lifestyle

On the Lifestyle of Ammonites

The bottom-related lifestyle of ammonites can unequivocally be demontrated by applied physics, using different methods.

1. Calculations

Calculations based on reasonable assumptions offer the best opportunity to achieve evidence whether ammonites were able to swim as well as to the behaviour of shells floating in the water column. A. Trueman (1941) was the first to carry out calculations concer- ning balanced positions of shells. As my own calculations also show, these positions are mainly dependent on the body chamber length (BCL). The balanced position follows from the positions of the centres of gravity and buoyancy. These centres are vertically superimposed. The BCL in ammonites is very varable. This fact was widely disregarded by palaeontologists. Apparently, nobody had serious doubts about their neutral buoyancy. It is regrettable that modern palaeontologists seem to be unable to deal with such tasks. Unfortunately, idees such as those propagated by Westermann are still in vogue. However, B. Ziegler (1967) in a study had already pointed to a close connection between sea depth and the occurrence of certain ammonite species.

As well as all previous workers A. Trueman had merely supposed that ammonites were neutrally buoyant. When I started my own calculations I did not use this assumption. Finally, this idea had never been substantiated. I had the intention to calculate as preci- sely as possible. The evalution of neutral buyancy was part of the results.

Normally coiled ammonites generally follow a logariothmic spiral, in this case two spirals are relevant: an outer and an inner spiral..

The formula describing the radius of such a spiral is :

Fig.1. Definition of parameters in a shell

A trapezoid as a general shape of a cross-section was put into the two spirals (fig. 1), allowing various cross-sections between square and triangle by variation of shell parameters. The volume of a segments grows with the growing angle j, generally 10°. In addition, I could vary the body chamber length. So, there were a lot of eligible possibilities, which allowed the calculation of almost any ammonite following a logarithmic spiral.

For each segment and by numerical integration for the whole shell these values were evaluated:

the surface of the shell, without a dorsal wall
the weight of the outer shell
the weight of septa
the total weight
the buoyancy
the difference between weight and buoyancy (complete animal and empty shell)
the ratio of shell weight to soft body weight
the position of centre of gravity and centre of buoyancy
orientation of the aperture
direction of the hyponomic backstrokerelative rotational stability (distance between the centres of gravity and buoyancy, related to the maximum diametre of the shell.

An important requirement were the assumptions to be used for a correct weight evaluation. I made these assumptions:

All specific weights were corrected by a factor of 1/1.026 to compensate for the difference between sea water and fresh water, since in this case the values of volume and buoyancy had the same numerical value.

Fig 2. Three ammonite shapes generated by the computer program

Fortunately, G. Westermann (1971) had carried out measurements of the thickness of outer shell and septa in many ammonite species. I have used these values, which I later found to be reliable, using a different method. Sin- ce I could not find data as to the number of septa per whorl I was grateful for the offer by G. Dietl to carry out a number of measurements in the Staatliches Museum für Naturkunde in Stuttgart

First of all I needed a scale of comparison. This could be achieved using a shell of the modern Nautilus, represented nowadays by several slightly differing species. Although its shell is considerably thicker than Westermann’s measurement in ammonites indicate, nevertheless the shell in Nautilus is smooth and void of ribs which would considerably contribute to the total weight. A comparison between the measured weight and the one calculated by the computer program showed that the calculated weight was slightly lo- wer. Therefore, I was sure that the weight of ammonites would not be too large; my assumptions were on the safe side. Starting with a radius of 0,5 mm 36 segments for each 10° angle increase up to a final diameter of 200 mm were calculated. The investigated parameters are intrinsic for judging a possible nectonic lifestyle, compiled in fig.3 in a somewhat simplified presen- tation. As can easily be deduced from fig.3, favourable conditions for a nectonic lifestyle are present only if the body chamber does not exceed a length of half a whorl. Only short body chambers can simultaneously fulfil all requirements valid for a swimming ecto- cochleate cephalopod. Certainly, it is not surprising that the modern Nautilus with a body chamber length of 120° -135° meets exactly the green region and fulfils all pre-conditions almost ideally. Nevertheless, Nautilus cannot be called a perfect swimmer. The red area characterizes the range of body chamber length occurring in normally coiled ammonites.

