 |
| Fig. 1: Two versions of “Proavis”, a
hypothetical proto-bird even more primitive than Archaeopteryx. The one
on the left is Gerhard Heilmann’s version from the 1920s, a thecodont that uses
scales to glide. The one on the right is Greg Paul’s from 2002, an arboreal
dinosaur similar to a tree kangaroo that uses airfoils derived from insulation
to soften its fall. Guess which vision won out. |
Birds are
dinosaurs. This is consensus now and has been at the very least since the 90s
(which are 20-30 years away now). While this is a fact that to our modern minds
seems obvious or even trivial, the road that led to this consensus was long, convoluted
and lined with some quite bizarre side-tracks. The purpose of this post,
similar to the one about pterosaurs, is to highlight some of these unorthodox ideas that
have now fallen into obscurity (though note that not all will be ridiculed
here, as some certainly have some merit). Before that however a little rundown
on the history of research on bird origins: The idea that birds might descend
from dinosaurs had already gained traction in the 1870s, as Thomas Henry Huxley
and the prominent paleontologists Edward Drinker Cope and Othniel Charles Marsh
endorsed this idea based on their research on dinosaurs and fossil birds. It
became a debated issue with no clear consensus reached. A great shift came then
in 1926 when Danish amateur-ornithologist Gerhard Heilmann released his The
Origin of Birds. In it he is only one sentence away from reaching the
conclusion that birds truly descend from coelurosaurian theropod dinosaurs, but
then takes a turn in the last second. The reason for this is that in the
dinosaurs known to him at the time the clavicle, a bone of the chest, was
either underdeveloped or absent, while this bone was strongly developed into a
furcula in birds. As Heilmann believed in Dollo’s Law (the idea that once a
trait was lost it could not re-evolve) he therefore concluded that birds could
not have descended from dinosaurs and must instead have been derived from
earlier “thecodont” reptiles similar to Euparkeria, which still had a
developed clavicle. Any similarity between dinosaurs and birds was therefore
due to convergence, though he still thought they must share a close common
ancestor. The problem with Heilmann’s thesis was that Oviraptor, a
dinosaur with clearly developed clavicles, was already known at the time, but
was unfortunately not studied well. Nonetheless, thanks to his writing and
artwork, Heilmann’s book was seen as the last word on bird origins, became wide
consensus and unintentionally shut down any further research on the topic. The
discovery of clavicles in Segisaurus in 1936 was blatantly ignored, as
were Percy Lowe’s (1935) and Nils Holmgren’s (1955) re-suggestions of a
dinosaurian bird-origin. This only changed in the 1970s when John Ostrom
compared the anatomy of Archaeopteryx with that of Deinonychus and
many new fossil discoveries were made, which allowed paleontologists to
robustly revive Huxley’s ideas of bird origins. An intense debate broke out in
the 80s and early 90s between dinosaur-experts and paleo-ornithologists who
still clung to the old Heilmann-idea. Cladistic methods however always favoured
the dinosaur-hypothesis and soon enough fossils of clearly feathered dinosaurs
were also discovered, basically annihilating any opposition. Today the
thecodont-hypothesis and its variations are seen as untenable (in part also
because thecodonts are not a thing anymore) and it is generally agreed that
birds are maniraptoran theropods, with their closest relatives being
deinonychosaurs such as Velociraptor and Stenonychosaurus.
Birds descend
from fish
 |
Fig. 2: If you
ever wondered what a mix between fish and poultry would taste like, I recommend
trying alligator meat. I always eat some when I visit Florida.
|
Of course, if you
want to be a Smarty McSmartpants, you could say that this statement is true (at
least depending on your definition of fish) in a broad sense. But what the
title refers to here is the hypothesis proposed by Chinese professor Tao Hai
that birds directly descend from flying fish. Yes, you heard that right.
“What Victorian-era silliness is this?” you might think to yourself, until I
tell you that this suggestion was made in the far and distant year of 1993. He
came to this conclusion after examining fossils from the Xinjiang region which
he called “proto-birds with fin-like wings”. According to Wolfgang Hassenpflug
and Gerald Kopp the fossils do indeed have some sort of resemblance to
decapitated bird-carcasses, but are in actuality normal fish which have become
bloated and disarticulated due to decomposition. There is a possibility that
they might be early flying fish, but they definitely are not bird ancestors. Apparently
Hai even made reconstructions of his bird-fish, but I have not been able to
find them. How exactly a teleost fish would be able to convergently evolve a
tetrapod body-plan and an avian lung-system was never explained by him and
frankly the idea seems so ridiculous that it would only be overtaken by me
proposing that birds descend from flying insects.
