This was a paper I wrote for my paleontology class. I learned later that the day I completed the paper – April 26, 2014 – Prof. Seilacher passed away. Let this be my tribute to an incredible scientist.
There is abundant evidence that an asteroid struck the Earth 65.5 million years ago. Sediments at the Cretaceous-Tertiary boundary possess exotic elements that probably came from space, and an enormous impact crater in the Yucatan was found to be about 65.5 million years old (Alvarez et al. 1980). There is also abundant evidence that an enormous volcano began to erupt about 67 million years ago in India. The pulses of eruption formed some of the longest lava flows in the geologic record, piled up almost 4 kilometers of lava on the Earth’s surface, and pumped out enough sulfur dioxide to choke a chain smoker (Keller et al. 2009). The non-avian dinosaurs disappeared from the fossil record about 65.5 million years ago, as well, leading paleontologists to suggest that either the asteroid, the volcanic eruption, or some mixture of both formed a killer cocktail.
But enough bout the non-avian dinosaurs: what about the avian dinosaurs, or the other organisms that made it through this deadly evolutionary choke point? Why did the gun of mass extinction allow them to live on? What made these organisms special?
There is a new interpretation of what caused the extinctions which, while not doing away with the killer events, puts more focus on the conditions of the creatures themselves at the time the killer events hit. According to this new interpretation, created by Dr. Adolph Seilacher of Kepler’s Tuebingen University, natural selection was allowed to proceed unhindered for extended periods of time. As organisms out-competed each other, their niches became progressively more narrow. Eventually, the creatures found themselves so dependent on the conditions to which they had adapted that changes wrought by “killer events” could devastate the biosphere. Those creatures that had been out-competed, and were left unspecialized, were able to adapt to the changed environment and make it through the extinction intact.
Because Seilacher’s theory is based on the work of his teacher, paleontologist Otto Heinrich Schindewolf, this report shall open with a discussion of Schindewolf’s hypotheses about evoloution. Seilacher’s theory will then be elucidated, which will indicate valuable lessons for paleontology.
Otto Schindewolf was a towering figure in mid-20th century paleontology. He was an expert on fossil invertebrates, and had seen just about every fossil cephalopod and cnidarian that existed during his time. In response to some clear patterns that he thought he saw in the fossil record, he wrote a book called Basic Questions in Paleontology, which laid out what he saw as the major challenges that faced paleontologists in the coming decades. Most of those challenges were related to his interpretation of the patterns of evolution (Schindewolf 1950).
Schindewolf believed there was some principle of evolution that drove lineages through a predisposed series of changes which led inexorably towards extinction. He did not think that organisms developed and kept features because those features made them fitter than their competitors. On the contrary, he thought that some novel features may have been disadvantageous, and perhaps deadly. He believed in an orthogenetic evolutionary progression which was not headed towards a desired outcome (i.e. not teleological), but rather was built in at the beginning. The stages of the theory, which Schindewolf called Typostrophe, will first be described, and then illustrated with his own example of the shelled cephalopods (Schindewolf 1950).
The first stage is what Schindewolf called typogenesis, or the formation of the type. 1 This stage begins with the appearance, out of thin air, of a new, basic form of organism which exhibits the seed-features that defined the type. After some time during which this lowly group of creatures supposedly made their existence, a short lived period of rapid evolution of its form took hold. New, dramatic deviations from the basic bauplan came into being. Schindewolf claimed that during this period of explosion of new manifestations of the type, it was difficult, if not impossible, to determine predecessor and successor species in the order of evolution. This was a period of experimentation with the possibilities that had already been built into the basic type structure.
The duration of typogenesis varies from lineage to lineage, but eventually the lineage transitions into the second stage, called typostasis. A relative few of the subtypes become dominant, while the greater mass of the rest disappear from the fossil record, as if they were discarded failed experiments. The few which persist continue on with a much slower rate of evolution. Each of these subtypes had developed advantageous variations on the basic features of the original bauplan. As time progresses, the advantageous appendages and structures in the organisms develop through successive generations, slowly becoming increasingly valuable as the subtypes move into the prime of their evolutionary existences.