Up to a BCL of 200° there is a almost linear relationship between BCL and apertural inclination. This means that the animal would be able to adjust its apertural position in a predictable manner by changing its BCL. A further increase of the BCL results in a chaotic change of the inclination of the aperture, which would not be acceptable to a swimming ammonite and its feeding habits. A similar behaviour can be stated for the direction of the thrust force. As in the present-day Nautilus the direction of the hyponomic thrust in a swimming ammonite should pass through the centre of gravity, since otherwise there would be a strong tendency of rotation. Therefore, the range of favourable BCLs is even more restricted. Even in case of neutrral buoyancy the hyponomic thrust would cause a downward movement.

A further limit is marked by low stability. The stability is defined by the distance between the centres of gravity and buoyancy in relation to the total diametre. The less this value is the easier will the shell rotate. Also here only a BCL of less than half a whorl is favourable. The stability has is lowest value just at a BCL of one whorl which we can frequently find in ammonites. Is’nt this curious?

Regarding all criteria simultaneously we find that in a swimming ammonite the body chamber in any case must not exceed 180°!

Fig 3. Characteristic features for judging the ability of a nectonic lifestyle. All requirements are well fulfilled in the modern Nautilus, but not at all in ammonites. Red hatching: body chamber length area found in ammonites, green hatching: body chamber length area urgently required for a nectonic lifestyle. At a body chamber length of 315°, a value that frequently can be found, we can read: 1. Orientation of the aperture: 22° upward, 2. Stability: 0,5% that is about 5% of Nautilus’ stability, 3. Direction of the hyponomic thrust: -50° that is a considerable downward component, 4. Ratio of weight to buoyancy: 115 %, that is the weight is higher than the buoyancy, it cannot be neutrally buoyant.

A particularly striking result is that ammonites do not at all take account of the requirements to be observed by a nectonic lifestyle. For example, the rotational stability of real ammonites is so low that they would start rotating in case of an intended locomotion by backstroke employing a power comparable to Nautilus. Moreover, there is a strange contrast between the actual body chamber length (BCL) and that one required for neutral buoyancy; there is no harmony at all. There are even indications that ammonites did avoid the region of imminent positive or neutral buoyancy. By the way, the outlined course of apertural orientation is in good agree- ment with the investigation by Trueman (1941) for a few selected forms. His investigation had already demonstrated remarkable differences concerning the life orientation between Nautilus and ammonites. Nevertheless, certain workers as for example G.E.G. Westermann and his supporters persist in transferring the orientation of Nautilus to all normally coiled ammonites.

So far, nobody besides me has ventured upon the calculation of neutral buoyancy. The reasons may be that for stratigraphical pur- poses this property as well as the mode of life is of little importance. The suitability as characteristic fossils for stratigraphic purpo- ses is not affected. On the other hand, the supporters of a nectonic lifestyle would not like to discuss this item. The early obtained capability of neutral buoyancy is beyond doubt for them. Maybe, there was a thorough conviction that a determination by calculation would never be feasible. However, the utilization of modern computers makes it rather easy to show that ammonites were too heavy to be neutrally buoyant. If a worker would have succeeded to demonstrate the contrary, he certainly would have published his results. Surely, there have been attempts.

The BCL differs from species to species. For example, in Dactylioceras commune shown in fig.4 it is generally one whorl. On the other hand, within a species there can be considerable differences. While in the gender Taramelliceras it is normally half a whorl, I have found a complete specimen with a remarkably short body chamber ( page Fotos). Apparently, the BCL has no meaning in con- nection with neutral buoyancy and may vary between individuals without any impact. Based on these calculations but also on other reasons I have become deeply convinced that ammonites must have been bottom- dwellers. In this respect I am in harmony with several other workers, for example G. Dietl (1978), who considered that heteromorph ammonites (Spiroceras) from the Dogger preferred still-water areas, where they lived on seaweed. Contrasting to ammonites Nautilus with a BCL of 120° - 135° fulfils all requirements of a swimming cephalopod almost ideally. All investigated parameters are situated in the favourable range, the stabilty near the maximum. Nevertheless, Nautilus is not at all a good swimmer. The locomotion is characterized by a remarkable pitching movement. Certainly, this animal does not cover large distances, and we must additionally keep in mind that the haemocyanin in his blood cannot well store oxygen. Nautilus is a poor swimmer.