Birds are the
closest relatives of mammals
 |
| Fig. 3: The stem-haemotherm, a hypothetical
ancestor to a clade that consists of only mammals and birds. |
In the nineteenth
century Richard Owen grouped birds and mammals together in the group
Haemothermia, suggesting that they both descend from a close common ancestor,
as they were the only warm-blooded vertebrates around. The two groups share
other superficial traits, such as insulation and four-chambered hearts, but
these are all clearly not inherited from a common ancestor but examples of
convergent evolution. Despite this some researchers have still tried to
maintain the Haemothermia/Haematothermia hypothesis. The first to resurrect it
was embryologist Søren Løvtrup (a critic of Darwinism who instead believes in
occasional saltation-events) in 1977, followed by Brian Gardiner in the 80s. Both
however largely based their conclusions on data obtained in 1866 and 1693 (!)
and their cladistic models only worked by excluding vast amounts of unignorable
fossil and anatomical data. Notorious David Peters’ recent suggestion that
mammals are the sister-group to archosaurs also has clear shades of this. Despite
how untenable the haemotherm-hypothesis is, Phillippe Janvier produced a
reconstruction in 1983 of what the stem-haemotherm, the common-ancestor of
birds and mammals, should look like, which you can see here. Needless to say,
such a bird-mammal has yet to show up in the fossil record and it never will.
It is however nice fuel for speculative evolution.
Birds are
crocodilian-descendants
 |
| Fig. 4: Effigia okeefeae as illustrated
by Raul Martin. These shuvosaurids show remarkable convergence with theropod
dinosaurs, so much that Shuvosaurus was once mistaken for an
ornithomimid. In reality these animals were more closely related to crocodiles. |
From the frankly
ridiculous suggestions we now come to ones that at least have (or had) some
merit. Birds, being archosaurs, are today the closest living relatives of
crocodilians and share many anatomical characteristics with them. These, we
know today, were given down by a distant common ancestor. For a brief time in
the 1970s however it was suggested that birds could have directly descended
from proto-crocodiles. This idea came from Alick Walker after his description
of Sphenosuchus in 1972. Sphenosuchus and its relatives, the
Triassic sphenosuchians, were a group of crocodylomorphs which are now thought
to have been the immediate ancestors of the crocodyliforms, including our
modern crocs. Despite this their anatomy was highly gracile, they had upright
limbs, could probably walk bipedally at least on occasion and likely had a high
metabolism. They in fact resembled theropod dinosaurs so much that some
sphenosuchians like Hallopus and Saltoposuchus were once mistaken
for early dinosaurs. Based on this and the assumption that sphenosuchians were
more capable climbers than dinosaurs, Walker picked them as the bird-ancestors.
At this point we should probably mention an important controversy in the
debates around bird origin: Did birds evolve flight by taking off from the
ground or by gliding from trees down? To some this had important implications
around the question if birds descended from dinosaurs or from other archosaurs,
as it was thought by some that dinosaurs were incapable climbers, while
“thecodonts” were seen as at least partially arboreal. Some researchers
thinking that bird-flight could have only evolved in an arboreal setting
therefore automatically excluded dinosaurs as candidates, despite any
anatomical similarities. This mutual exclusion became nil however after
paleontologists like Gregory S. Paul and Sankar Chatterjee convincingly
demonstrated that theropods like Sinornithosaurus or Ornitholestes would
have been perfectly capable of climbing trees in the same way as cats or tree
kangaroos do today. In fact, they would have been better at doing so than
sphenosuchians, which actually lacked many of the arboreal qualities of earlier
archosaurs such as Euparkeria. Some of the other characteristics Walker
chose to argue for a sphenosuchian-bird link also turned out to not be
convincing. While crocodilians and modern birds do share some interesting
similarities in their skull-bones, like a double-headed, pneumatic quadrate, these
are not present in early birds such as Archaeopteryx and therefore
represent a case of convergent evolution. Even more importantly is that
crocodilians and birds share a completely different ankle-structure, so much so
that this is the defining characteristic which separates croc-line from
bird-line archosaurs. Walker himself has recanted his hypothesis in 1985.
Birds descend
from pterosaurs
 |
| Fig. 5: Archaeopteryx and Rhamphorhynchus
certainly would not enjoy a family reunion. |
Birds and
pterosaurs share many characteristics, such as the avian-style ankle, hollow
bones, filamentous body-coverings, beaks and of course the capability of
powered flight. That they share a direct ancestor-descendant-relationship is
therefore an intuitive but wrong assumption made by pop-culture, laypeople and
children (though I have witnessed moments in museums where even six-year-olds
were able to correct their parents about this, which I can only hope to achieve
one day when I have kids). In the pre-Heilmann era a close relationship between
pterosaurs and birds (though not necessarily are direct ancestry) was however a
very popular, legit hypothesis and was seen as the most viable alternative to Huxley’s
dinosaur-bird link. It was championed by figures such as St. Georges Mivart and
Harry Govier Seeley, the latter thought they were each other’s closest
relatives with a common ancestor living somewhen in the late Paleozoic. An
earlier even more unusual suggestion was made by Samuel Thomas von Soemerring,
one of the original workers on Pterodactylus. In 1810 he proposed that pterosaurs
represent an intermediate stage between birds and bats (though not necessarily
in an evolutionary sense as Darwin had not published his theories yet). Any
such notions were already carefully criticised by Huxley in the 1870s and
thankfully whisked away by Heilmann in his 1926 work. It has since become obvious
that most similarities shared by the two groups convergently evolved due to the
adaptation to powered flight. In 1983 Larry Martin nonetheless suggested that Scleromochlus,
an archosaur closely related to pterosaurs, was also an especially close
relative of birds, but this has largely been ignored or dismissed for good
reason. How many of the similarities between birds and pterosaurs are truly convergent is still an
interesting question. Recent work done of pterosaur pycnofibers heavily
suggests that these structures are homologous to the proto-feathers of
dinosaurs and might be called such. The last common ancestor between dinosaurs
and pterosaurs may therefore already have been quite a birdy animal.
