors may learn to glide.

Click here or on the image to play.