Fig. 4. Median section through the shell of Nautilus respec- tively through an ammonite demonstrating the typical diffe- rences of features. The body chamber of Nautilus is unusu- ally short with 120° - 135°, in ammonites it is often very long and then worm-like. The septal walls are concave towards the aperture in Nautilus, in ammonites convex. The siphonal tube has a central position in Nautilus, whereas it is ventral in ammonites.

Furthermore, it became clear to me by these calculations that Nautilus is unique among all shelled cephalopods. Nautilus has the largest spiral constant among all forms, that is it has the largest increase in diametre and cross-section per whorl. The shell is tightly coiled with a large cross-section. From these features results the short body chamber. But it must be stressed that Nautilus is a modern animal which cannot be used as an analogue for forms that have disappeared 65 million years ago. Even the assumption the ancestors of Nautilus were swimmers has never been demonstrated.

2. Alleged hints to swimming abilities, growth of epizoans on empty shells

The main argument for a nectonic mode of life has already early been derived from the general resemblance of ammonites to the mo- dern Nautilus which can obtain neutral buoyancy and move by means of its hyponomic thrust. Nevertheless, as mentioned above the motility of Nautilus is considerably restricted, and he cannot swim from one border of its region of occurrence to the opposite. Nauti- lus shells differ considerably from ammonites by the rapidly increasing spiral, short body chamber, smooth shell, and simple sutu- res. Since ammonites have a logarithmically coiled shell as well, they should therefore likewise have been swimmers. Certainly, this is a very simple argumentation.

As David K. Jacobs from the USA would have us believe this capability was present already since the first occurrence of ectococh- leate cephalopods during the Cambrian period. In his polemic review of my paper on hydrostatics of ammonites (1999) he wrote

˜...Clearly the chambered shell and siphuncle are complex and costly to maintain and where they occur in all modern cephalopods they serve to generate neutral buoyancy. Thus it is reasonable to assume, as most workers in the field do, that fossil cephalopods with chambered shells could achieve neutral buoyancy and that this was the function of the chambered shell and siphuncle since their evolutionary inception in Cambrian forms. Thus this interpretation is well established and reasonable given the range of observations across the modern cephalopods. Broad claims to the contrary are arguments against an established principle or null model. It is not up to other cephalopod workers to justify this model which is well supported by a broad body of evidence....

Really not?

Almost every feature such as the complicated sutures, the phragmocone, a seemingly streamlined shape, etc. were regarded in connection with the requirements of neutral buoyancy and a nectonic lifestyle. Alternatives were disregarded by most workers. Con- tradictory evidence is intentionally ignored in an unscientific manner by the obstinate supporters of a nectonic lifestyle, though in contrast to critical and unbiased palaeontologists. The world-wide occurrence of many species is regarded by many workers as caused by a nectonic lifestyle. On the contrary, ammo- nites are easily distributed over large distances by planktonic larvae. Shigeta (1993) has indicated by calculations that up to a dia- metre of 2,5 mm larvae could stay in a planktic layer, only the growing shell weight with a further growing diametre made them des- cend to the ground. For example, a new mussel in the Lake of Constance has spread across the whole lake within a few years, with an average speed of approximately 10 km per year. Using this spreading speed we can easily estimate that a world-wide distribution is possible within a few hundred years. All arguments supporting a nectonic mode of life are ambiguous and cannot be used as unequivocal evidence. Often the wish is the father of the idea concerning such models and alleged proof. A settlement by epizoans such as oysters on both sides of Lytoceras shells from the Posidonia-shales (Toarcian) was used by Seil- acher (1982) as evidence that ammonites did actively swim (fig.5). Since the apertural area was free of settlers he opined that the living animal was still within the shell and kept the aperture free using its tentacles. Similar conditions as for the complete animal I found by my calculations are valid for empty shells, except that neutral buoyancy is still present at approximately 30 degrees longer body chambers. Taking into account this fact it is more probable that the shells shown in fig.5 were empty, drifted at the water surface, and could tolerate the settlement by epibionts for some time until finally the growing weight due to flooded chambers made them sink to the ground. The outlined water line even allows an estimation of the actual body chamber length. It is about 240 degrees which is in good agreement with the BCL known from real lytoceratids. A. Seilacher had wonderful material at his disposal, unfortunately, he did not interpret it in an unbiased manner. The main growth direction makes clear that the shells did no more change their apertural orientation and, therefore, must have been empty. Furthermore, it is probable that they drifted at the water surface with the aperture and in some cases upper shell parts not continuously covered by water. There are two possible reasons for

