Paleoecology: Jurassic Solnhofen Fauna

Archaeopteryx lithographica is among the most famous and beautiful of all fossils. Not only are the available specimens exquisitely preserved in limestone, they attest to what was once unthinkable; a dinosaur with feathers. While the first feather of Archaeopteryx was discovered in 1860, an mostly-complete specimen (the London Specimen) was not uncovered until a year later. When the first Archaeopteryx was found, however, it sparked a debate about “transitional fossils” that still goes on to this day, seemingly showing up on cue to support Charles Darwin’s recently published On the Origin of Species. Although the fossil is amazing in of itself, paleontology should not be relegated to mere “stamp collecting”; we need to understand the ecology in which Archaeopteryx found itself if we’re to fully understand its significance.

A beautiful drawing of the Berlin specimen of Archaeopteryx as featured in Gerhard Heilmann’s The Origin of Birds.

If Solnhofen sounds familiar to you, it should; every Archaeopteryx lithographica fossil ever discovered has been found near the town in limestone deposits that exquisitely preserve detail in fossils. Such sites of magnificent fossil preservation are known as lagerstätten, which is further broken down into two categories: Konzentrat-Lagerstätten are deposits that are essentially collections of disarticulated bones (i.e. a bone bed) and Konservat-Lagerstätten is an area where soft parts of the long-dead animals are preserved as casts or impressions, and the latter is the kind of preservation seen in the Solnhofen limestone. While this may seem to be a lot of technical jargon that has little to do with our discussion here, the preservation of the late Jurassic creatures (more specifically from what is known as the Kimmeridgian stage, from approximately 155 million years ago to approximately 150 million years ago) can tell us a bit about the conditions in which these animals died, primarily that they were covered quickly by sediments with little oxygen content, therefore slowing decomposition by bacteria and allowing the various molds and casts to form. What we have to determine now is what sort of habitat would allow for such anoxic sediments to cover deceased creatures without interference from scavengers.

During the late Jurassic the world was certainly a different place, and what is now Solnhofen is now understood to have been part of an archipelago on the edge of the Tethys Sea, a body of water that existed between a landmass of northern hemisphere continents (Laurasia) and southern hemisphere continents (Gondwana) during the late Jurassic (but through time continued to shrink until it ceased to exist). Along this archipelago were lagoons that were fairly isolated from the sea by coral reefs, allowing the lagoons to develop such a high salinity that little could live in the concentrated saline waters. There is some debate about where the division occurred between habitable water and the anoxic level (some arguing a division in the water column, others that the anoxic level was below the first level of sediment), but regardless of level it is apparent that once organisms settled to the bottom they received little disturbance from scavengers or abiotic factors. This is consistent with the observation that the limestone found today from such lagoons do not reflect much disturbance, although the layers were likely deposited as a result of storms bringing new sedimentary material into the lagoons where it eventually settled. How did dead Archaeopteryx find their way into such a lagoon? Its been proposed that they could have fallen into the water while flying and drowned, that they could have died further inland and been swept into the lagoon by a storm or by a river, or found their way in another way. The fact that the fossils are so well preserved, however, suggests that they only experienced minimal disturbance and makes it unlikely that they were carried any long distance by a river or by way of anything that might exert a lot of mechanical action on the corpses.

While Archaeopteryx may be the most famous of the fossils recovered from the Bavarian limestone, the same lagoons that preserved the early bird also preserved various other creatures. Wonderfully preserved dragonfly fossils also come from the lagerstätten (suggesting that the lagoon was not far from land), as well as various crustaceans, echinoderms, cnidarians, insects, pterosaurs, reptiles, etc.

While it is certainly important to consider the niches and lifestyles of all the creatures preserved in limestone, reconstructing the ancient lagoon habitat may give us some important clues as to the origins of birds, specifically Archaeopteryx. Indeed, one of the most puzzling aspects of the Solnhofen limestone is that Archaeopteryx was not left alone on some isolated isle to evolve without competition or threat; pterosaurs are exceedingly abundant in the Solnhofen limestone, the first pterosaur ever discovered coming out of the rock at Solnhofen by the hand of Cosimo Collini in 1784. Species known from Solnhofen include Ctenochasma gracile, Ctenochasma porocristata, Gnathosaurus subulatus, Germanodactylus cristatus, Germanodactylus rhamphastinus, Anurognathus ammoni, as well of members of the genera Pterodactylus and Rhamphorhynchus. Some studies (Bennett, 1996) have suggested that the species diversity of the pterosaurs preserved in the limestone deposits may not be as great as previously assumed, some smaller “species” being different growth stages of large species, but regardless of this it is still true that the islands inhabited by Archaeopteryx also harbored large numbers of various pterosaurs, regardless of whether they were transients or had nesting sites on the islands (although given the various growth stages it’s reasonable to suggest that they stayed in one place, or at least returned to the same site at a regular interval).