Though typostasis tends to be the longest stage, it also does not last forever, but eventually transitions into the third and last stage, called typolysis. After the subtypes experience their heyday of success, their forms go into a kind of old age, or senscence. Their forms begin to fall apart. It is often observed that organisms in this stage become gigantic relative to other organisms, as were the brachiosaurs or woolly mammoths. Features which had formed the crux of the type’s survival continue to develop past advantage, and become burdens on their carriers. The geometry of the bauplan begins to unravel, sometimes literally as in the coiled cephalopods. Some of the features may become enormous, such as the antlers on the Irish elk (see figure 2). Schindewolf describes these organisms as becoming decadent, and generating almost another typogenesis of wild deviations from the original bauplan while the type falls apart. The inevitable outcome of this period is the extinction of the type.
Schindewolf believed that all organisms were caught within one of the three stages of typogenesis. To be blunt, Schindewolf was anti-Darwinian. He thought that as soon as the prototypical organism came onto the stage of geology, a path of evolution towards extinction was built in. This hypothesis is one form of orthogenesis, in which there is some factor within the lineage that propels evolution to future forms. Creationism is an orthogenenetic theory which declares that a future, desired morphology is the teleological cause which drags all prior states through the steps of evolution to that desired state. Typostrophe is an orthogenenetic theory which blames initial conditions within the prototype organism for all successive steps. In creationism, the type knows where its headed, and takes appropriate steps; in typostrophe, the type doesn’t know where the endpoint will lie, but each evolutionary step taken is bounded by the possibilities within the type.
In a typostrophic progression, the advantages that the subtypes found in their evolved type-features were less a product of environmental pressures than of internal drive. It was by chance that the features were advantageous in their environments. All lineages, however, were ultimately doomed to extinction as their former advantages grew into severe burdens and their bauplans became decadent. For Schindewolf, no outside factor need intervene to cause extinction – it is the inevitable result of typolysis.
3. CASE STUDY: SHELLED CEPHALOPODA
It is important to understand the last stage in Schindewolf’s theory, typolysis, in order to see how Seilacher develops his own hypothesis. It is also important to recognize that Schindewolf’s interpretation relied on the observed fossil record.
By observing, for example, the Ammonoid cephalopod lineage, we may further understand the typolysis stage. The Ammonoids sprang up during an earlier period of typogenesis, from which also arose the Orthocerida and the Nautilloidea. They did not come into dominance until their predecessors, the Goniatitida and the Ceratitida, blew themselves out. By mid-Cretaceous, ammonites were the dominant form of marine cephalopod. They had tightly curled, ornamented shells, with very complex sutures that some believe may have given them an advantage in both swimming strength and resistance to water pressure (e.g. Schindewolf 1950, p. 139, Benton & Harper 2009, p. 351). As the lineage evolved, the shells became more ornamented, and the sutures became ever more intricately complex.
As the end of the Mesozoic drew near, the evolution of the ammonites began to take new shape. The shells began to uncurl and to re-curl in strange directions. We get the so-called heteromorph Ammonoidea, such as Nipponites and Scaphites. Besides the uncurling, in some species we see the suture crenulations smooth out, such as in Tissotia the “Cretaceous ceratite.” Some ammonites grew to enormous dimensions as well, such as the Pachydiscus. For Schindewolf, unless the ammonoids experienced a burst of typogenesis, the appearance of these degenerate forms were the end of the line: “All these overspecialized, degenerate forms were incapable of further evolution, and since there was no vigorous revival of ammonite stock at the Cretaceous-Tertiary boundary, their fate was sealed.” (Schindewolf, 1950, p. 144)
All shelled cephalopods except for the nautiloids disappeared by the Tertiary Period. Schindewolf wasted no time hunting for a killer event, since he thought that typostrophe had, once again, run its course.
4. SEILACHER’S HYPOTHESIS: THE GOLDEN AGE
There are a few obvious problems with Schindewolf’s theory. Let it be assumed for a moment that perhaps it is possible that there is some “evolutionary lifespan” built into a taxonomic type of organism, and that after its heyday of enjoying unique abilities, the type becomes decadent and dies off. It should be expected, in that case, that there be a somewhat regular rhythm of extinction throughout the fossil record, since one taxon doesn’t really know or care where another taxon is in its evolutionary life cycle. Were this the case, how would mass extinctions be explained? At these times, there must have been a large number of taxa that were miraculously in sync at the end of typolysis.
