Archaeopteryx and the Solnhofen Lagoon

From: Stan Friesen [swf@ElSegundoCA.ATTGIS.COM]

From: Jeffrey Martz [martz@holly.ColoState.EDU]
> I'd like a little clarification on exactly what sort of depositional
> environment the Solnholfen limestone is and how the Archaeopteryx's
> ended up there. I've kind of had the impression that the Solnholfen
> were a series of offshore islands, and the Archaeopteryx and
> pterosaurs and all the other nifty critters were oreserved in a lagoon
> between the islands and the mainland. Is this even remotely correct?

Aproximately.

The Solnhofen itself is the *lagoonal* deposits. It was a more or less stagnant lagoon, with anoxic bottom waters. It preserved a great many fossils - mostly of marine animals: fish, ichthyosaurs, and similar beasts.

There really was no "mainland" - all of Europe was a bunch of islands at the time. I believe that the lagoon was between an outer coral reef and a large island.

From time to time, due to various causes, a land animal would float out into the lagoon, and get buried along with the marine stuff. These land animals include such things as Compsognathus, one of the smallest known dinosaurs. In this regard Archaeopteryx is not all that unusual, just one more land animal that died and ended up floating in the lagoon.

> I'm asking to try and get a better idea of what the Archaeopteryxs
> were doing there, particularly if they were supposed to be arboreal.
> I've heard two ideas:
> 1) The Archaeopteryxs were living on the islands (or at least visiting
> the islands) and died nearby, in the lagoon.

Note the islands were on the *land* side of the lagoon, not the seaward side. Also, they may have entered the lagoon *after* death, by floating down stream during the rainy season.

> 2) The Archaes were living inland on the mainland, died and fell in a
> river, and got washed out to the lagoon.

The "mainland" was just a large island.

In all probability both scenarios happened from time to time.

> Also, It seems more likely to me that the Archaeopteryx died very
> near the lagoon, possibly falling right into it when they died or at
> least dying on the beach and getting washed in, simply for the reason
> that I find it hard to believe that a little fragile animal like
> Archaeopteryx could get washed a ways down a river and end up often
> (usually?) with minimal disarticulation and feathers relatively
> unrumpled by the time they reached the lagoon.

Actually, before serious decay sets in they would break up very little while floating. Whole carcasses can be moved quite some ways by river action.

Also, "inland" need only have been a couple of miles, or perhaps a few tens of miles. These islands were at most a few hundred miles across.

> Also, how common is Archaeopteryx relative to other tetrapod
> species like pterosaurs and Compsognathus at the Solnhofen?

It is more common than Compsognathus (seven versus at two or three specimens). It is less common than Pterodactylus. But then I think Pterodactylus (or at least some species of it) may have been the Mesozoic equivalent of swallows, which I have seen swooping about above a lake catching insects, and even dipping into the water to catch water-striders and the like.

> I mean
> counts of individual species, not a percentage of the total number of
> species. In other words, what does its rarity or commonality say
> about if it was a rare an accidental visitor, or a native of the
> area?

It is rare enough compared to Pterodactylus to make it an occasional relative to P. Even P. is rare compared to the marine fossils, however.

From: bk090@freenet.carleton.ca (David Brez Carlisle)

The lagoon cannot have been totally anoxic and stagnant, or else there would be no fossils of marine animals. It is likely that the surface layers were oxygenated and supported a copious algal flora, but the deeper parts were anoxic and stagnant, like the depths of several water bodies, including the Black Sea. Any unfortunate creature that dived too deeply would be in trouble, but most fossils are almost certainly remains of creatures that died from other causes and then sank into the depths from the oxygenated surface layers, after death.

From: Robert.J.Meyerson@uwrf.edu (Rob Meyerson)

Benthonic animals at least. Fish would be present whether or not the sea floor had oxygen.

I would add that if Solnhofen was truly anoxic, we would expect the rock to be of dark colors, due to the abundant organic matter (what scavenger is going to feed where it can't breath?).

From: Adam Yates [zooamy@zoo.latrobe.edu.au]

Neither can this be the full story for many of the marine animas preserved are benthic organisms (ie. bottom dwellers) eg. Limulids, various crustaceans and a small crinoid - the most abundant of solnhoffen fossils. This suggests that the boundary between oxic and anoxic water (which can be very sharp) was BELOW the sediment - water interface at least some of the time. The absence of any infaunal animals (animals living in the sediment such as bivalves) further suggests that the oxic - anoxic boundary was not far below the sediment surface (probably a few millimeters). Of course there would have been environmental fluctuations when the boundary would have risen up into the water column. It would be at these times most of the fossils would be preserved.