Birds descend
from ornithischian dinosaurs
 |
| Fig. 6: The fuzzy ornithischian dinosaur Kulindadromeus,
drawn by Tom Parker. |
The, albeit
superficial, similarities between birds and some ornithischian dinosaurs,
especially the aptly named ornithopods, is fascinating and something I feel is
underappreciated. Both have a retroverted pubis, three-toed feet at the end of
long bipedal legs, long necks and beaks. It is not hard to imagine that many
smaller ornithopods had lifestyles and behaviours similar to those of modern
ground-birds and landfowls. Until the 70s it was even thought that some
ornithopods were arboreal and had a retroverted first toe like modern perching
birds. The heterodontosaur Manidens may have been a genuine tree-dweller.
That birds descend from the bird-hipped dinosaurs is therefore again an
assumption that could be intuitively made, but will confuse a lot of laypeople,
as birds counterintuitively descend from Saurischia, the lizard-hipped
dinosaurs. The similarities to ornithischians are largely due to convergence or
common ancestry. Nonetheless there have been serious suggestions, made by some
quite prominent figures, that the former is the case. The first was made by
Huxley after comparing Archaeopteryx with the ornithopod Hypsilophodon,
mainly based on the shape of their pubis, though he saw a relationship to
theropods such as Compsognathus as equally likely. This was further
elaborated on by Othniel Charles Marsh in his 1896 monograph The Dinosaurs
of North America where he writes about ornithopods:
“The various
dinosaurs thus briefly referred to under their respective genera have many
other points of interest that can not be here discussed, but their resemblance
to birds is worthy of some notice. This is apparent in all of them, but in the
diminutive forms the similarity becomes most striking. In all the latter the
tibia is longer than the femur, a strong avian character, and one seen in
dinosaurs only in the small bird-like forms. In Nanosaurus nearly all,
if not all, the bones preserved might have pertained to a bird, and the teeth
are no evidence against this idea [Marsh had previously discovered the
toothed birds Hesperornis and Ichthyornis]. In the absence of
feathers an anatomist could hardly state positively whether this was a
bird-like reptile or a reptilian bird.” (Marsh 1896 p. 201)
Especially the
last sentence is interesting, as it might raise the question if Nanosaurus
may have been feathered in life. While I am not sure if that is what Marsh
wanted to insinuate, if he did, he would have predicted fossil discoveries made
over 100 years in the future. Since 2010 ornithischian dinosaurs have been
discovered with structures very similar to and likely homologous with feathers,
most important among these being Kulindadromeus, a basal neornithischian
just like Nanosaurus. The bird-ornithischian link was last revived in
1970 by Peter Galton. He however had to admit that ornithischians were far too
derived for them to be the direct ancestors and instead just proposed a close
common ancestor. He later recanted his idea. Ironically, this may not have been
too far off the mark, given the recent Ornithoscelida hypothesis.
Birds
originated in the Triassic
 |
| Fig. 7: Pretty much everything in this picture is
wonky as hell. The anatomy of the Postosuchus is completely off (though
at least they got the bipedalism right), Technosaurus is considered a
dubious genus now, Shuvosaurus was a croc-line archosaur and Protoavis
was not a real taxon, let alone a Triassic bird. The limestone also looks suspiciously like a brick wall. |
When birds first
started appearing was a controversial question for quite some time. In the
early days of paleontology it was first generally thought that they only
appeared in the Cenozoic, but then the discovery of Archaeopteryx
extended the timeline all the way into the Late Jurassic. But were there also birds
long before Archaeopteryx? The first such claim came from Edward
Hitchcock in 1848, 12 years before the Urvogel was even discovered. He
identified giant three-toed footprints of the Triassic New Red Sandstone as
pertaining to gigantic moa-like waterfowl, but these later turned out to have
been produced by basal sauropodomorphs. Some later paleontologists, who wanted
to argue against a dinosaurian origin of birds, also used Triassic footprints
as evidence for a pre-dinosaurian origin, but many of these footprints were as
large as a human’s, if not larger, so they likely made the same mistake as
Hitchcock. In the 70s Paul Ellenberger claimed that birds descend from the
Triassic reptile Cosesaurus, but none of the subsequent studies were
able to confirm his observations of bird-like features in this animal,
including feathers, and it is now regarded as a protorosaur related to Tanystropheus.
The most recent suggestion of Triassic birds came from Sankar Chatterjee, who
in 1991 discovered some interesting bones in the Texan Dockum Beds (I have
written more about this here).