For example, people die all the time, sometimes from injury, but usually from complications of old age. If one year came along where more than, say, 60% of the people on Earth happened to die off from old age, an incredible cause would have to be called upon to explain the event. Perhaps some kind of a killer event took out the old people, such as a disease or an asteroid impact. Even if some such event were identified, it would still have to be explained why so many people happened to be old at that moment. What miraculous timing! It should be clear that typostrophe must invoke very unlikely coincidences, in order to take into account these mass extinctions.
But, if it is assumed that at least Schindewolf’s observations were accurate, then prior to mass extinctions we must see a large proportion of organisms sporting what he would call decadent forms: heteromorph ammonites, gigantic dinosaurs, brachiopods with elaborate systems of spines, etc. An impending mass extinction’s coincidence with widespread decadence of lineages must have some causal connection. But surely the cause cannot lie in an orthogenetic theory of evolution.
Schindewolf’s student Adolf Seilacher has come to Darwin’s defense. Seilacher has proposed that these periods prior to extinction, which he calls “Golden Ages,” are generated by external factors, not orthogenesis. Each evolutionary step taken by a given clade must always lead to increased fitness. Therefore, each morphology found in the fossil record must be interpreted as having been perfectly adapted to its environment and lifestyle. Deviations from established bauplans must indicate special aptitude for certain niches. Extreme deviations, such as in the heteromorph organisms, had to come about through a series of evolutionary moves towards better adaptation, and must indicate extreme specialization. (Seilacher, 2013b)
In Seilacher’s view, it is this extreme specialization that made the so-called decadent forms so susceptible to extinction. These extreme subtaxa were so intricately adapted to their niche, that even small changes could have profound effects on their ability to survive. Let us assume for a moment that the environment were suddenly changed in some significant way by the eruption of a large igneous province, the impact of a large asteroid, or some other catastrophic event. Organisms that had been extremely adapted to the previous environment would suddenly find themselves terminally unsuited to the new environment. They would be “like fish out of water,” and may never recover. Those organisms that were not yet extremely specialized could then come into the vacated niches and take over.
For example, let us look back at the heteromorph ammonites. Schindewolf saw the late-Cretaceous ammonoid lineage as having long passed the sweet spot of useful traits, with its curves and crenulations becoming twisted and undone in very unuseful ways. Seilacher instead sees each unfolding as a step toward higher adaptation, except that we need to discover what the organism was actually adapting to. If the ammonites are seen as predators that move like modern nautiloids, squids, or octopi, then the wildly curving, sometimes gigantic shells must have been terrible impediments. However, if the organisms lived a lifestyle different than these modern relatives, then these shapes may actually have been useful.
Seilacher and Michael LaBarbera (1995) proposed that ammonites were not vicious predators, but were rather plankton feeders (see figure 5). In their model, the last septal wall in the shell wasn’t calcified, but was a flexible membrane the ammonite could control like a balloon. With the flexible wall pushed in, the gas within the shell chambers would compress, and the animal could use gravity to sink down. With the wall relaxed, the gas would once again expand, and the animal could float back up. Up and down, the ammonite could easily scan the water column for plankton to eat, while using a minimal amount of energy for propulsion. Seilacher and LaBarbera point out that the more extreme crenulations within the septal walls would form better attachment points for the last flexible wall. Wild deviations of shell shape, even the Nipponites, may become very useful in such a lifestyle, especially since hydrodynamics is of far less importance to such a mode of existence.
