Tuesday, April 17, 2018

Archaeopteryx: Beast of the Week

This week we will be checking out an extremely famous little dinosaur that has produced arguably one of the most important fossils in history.  Let's check out Archaeopteryx!

Archaeopyeryx was a small, meat-eating dinosaur that lived in what is now Germany during the late Jurassic Period, about 150 million years ago.  The largest specimens measure a little under two feet long from nose to tail. The genus name translates to "Ancient Wing".  The most widely used species name, lithographica, in reference to a lithograph since all the Archaeopteryx fossils are preserved in flat slabs of rock.

Life reconstruction of Archaeopterx defending a fishy meal from a Pterodactylus.  Watercolor by Christopher DiPiazza.

Archaeopteryx was the first fossil discovered that showed visible evidence of feathers.  In fact, the first ever unearthed specimen of this dinosaur was simply just one preserved feather!  It wasn't until a year later that an actual skeleton with feathers was found.  These beautifully preserved fossils showed that Archaeopteryx had long wing feathers growing out of its arms, thick feathers on its legs down to the ankles, and a row of long feathers arranged on each side of its tail, which was long, like that of a non-avian dinosaur's.  Eventually a total of twelve specimens of this little dinosaur would be unearthed and had paleontologists and biologists alike talking.  Archaeopteryx, like birds today, had a fused clavicle (wishbone) and the first toe on each foot appears as if it could have possibly flexed in the opposite direction of its other toes, in a grasping action.  Many birds today use this same adaptation to perch.  That being said, Archaeopteryx had many features that are closer to non-avian dinosaurs than to birds.  It had teeth in its jaws, three long fingers tipped with hooked claws on each hand (although some modern birds do have claws on their wings, but not as long as Archaeopteryx's) and it had a long, bony tail. (birds have short, highly reduced tails.)  Another interesting feature is that by looking closely at the bones of all the Archaeopteryx specimens on the fossil record, paleontologists determined that Archaeopteryx would have grown a bit more slowly than many birds today, which usually hit maturity within several weeks.  Archaeopteryx would have taken almost an entire year to reach the size it was when it died.  Because of this combination of features, Archaeopteryx was and still is considered a very important fossil that plays a part in our understanding of evolution. It was discovered shortly after Charles Darwin's Origin of Species was published, and further strengthened the idea of long term descent with modification by posing as an almost perfect "transitional fossil" between birds and other reptiles.

Most complete of the Archaeopteryx specimens on the fossil record, now on display in Berlin, Germany.  Note the long, bony tail and the imprints of long feathers on the tail, legs, and wings.

So was Archaeopteryx truly a bird?  Since its discovery it was long  considered so, often posing as "the first bird" or the "ancestor of all living birds".  Since it's discovery there have been at least a few other dinosaurs that might take the title as "oldest bird" known to science, however.  There was also some suspicion among some paleontologists that Archaeopteryx may have been more closely related to dromaeosaur dinosaurs, like Velociraptor, than to birds.  Most recent studies, however, place Archaeopteryx somewhere in between.  It likely wasn't a true direct ancestor of the birds we see today, but rather was probably more basal than what we would consider the earliest birds.  A sort of "stem-bird" or "proto-bird" if you will.

Fast little sketch I did comparing the tails of modern birds to Archaeopteryx. Internal vertebra are in blue.  Note how the "tails" of modern birds are really just feathers growing out of a stump.  Archaeopteryx, on the other hand, had a true tail supported by its skeleton with feathers growing from either side of it.  The arrangement always reminded me of a fern.

Archaeopteryx has been the victim of some prejudice with regards of its life reconstructions over the years.  Because it was so birdlike in its appearance, it is often depicted as such.  Countless artists have recreated Archaeopteryx perching in trees or flying in the air, like a songbird.  It is also very common to see reconstructions of this little dinosaur with very brightly colored feathers to really drive home the plumage aspect, despite the fact that for every bright yellow or blue bird alive today, there are plenty of brown and black ones.  But how accurate are these depictions?