So what were all these pterosaurs feeding on? For there to be such a large number of them there must have been adequate food, and given their location and mouth adaptations, they are considered by many to be analogues of modern shorebirds. I know nothing of their nesting behavior, but it is interesting to ponder what it may have looked like in the late Jurassic, with larger pterosaurs arriving early (like modern Herring and Greater Black-Backed Gulls), taking the prime spots that are least flood-prone, smaller pterosaurs playing to role of modern Black Skimmers and Common Terns, being pushed out to the side, with prehistoric equivalents of Plovers being forced to nest in flood-prone marshes. Indeed, in my head I almost have an image of the salt marshes I’ve seen along Barnegat Bay and Island Beach State Park here in New Jersey, with some shrubs and bushes but few trees. This isn’t to say there isn’t rich avian fauna in the area, however, plenty of birds (like Red-Winged Blackbirds) hopping among the branches of various woody bushes and shrubs.

It’s also important to note here that the term “shrub” conjures up an image of hedges or woody plants lining suburban driveways, but shrubs can grow to be quite large, sometimes being confused with trees. What makes a shrub a shrub is that it usually doesn’t grow over 18 feet high and branches out much lower than trees do, but it still provides ample habitat for flying organisms to find homes in. While the first image I ever saw of Archaeopteryx was of it climbing a tree, such a scene is unlikely to have played out in real life; there doesn’t seem to be any evidence of high trees to leap down from as is so-often depicted. This doesn’t mean, however, that this bird didn’t climb, and as I already mentioned the existing shrubs would have provided a lot of cover for Archaeopteryx.

An Osprey coming back to its platform at Island Beach State Park, New Jersey. Note the salt-marsh habitat below the bird. (Photo: Brian Switek)

So, while pterosaurs had a large presence along the islands, it does not seem that they were in direct competition with Archaeopteryx or “suppressing” its evolution. Thinking back again to Island Beach State Park, along the shoreline and the salt marshes gulls and shorebirds are ever-present. If you cross of the dunes to the other side of a road running along the park, however, you’ll soon encounter an area of thick shrubs and brush inhabited by finches, Red-Winged Blackbirds, Boat-Tailed Grackles, various warblers, and other birds most people don’t readily associate with the shore. They’re inhabiting an entirely different niche and habitat than the large gulls, and so there is not a conflict. Could Archaeopteryx have had a similar relationship with the pterosaurs of its day? I think its reasonable to think so, although there is always more work to be done.

We should be careful not to think about Archaeopteryx too narrowly, however. While the cursorial vs. arboreal debate rages on, there is no guarantee that this creature exclusively lived in trees or ran along the ground. Indeed, the recently revealed Thermopolis specimen has some anatomical details of its foot that seem to link it with dromeosaurids like Deinonychus, which may mean that it spent more time on the ground than previously thought. While the prospect of Archaeopteryx “hyper-extending” one of its toes off the ground like some of its evolutionary relatives is an interesting revelation, to me it does not signal the death knell for a cursorial or arboreal lifestyle; Archaeopteryx could have held that toe off the ground while walking, or perhaps it could have used it as a sort of spike to help it climb or gain purchase on a branch. The answer as to what this bird did with most of its time still eludes us.

Now that what Archaeopteryx was doing and how it came to be preserved has been considered, I have to wonder how it came to exist on the islands in the first place. Did the wings (and for that matter, feathers) that Archaeopteryx bears evolve elsewhere, in a forest somewhere, the birds only coming to inhabit the islands later on? Did the “urvogel” leave any descendants or did it die out on the island chain (was it at the base of a branching tree or the terminal end of one of the branches)? I can’t help but wonder if the Archaeopteryx fossils we’ve found so far are representative of a population that survived on the islands for some time but ultimately died-out, populations that remained in forests or other habitats continuing to evolve and give rise to other groups. Keep in mind that this post is rife with speculation from someone who isn’t a respected member of the paleontological field and could be wrong about any number of my hypotheses, but still I do wonder about exactly what place the Solnhofen birds have on the larger Tree of Life.

Sources consulted for this article:

Solnhofen Limestone – Wikipedia
Lagerstätte – Wikipedia
Kimmeridgian – Wikipedia
Archaeopteryx – Wikipedia
Tethys Sea – Wikipedia
Pterosaur – Wikipedia
Pterodactylus – Wikipedia
Rhamphorhynchus – Wikipedia
Shrub – Wikipedia
Fossils from the Solnhofen Limestone – Virtual Fossil Museum
The Solnhofen Lithographic Limestone – University of Bristol
Upper Jurassic Pterosaurs – Pterosaur Database
“Year-classes of pterosaurs from the Solnhofen Limestone of Germany: taxonomic and systematic implications” (Abstract) – Journal of Vertebrate Paleontology 16(3):432-444
Archaeopteryx and the Solnhofen Lagoon – Dinosauria On-Line
Archaeopteryx: The Thermopolis Specimen – Wyoming Dinosaur Center

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