With this lifestyle, the heteromorph ammonites could have become extremely successful. They would also have become extremely sensitive to changes in the environment. Seilacher and LaBarbera suggest that the shockwave from an asteroid impact could have severely modified the planktonic zone for long enough to drive the ammonites out of existence. Alternatively, a sharp reduction in photosynthesis among plankton could have been enough of a stress to end the ammonite lineage. (Seilacher & LaBarbera, 1995)
So much for a single lineage, but how do multiple clades simultaneously, sometimes globally, come to such a state of extreme specialization, and thus extreme vulnerability? Seilacher argues that a long period of relative environmental stasis were necessary for mass extinction events to have occurred. He notes that each of the major extinctions were preceded by long global hothouse climates. It would have been during these extended greenhouse events that organisms across the kingdoms could proceed towards unprecedented specialization, since the environment was not changing much. Thus, heteromorphy would tend to become ubiquitous as long as the warm climate endured, until change struck. At that point, what the organisms had experienced as their unique strengths would become their Achilles’ Heels. (Seilacher, 2013a)
Seilacher is making a conclusion drawn from basic Darwinian principles. The degree of adaptation and specialization reached by a clade is tied to how long the clade’s environment remains unchanged. A corollary of this conclusion is that ecosystems populated with strangely shaped, extremely specialized and successful organisms must exist in environments that have been very static over a long period of time. Conversely, environments that have been in constant conditions should be inhabited by wildly diverse and specialized organisms.
This is apparently the case on the ocean floor. The ocean floor does not experience an alternation of day and night, weather, temperature shifts, sea level change, or any other large shifts of its environment. Immigration of new species is also limited, since the physical conditions are so oppressive. The organisms that inhabit this zone are free to become very highly tuned to the necessities of their lifestyle, and we should expect to find a large variety of extremely strange forms. Indeed, this is exactly what is found. Seilacher asserts that the ocean floor in particular was also exempt from several extinction events, including the KT event. This would explain why so-called “Lazarus taxa” tend to be re-discovered there, living happy, calm lives (Seilacher, 1998).
We’ve seen that the coincidence of wild evolutionary forms with mass extinctions can be explained using Darwinian evolutionary principles. Over long periods of environmental stasis, organisms are free to pursue adaptive paths to extreme specializations. This leaves clades around the globe with Achilles’ Heels that can be exploited to their detriment by major events which change the environment. Therefore, specific killer events cause mass extinctions, but only when the biosphere has had time to prepare itself.
A hot button question for paleontologists is “what event caused such and such mass extinction?” A more important question that Seilacher’s theory provokes, though, is “what causes these prolonged hothouse periods, during which extreme overspecialization can occur?” Seilacher himself suggests that the answer may lie within the domain of galactic astrophysics. It may be that our climate is largely driven by what is happening in our arm of the Milky Way, and that there may be areas of the Milky Way that our solar system passes through which are more conducive to hothouse climates. Indeed, this leaves open the possibility that killer events could be more common than we think, but they only have cataclysmic effects when the biosphere is conditioned properly.
Alvarez, L. W., Alvarez, W., Asaro, F., Michel, H. V., 1980, Extraterrestrial cause for the cretaceous-tertiary extinction: Science, v. 208, no. 4448, p. 1095-1108.
Keller, G., Sahni, A., Bajpai, S., 2009, Deccan volcanism, the KT mass extinction and dinosaurs: Journal of Biosciences, v. 34, no. 5, p. 709-728.
Schindewolf, O. H., 1993, Basic Questions in Paleontology: Geologic Time, Organic Evolution, and Biological Systematics: Chicago, The University of Chicago Press, 467 p. Originally published as Grundfragen der Palaontologie (1950).
Schindewolf, O. H., 1953, Uber die Faunenwende vom Palaozoikum zum Mesozoikum: Zeitschrift der Deutschen Geologischen Gesellschaft, Bd. 105, No. 11, p. 153-182.
Seilacher, A., 2013a, Cyclism revisited: extinction and ‘Achilles’ Heels’ keep diversification in check on macroevolutionary time scales: Historical Biology, v. 25, no. 2, p. 239-250.
Seilacher, A., 1998, Patterns of macroevolution: how to be prepared for extinction: Comptes rendus de l’Academie des sciences, ser. IIA. v. 327, p. 431-440.
Seilacher, A., 2013b, Patterns of macroevolution through the Phanerozoic: Palaeontology, v. 56, part 6, p. 1273-1283.
Seilacher, A. and LaBarbera, M., 1995, Ammonites as Cartesian Divers: PALAIOS, v. 10, no. 6, p. 493-506.
Benton, M. J. & Harper, A. T., 2009, Introduction to Paleobiology and the Fossil Record: United Kingdom, Wiley-Blackwell, 592 p.
1 Here, by type, or bauplan, we mean a general body plan that defines the boundaries of a group of evolutionarily related organisms.