He called it Protoavis texensis and reconstructed it as a Triassic
proto-bird even more derived than Archaeopteryx. Notably though,
Chatterjee still argued in favour of birds descending from maniraptoran
theropod dinosaurs, but in order to do so he had to move the origin of that
clade all the way back into the Triassic. In the process he accidentally
classified Shuvosaurus as a Triassic ornithomimosaur. It later turned
out to have actually been a pseudosuchian crocodile-relative. After further
examination by other workers, Chatterjee’s Protoavis turned out to not
even be a genuine taxon and many of the bird-like characteristics he ascribed
to it could not be found. Instead it was a fossil chimaera of one or more early
theropods and a drepanosaur which died at the same spot likely due to a
monsoonal flash flood. Modern consensus still is that the bird-lineage
originated somewhen in the mid-to-late Jurassic, with crown-group birds only
appearing in the Late Cretaceous.
Only some
birds descend from dinosaurs
 |
Fig. 8: Struthiomimus,
illustrated by Gerhard Heilmann with some very ostrich-like behaviour. Take
special note of the one scratching itself in the background.
|
At times the idea
that all birds descend from the same ancestor has also been called into
question. In the late nineteenth century Carl Vogt, St. George Mivart and
Robert Wiedersheim for example believed that Archaeopteryx and modern
flying birds descended or were closely related to pterosaurs, while flightless
ratites, such as ostriches, directly descended from dinosaurs. A similar belief
was also held by Percy Lowe and Baron Franz Nopsca, who believed ostriches and rheas directly
descended from the ornithomimd theropods, such as Struthiomimus, Ornithomimus
or Gallimimus, with Hesperornis serving as an intermediate
stage. In this view, ratites did not secondarily lose the ability to fly, but
never evolved flight in the first place and supposedly evolved their wings for
different purposes. Needless to say, this view never caught on, but when I was
a kid I remember having a German dinosaur book, Das Grosse Buch der Saurier,
that actually still presented this idea in a serious manner. The book was
originally written in 1989 by a certain Peter Klepsch, of whom I could not find
more information, and was beautifully illustrated by Peter Thiemeyer, an
accomplished fantasy-artist. The version I own was a reprint from 2000. I
scanned and translated the anachronistic section below.
 |
Fig. 9: “For a
long time animals with scales were regarded as reptiles and those with feathers
as birds. But since in the meantime dinosaurs with feathers have been
discovered, some scientists order animals into the reptiles or birds according
to their ability to fly. Emus, cassowaries, kiwis, rheas and ostriches never
flied though and do not descend from flying animals. They are according to the
latest understanding either the closest relatives of the dinosaurs or still
belong to them today.”
|
Birds descend
from theropods other than maniraptorans
 |
| Fig. 10: Climbing, feathered Ornitholestes,
illustrated by Greg Paul in 1988. The idea that Ornitholestes had a
nasal crest is discarded today and was based on a crushed skull. |
Even among people
who agreed that birds are a group of theropods it was not always universally
agreed upon from which theropods they came from. Before it was realized that it
was a chimaera, Greg Paul in 1988 briefly considered the possibility that Protoavis
and all later birds descend from the herrerasaurs, but he saw this as unlikely. In
1985 Michael Raath also proposed that coelophysoids were especially close
relatives of birds, but this was dismissed as these theropods had short arms
and were still relatively archaic. Even after Thomas Huxley, Compsognathus still
gained a lot of attention as a possible close relative of Archaeopteryx,
mainly because they lived in the same place and had roughly similar skeletons
(to the point where one Archaeopteryx preserved without feathers was
originally identified as a Compsognathus), but this dinosaur was too
basal of a coelurosaur for that to be the case. A somewhat bizarre idea came
from Andrzej Elzanowski and Peter Wellnhofer in 1992/3, who concluded that
troodonts, spinosaurs and birds form a monophyletic clade. Birds and troodonts
being close relatives is obvious, but the titanic Spinosaurus being an
especially close relative of Archaeopteryx seems rather hard to believe.
Though the way shoebill storks look at me at the zoo sometimes makes me doubt
my assertions.
Some
dinosaurs are actually secondarily flightless dino-birds
 |
| Fig. 11: Are dinosaurs like Tarbosaurus and Therizinosaurus actually secondarily flightless? It is more plausible than you
think. |
One of the most
fascinating and plausible ideas discussed in this post comes from Gregory S.
Paul. First in his 1988 book Predatory Dinosaurs of the World and later
in more detail in his 2002 book Dinosaurs of the Air he proposed the
idea that some of the dinosaurs we conventionally think of as non-avian are
actually secondarily flightless and descend from flying dino-birds more derived
than Archaeopteryx. Several lines of evidence make this idea attractive:
- Archaeopteryx is, skeleton-wise, not really more derived than other
maniraptoran dinosaurs, essentially being just a miniature version of Deinonychus.
Some supposedly non-avian dinosaurs on the other hand even have skeletal features more similar to
modern birds than Archaeopteryx.
- Many of the most
bird-like dinosaurs, including Deinonychus, start appearing in the
Cretaceous, after Archaeopteryx.
- The most basal
dromaeosaurs and troodonts were forms such as Microraptor, which were
small and probably capable of flying or gliding, and the lineages grew larger
and less bird-like as time wore on. Even the oldest therizinosaurs and tyrannosaurs were small, bird-like animals.