First let's look at the habitat Archaeopteryx would have lived in when alive.  The part of Germany its fossils were found in would have been mostly underwater during the late Jurassic, save for a series of close islands.  These islands would have been pretty dry, inhabited by mostly low growing plants that could survive dry, sandy soil and lots of sun.  There doesn't appear to have been any large animals living here at the time.  Archaeopteryx's neighbors were all relatively small, including the pterosaurs, Pterodactylus, Aerodactylus, and Rhamphorhyncus, as well as the slightly larger dinosaur, Compsognathus.  In the interior of these islands were fresh and brackish lagoons.  These lagoons were where all the fossilized Archaeopteryx, and other animal specimens from this place that have been discovered, ended up shortly after they died.  There they remained, mostly undisturbed, being covered by layers of mud, to be fossilized over time.  Because of what we know about this environment, Archaeopteryx wouldn't have had many tall trees to roost in, and likely would have been more comfortable running around on the ground, like most theropod dinosaurs of the time.

So we know Archaeopteryx was probably comfortable running around on the ground...but it still had those big wings.  Could it fly?  This is also the subject of much debate.  First thing to note is that Archaeopteryx' wing feathers were what we call asymmetrical.  This means that the barbs on the front side of the feather vein are more narrow than those on the back side.  This is a characteristic seen in modern birds that can fly, to reduce drag while in the air.  The tail feathers of Archaeopteryx were also very broad, which would have provided extra lift and maneuverability if it was in the air.  Archaeopteryx' brain was large compared to the rest of its body.  The parts of the brain associated with sight and coordination were strongly developed, which are both qualities seen in modern birds that enable them to fly.  However, Archaeopteryx's arms weren't capable of being raised past its back.  So if it were to flap its wings, it couldn't perform the strong upstroke that flying birds do today.  It also lacked a keel bone, the extended part of a bird's breastbone that is the attachment site for all the breast muscles that allows most birds to perform powered flight.  (Fried Archeopteryx would have been lacking in the white meat department.)  That being said this doesn't mean Archaeopteryx couldn't have had enough muscle in its arms and chest to get the job done.  As of now, most paleontologists would guess that Archaeopteryx, given its combination of traits, was probably capable of limited flight at best.  It was by no means an areal master, like many of the birds you see today, but maybe could fly for short distances if it had to.  I would imagine its flight ability similar to that of a pheasant or chicken for a possible living comparison.  Great at running, but also capable of short bursts of flight if need be.

Archaeopteryx skeletal mount on display at the Academy of Natural Sciences in Philadelphia, PA.  Note the lack of prominent keel bone, that would be present in most bird skeletons.  Skeletal reconstruction sculpted by Larry Martin and mounted by Bruce Mohn.

As for the colors, believe it or not we can confirm what color Archaeopteryx was!...well...actually we can confirm what color one of its feathers was.  You see, most of the Archaeopteryx specimens preserved only imprints of the feathers, but not the feathers, themselves.  That is except for one isolated feather specimen that appears to have been a covert, that would have grown from the top of the wing.  By using electron microscopes and x-rays, paleontologists were able to detect melanosomes that survived the fossilization process.  Melanosomes are organelles in the cells of feathers that determine what color the feathers are.  The visible color of the feather depends on the shape of the melanosomes.  So by looking at the shapes of the melanosomes seen in the Archaeopteryx feather, and then comparing them to the melanosomes in feathers of modern birds that we can visibly see the colors of, it was determined that this Archaeopteryx feather was...black!  Specifically matte black, not shiny or iridescent black. (which we have found in other prehistoric dinosaur feathers!) So there you have it...at least one feather on the wing of Archaeopteryx was black.  The colors on the rest of the body are still a mystery, however.

Isolated wing feather associated with Archaeopteryx that provided data on what coloration it probably was in life.

So when alive, Archaeopteryx was at least partially black in coloration and running around on the sandy beaches and shallow scrubby forests of its island habitat.  When it wanted to, it could fly short distances up into a short tree or across a small lake or pond to escape predators or to chase down prey.  It likely would have been hunting small animals, like insects, lizards, and possibly small fish.  At the same time it would have needed to look out for predators, like the slightly larger dinosaur, Compsognathus, or competitors, like the pterosaurs that frequented the islands it lived on.

That is all for this week!  As always feel free to leave a comment below!


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1 comment:

  1. Hi Chris: Slight correction. The Academy of Natural Sciences Archaeopteryx skeleton was mounted by me, but the pieces were sculpted by Larry Martin. I heavily reworked it to enable that pose and added sclerotic eye rings that I had sculpted. I produced my far more accurate and complete skeletal model after seeing now much room there was for improvement in the Larry Martin skeleton. I can provide a pic of that if you want to edit your post.