- These Cretaceous maniraptorans possess many flight-related features, such as
wing-folding-mechanisms, asymmetrical feathers, large sterna, ossified sternal
ribs, binocular vision and large braincases, that are hard to explain in a
terrestrial context and differ from previous terrestrial theropods, but make
more sense if these animals descended from an ancestor capable of flight.
- Secondary
flightlessness has evolved in birds on multiple occasions, even during the
Mesozoic, examples being Gargantuavis and Hesperornis.
While Paul never
declared any dinosaur-group with certainty as neoflightless dino-birds,
possible candidates are the dromaeosaurs, troodonts, oviraptorosaurs,
alvarezsaurs, but also therizinosaurs (based on their reduced tail, posture and
arm-folding-mechanism) and even tyrannosaurs (based on their reduced,
ratite-like forelimbs and binocular vision). The problem with assessing these
possibilities, Paul himself admitted, is that during the process of flight-loss
traits are lost or reversed which would have given more clues about
ancestor-descendant relationships. We know from modern ratites that once flight
is lost, details of the pectoral girdle and limbs revert back to a dinosaurian
state. An incomplete skeleton of an ostrich could under certain circumstances
therefore be misclassified as a non-avian dinosaur. Something quite similar happened with Balaur bondoc, a flightless avialan from Hateg island that was at first identified as a velociraptorine. Even more uncertainties are
introduced once we consider the possibilities that certain dinosaur groups may
have evolved flight and neoflightlessness independently from each other. Most
cladistic work does not strongly support Paul’s idea and recent finds have
shown that troodonts and dromaeosaurs have existed in the Jurassic before Archaeopteryx,
but unlike the other ideas discussed here, his hypothesis does have some
significant following among respected paleontologists, such as Michael J.
Benton. In his renowned textbook Vertebrate Paleontology Benton notably classifies oviraptorosaurs as flightless birds and there are many
anatomical oddities among these dinosaurs, especially among forms like Caudipteryx,
which are hard to explain without a secondary loss of flight. Even famous evolutionary biologist Stephen Jay Gould had some support for the idea. Depending on
future fossil finds of small theropods from the Jurassic, it remains to be seen
how well Paul’s hypothesis will do. Some analyses have recovered a slightly
different scenario, which is that dromaeosaurs like Microraptor or Rahonavis
evolved flight independently of birds. All three could nonetheless still have
descended from a common gliding ancestor. One thing I have curiously never seen
anyone, not even Paul, address is that the neoflightless hypothesis could
explain one of the strangest anatomical oddities in therizinosaurs. Unlike any
other non-avian theropod, these scythe-turkeys have a four-toed foot, with the
first toe facing forward and touching the ground. I propose that, based on basal therizinosaurs
like Beipiaosaurus, if these dinosaurs really do descend from flying or
at the very least arboreal, tree-sloth like ancestors, they might have
originally had a retroverted first toe like modern perching birds, which, as
they grew in size, was re-adapted to add further support to the feet. The double sickle-claws of Balaur are proof that something like this can happen with secondarily flightless dino-birds. (Edit: I have been informed that the most basal therizinosaurs had ordinary feet, so this renders my pet theory into total bunk. Oh well, at least I tried).
Dinosaurs
descend from birds
 |
| Fig. 12: Paul’s illustration of a feathered proto-dinosaur Lagosuchus
bothering a proto-mammal. |
Now what if we all
got this backwards? Seemingly taking Greg Paul’s idea to an extreme, George
Olshevsky argued in 1994 that all dinosaurs descend from Triassic small,
feathered, bird-like animals capable of climbing or even flying that then
became secondarily flightless. He essentially modified an idea had by Othenio
Abel in 1911, who suggested that birds and dinosaurs shared a close, common
arboreal ancestor. In this view, even giants like Brachiosaurus and Stegosaurus
would be flightless proto-birds. As possible ancestors of this weird
bird-dino-clade he suggested Protoavis, which in turn would have evolved
from bizarre reptiles such as Longisquama. Needless to say, this did not
stand the test of time, as Protoavis was not a real taxon, Longisquama
had nothing to do with bird evolution (as explained further below) and the
earliest dinosaurs generally do not show signs of having been arboreal or
secondarily flightless. There is nonetheless still some merit to speculating
that the ancestors of dinosaurs may have been partially arboreal and feathered.
Their close relatives are the pterosaurs, which as mentioned earlier may have
already possessed proto-feathers, and most would agree that these archosaurs
evolved flight from arboreal ancestors. Even more intriguing are the
discoveries of new dinosauromorphs such as Kongonaphon kely, which was
described just this year. This lagerpetid stood just 10 centimeters tall and
possessed an anatomy reminiscent of a jerboa. If it was also as metabolically
active as one it surely would have needed some form of insulation. It heavily
suggests that the ancestors of both pterosaurs and dinosaurs went through a
size-squeeze at one point in their evolution, which lead to the development of
proto-feathers and other anatomical quirks in the two groups. Could the cause
of this squeeze have been an adaptation to living in the trees? Early
ornithodirans like Kongonaphon, Scleromochlus or Lagosuchus
do not show any notable specializations for arboreality, though it is not inconceivable
that they could have occasionally climbed trees in the manner of modern tree
kangaroos. This would have helped them avoid competition with sphenosuchians
and other croc-line archosaurs which were dominant on the ground throughout
most of the Triassic.
Alan Feduccia
in general
 |
| Fig. 13: Drepanosaurus is a fascinating
animal with a bird-like head but an entirely un-bird-like body. It is about as well-suited for being a bird-ancestor as a tortoise. Those have beaks too, you know. |
Those who have
read the previous pterosaur post will notice the obvious parallel here, as I
ended that one with a section on David Peters. It would however be unfair to
directly compare Alan Feduccia to Peters, as the latter is basically a hack
fraud from outside the main scientific community. Feduccia on the other hand
used to be a respected authority in paleoornithology, but alas fell from grace
due to his increasingly unscientific insistence to this day that birds could
not possibly have descended from dinosaurs. In his 1980 book The Age of
Birds his view on bird origins was still largely agnostic with being open
to the dinosaur-hypothesis, but preferring Heilmann’s thecodont-hypothesis (a
still respectable opinion at the time). This tone drastically changed in 1996
with his The Origin and Evolution of Birds, in which he called the
dinosaur-hypothesis a “dogma” that other paleontologists just blindly follow
(in reality it had become consensus due to robust cladistic studies). I will
now list his main arguments and their respective refutations:
- Birds could not
have possibly evolved from theropod dinosaurs, as the theropod hand consists of
digits I-III (corresponding with our thumb, index- and middle-finger) while the
hand of birds consists of digits II-IV (corresponding to our index-, middle and
ring-finger). The problem with this argument is that we actually do not know if
the latter statement is true. It is merely an assumption born from embryology,
as it is the norm for tetrapods to first lose their outermost digits during
evolution. But just because this is the norm does not mean that this is always
the case and no embryologist has ever said with confidence that the bird-hand
truly consists of digits II-IV. Instead both embryological and fossil evidence
heavily suggest that birds truly possess digits I-III and that this condition
was inherited from theropods who first lost digits IV-V.
- Birds descend from
Triassic reptiles such as Longisquama, Megalancosaurus and Drepanosaurus.
The former because it possesses feather-like appendages on its back and a
furcula-like breastbone, the latter because of their eerily bird-like heads. While
Longisquama is certainly striking with its ornamentation we have
actually no idea what these structures are and it seems very unlikely that they
are related to feathers. Some have suggested that they are not even part of the
animal but actually vegetation that was preserved with it. If they are part of
the animal, the resemblance to feathers is only superficial and very divergent
in the finer detail and no living or fossil bird has comparable structures on
its back. Most regard these today as extravagant display-scales. That they were
used for gliding, like some have assumed, has also become untenable, as Longisquama
had apparently only one singular row of these along the back. Its
breastbone is not preserved well enough to determine if it was a furcula or
just a pair of clavicles like earlier reptiles had, which is ironic as Feduccia
does not consider the presence of very obvious furculae in theropods as
evidence that they are related to birds (even though only birds and other
theropods have this bone). Drepanosaurs like Megalancosaurus do have
remarkably bird-like heads, but the rest of the body could not be more
different. They possessed monkey-like prehensile tails with large hooks at the
end and clamp-shaped hands like chameleons, which bear no resemblance to bird-hands
at all. The forelimb of Drepanosaurus has been found to be so odd that
its radius, ulna and a carpal formed a triangle, making it unable to swivel its
forearm, let alone evolve a bird-like wing. Their heads are therefore a case of
convergent evolution. Perhaps even more concerning is that none of these
animals are considered archosaurs nowadays, while birds are by definition
archosaurs. Instead cladistic analyses find these three to either be related to
protorosaurs like Tanystropheus or to even be basal diapsids. The latter assumption would make them less closely related to birds than even snakes.
- The most bird-like
dinosaurs start appearing only after Archaeopteryx evolves, therefore
birds could not have descended from these dinosaurs. This “temporal-paradox” is
essentially a strawman-argument used by Feduccia, as no paleontologist ever
claimed that birds directly descend from Cretaceous forms like Deinonychus,
but rather that these represent relics from an earlier time in evolution. The
“paradox” has been largely resolved by Jurassic maniraptoran fossils even older
than Archaeopteryx, such as Anchiornis and Scansoriopteryx.
Feduccia curiously seems oblivious to the glaring temporal gap in his own
hypothesis, which is that drepanosaurs (as well as any other non-dinosaurian
candidate for bird-ancestry) went extinct at least 60 million years before Archaeopteryx
showed up.
- Birds evolved
flight from climbing ancestors, while dinosaurs were purely terrestrial and therefore could not have been their ancestors. As
mentioned earlier, while the former is likely true, the latter is not as many
smaller dinosaurs would have been perfectly able to climb trees and lead
scansorial lives. The maniraptoran group scansoriopterygidae is even named
after their climbing abilities.
- Any similarity
between birds and dinosaurs is purely due to convergent evolution. This point
is defeated by Feduccia’s aforementioned assertion that birds evolved from
arboreal ancestors, while dinosaurs always lived on the ground and were unable
to climb. For convergent evolution to take place, two unrelated animals
generally need to have the same lifestyle and live in the same environments.
How would purely terrestrial dinosaurs and purely arboreal birds then be able
to converge on an almost identical anatomy? Feduccia never offered an
explanation for this and also did not explain why dinosaurs and birds resemble
each other even in the microstructure of their bones and eggshells, a feature
that is nearly impossible to explain through convergence.
Feduccia, being a
paleoornithologist, was not an expert on dinosaurs, and many of his assertions
seem to be arguments from ignorance. This is evident by his anachronistic view
of dinosaurs being a polyphyletic group of unrelated, exothermic terrible
lizards and many anatomical errors he makes. He also still believed that
feathers evolved from scales and purely in an aerodynamic context, not for
insulation, something which was already disproven at the time by genetic and
embryological studies (feathers are homologous with crocodilian scutes, not
scales, and first evolved for insulation).While his work received praise from
ornithologists at the time it was obviously panned by paleontologists.
Ironically in the same year Sinosauropteryx, the first known non-avian
dinosaur with proto-feathers preserved, was discovered. What was Feduccia’s
response to that? First he called it the “Piltdown Dinosaur” (comparing it to the
Piltdown Man and thereby insinuating that it was a fraud, a serious and
unfounded allegation) and then claimed that its filaments are not actually
feathers, but fossilized muscle-fibers or a collagen frill like a lizard’s.
While geochemical analyses could not be done on the Sinosauropteryx
fossils, they were able to be carried out by Mary Schweitzer on identical
structures in the alvarezsaur Shuvuuia in 1999. These studies found that
the structures had a hollow central shaft and were keratinous in origin,
meaning they were feathers and not collagen-fibres. A 2017 study carried out by
Fiann Smithwick directly compared Sinosauropteryx’s filaments with the
fossilized collagen-fibres on the ichthyosaur Stenopterygius and found
no similarities. The supposed frill around Sinosauropteryx which
Feduccia wanted to identify also turned out to be an artifact of
fossil-preparation. Despite all this, as far as I am aware, Feduccia still uses
the collagen-fibre argument whenever proto-feathers are discovered on dinosaurs
such as Yutyrannus or Beipiaosaurus. Once it became undeniable
that dromaeosaurs and other maniraptoran dinosaurs possessed clearly avian
pennaceous feathers he did a complete 180 degree turn and, after years of
saying that these dinosaurs had absolutely no relation to birds, declared that
the maniraptorans are actually not dinosaurs at all, but secondarily flightless
birds descended from drepanosaurs. Theropods would therefore be polyphyletic
and the likes of Velociraptor or Deinonychus would be non-dinosaurs that just convergently look like theropod dinosaurs. While he
details this the most in his 2012 book Riddle of the Feathered Dragons,
he came to this conclusion around 2002. This is curiously the same year in which
Gregory S. Paul published his Dinosaurs of the Air. Indeed, it seems
very much like Feduccia’s current position is a bastardization of Paul’s and
Olshevsky’s neoflightless hypotheses, which he misappropriated to weasel his
way out of admitting that he was wrong and that birds are dinosaurs. Funnily
enough, Feduccia is so predictable that Paul already knew in 2002 that this
would happen. Back then he wrote:
“That argument
that avepectoran dinosaurs [what Paul calls maniraptoriformes] were
birds has not been presented but can nevertheless be anticipated. Let us assume
that feathered Caudipteryx is an oviraptorosaur relative and, further,
unambiguous contour feathers are found in the likes of dromaeosaurs, troodonts,
oviraptorosaurs, or therizinosaurs [this has indeed come to pass at least for the first three]. According to the hypothesis that feathered
archosaurs are birds with fingers II-IV rather than dinosaurs with digits
I-III, the avepectorans were members of the bird clade, a clade that evolved
independently from theropods that lacked ossified sternal plates! The problem
with this notion is that avepectorans share far too many derived
characteristics with other predatory dinosaurs for us to accept the extreme
degree of convergence required to produce this version of dinosaur polyphyly.
The closest known sister taxa to avepectorans clearly are less-derived
averostran avetheropods such as Allosaurus, Ornitholestes and Coelurus.
Consider that opponents of the dinosaur-bird link have long thought that the
teeth of dromaeosaurs and troodonts are nonavian, so making these avepectorans
post-urvogels only serves to reinforce the link. The presence of feathers in
nonavepectoran dinosaurs also works against the possibility that dinosaurs were
not monophyletic.” (Paul
2002, p. 217-218)
Feduccia’s current
BANDit-movement (short for Bird Are Not Dinosaurs) was therefore dead on
arrival and has only become more untenable, with the discovery of feathers in
even more compsognathids (which clearly were not secondarily flightless), the
large Tyrannosaurus-relative Yutyrannus and ornithischians like Kulindadromeus
and Psittacosaurus, as well as numerous cladistic analyses agreeing
that Dinosauria is a monophyletic group (Feduccia’s ideas in fact only work if
proper cladistic methods are completely disregarded). If we were then to follow
his logic to the inevitable conclusion, Tyrannosaurus and Triceratops
would not be dinosaurs at all, but secondarily flightless
drepanosaur-descendants. In the early 2000s even the ornithological community
turned on him for this nonsense. The harshest critique came from Richard Prum
of the American Ornithologist’s Union, who argued that Feduccia’s methodology
is downright pseudoscientific, as he has clearly already made a conclusion
beforehand, which is that birds could not have possibly descended from those
icky reptilian behemoths, and tries to bend reality to fit his conclusion. The
way in which he is willing to constantly contradict himself and immediately
move the goalposts once disproven, as well as the extreme denial, has even been compared to the strategies used
by young-earth-creationists. His rhetoric is especially reminiscent of the "Teach the controversy"-attitude of intelligent design advocates. This is not helped by the fact that actual
creationists have used some of Feduccia’s work to argue against birds evolving
from reptiles. Feduccia’s last major argument in 2014 was that maniraptorans,
through Scansoriopteryx, descend from the Triassic pterosaur-relative Scleromochlus
(an idea he likely borrowed from Larry Martin, who unfortunately died in 2013
due to cancer). If somewhen in the future Feduccia will go further and argue
that birds descend from pterosaurs, then even Gerhard Heilmann will be rolling
in his grave.
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Related
Posts:
Literary
Sources:
- Bakker, Robert
Thomas: The Dinosaur Heresies. New Theories Unlocking The Mystery of the
Dinosaurs and Their Extinction, New York 1986.
- Benton, Michael
James: Vertebrate Paleontology, 1990 (3. Edition from 2005, German
translation).
- Chatterjee,
Sankar: The Rise of Birds. 225 Million years of Evolution, Baltimore 1997.
- Desmond, Adrian:
The Hot-Blooded Dinosaurs. A revolution in Paleontology, London 1975.
- Heilmann, Gerhard:
The Origin of Birds, London 1926.
- Klepsch, Peter: Das grosse Buch der Saurier,
Ravensburg 1989 (2000 reprint).
- Knight, Charles Robert: Life through the Ages, New York 1946 (Commemorative Edition).
- Marsh, Othniel
Charles: The Dinosaurs of North America, Washington 1896.
- Norman, David: The
Illustrated Encyclopedia of Dinosaurs, London 1985.
- Paul, Gregory
Scott: Predatory Dinosaurs of the World. A Complete Illustrated Guide, New York
1988.
- Paul, Gregory
Scott: Dinosaurs of the Air. The Evolution and Loss of Flight in Dinosaurs and
Birds, Baltimore 2002.
- Feduccia, Alan: The
Age of Birds, Cambridge 1980.
- Feduccia, Alan:
The Origin and Evolution of Birds, New Haven 1996.
- Feduccia, Alan:
Riddle of the Feathered Dragons, New Haven 2012.
- White, Steve: Dinosaur Art. The World's Greatest Paleoart, London 2012.
- Witton, Mark: Recreating an Age of Reptiles, Marlborough 2017.
Papers:
- Hassenpflug,
Wolfgang/Kopp, Gerald (1997): Some New Discoveries About the Groups of
Palaeoecological Geography of Xinjiang and the Study of Them
- Huxley, Thomas Henry
(1868): On the animals which are most nearly intermediate
between birds and reptiles
- Janvier, Phillipe
(1984): El divorcio del ave y del cocodrilo, in: Mundo Cientifico 32,
14-16.
- Olshevsky, George
(1994): A Revision of the Parainfraclass Archosauria Cope,
1869, Excluding the Advanced Crocodylia
- Prum, Richard (2002): Why Ornithologists Should Care About The Theropod Origin of Birds
- Prum, Richard (2003): Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia (2002), in: The Auk, vol. 120, p. 550-561.
- Schweitzer,
Mary Higby et al. (1999): Beta-keratin specific immunological reactivity
in feather-like structures of the Cretaceous alvarezsaurid, Shuvuuia
deserti, in: Journal of Experimental Zoology (Mol Dev Evol), 285,
p. 146-157.
- Smithwick, Fiann et al. (2017): On
the purported presence of fossilised collagen fibres in an ichthyosaur and a
theropod dinosaur
- Yang et al. (2019): Pterosaur
integumentary structures with complex feather-like branching
Online
Sources/Further Reading:
Image
Sources:
- Fig. 1 left: Heilmann 1926, p. 200.
- Fig. 1 right: Paul 2002, p. 129.
- Fig. 2: Wikimedia.
- Fig. 3: Janvier 1984.
- Fig. 4: White 2012, p. 176.
- Fig. 5: Knight 1946, p. 17.
- Fig. 6: Wikimedia.
- Fig. 7: Chatterjee 1997, p. 42.
- Fig. 8: Heilmann 1926, p. 184.
- Fig. 9: Klepsch 1989, p. 49
- Fig. 10: Paul 1988, p. 211.
- Fig. 11: Paul 2002, p. 291.
- Fig. 12: Paul 1988, p. 241.
- Fig. 13: Witton 2017, p. 78.