Fig.5.Settlers on Lytoceras shells demonstrating the former orienta- tion during settlement, but not necessarily during life, after A. Seilacher (1982)

this fact: It is conceivable that the drifting shell moved up and down due to wave movements, or it is possible that a bubble of gases of the decaying soft body remained in the rear part of the body chamber thus making the shell lighter. This is not possible in Nautilus with its short body chamber. Nevertheless, Nautilus shells can be buoyant for several months and drift over large distances. A bubble of decay gases can also serve as an explanation for the occurrence of large quantities of ammonite shells in sandy litoral sediments, for example the well-known Dactylioceras-bed of the Franconian Alb. This species (fig.4) has a very long body chamber which could easily contain a gas bubble. Obviously, enormous quantities of empty shells were washed ashore, although presumably not over large distances. The complete absence of soft parts in ammonite shells from the Solnhofen limestones, often containing the aptychus, makes likely that such shells drifted empty. As well preserved soft parts of coleoids demonstrate, preservation was generally possible in these limestones. The absence of soft parts would hardly be understandable in case of a nectonic lifestyle. The growth of oysters etc. on shells is no unequivocal indication of a settlement during life, in particular if only one generation of settlers is found. This kind of settlement occurs nowadays also in Nautilus.

However, certainly it was not the normal fate of an ammonite shell to ascend to the surface. In general, empty shells were too heavy and fell to the ground where the animal died. Such shells are not uniformly distributed in the sediment, but are found clearly accumu- lated in some areas with a special relief. For instance, they are often closely related to the sponge-algal reefs of the Swabian Alb and particularly to the reef debris. Empty shells could even occasionally remain standing upright on the ground for some time. There- fore, the aptychus could be preserved within the shell or in the immediate vicinity also in this way. Alternatively, shells could be moved by near-bottom currents and could be aligned in grooves like tiles. This kind of preservation is known from the Posidonia shales. Frequently, the aptychus is still preserved in the body chamber. In forms with long body chambers it could not drop out of it, accor- ding to the upward apertural orientation. After death and decay of the animal occasionally such shells could ascend to the surface and drift considerable periods and distances.

Fig.6. An ammonite shell settled by serpulae. The aptychus has slided down to the lowest position. Probably, the shell remained standing on the ground for some time, and the serpulae grew upwards, after A. Seilacher (1960), slightly modified.

Fig. 6 presents such a shell from the Posidonia shales which is settled by serpulae on both flanks showing a preferred growth direction. The aptychus within the shell would mark the lowest position. Therefore, it is probable that this shell remained standing on the ground for a some time with the settlers growing upward.

3. Evidence of a missing ability to swim or to achieve neutral buoyancy : settlers on the shell during shell growth and shell injuries

However, it is quite a different story if settlers are concealed by the growing shell, as reported by S. Rein (1996) in ceratites. The strong growth of Ostracina placunopsis on such shells convinced him entirely that ceratites could not have been swimmers, since the considerable additional weight and problems concerning the formation of new chambers would not have allowed this lifestyle. Overgrown representatives of the genders Ceratites and Germanonautilus from the Thuringian Muschelkalk allow observations which are hardly possible elsewhere.

Fig. 7. Ceratites completely overgrown during life by settlers., from S. Rein (1997).

Relatively abundant are specimens with large areas covered by mussels or oysters. The settlement is not restricted to the adult shell, but affects also early whorls, and the growing cephalopod overgrew the settlers. Although the shells of epizoans contribute to the buoyancy, the total weight is even more increased since the settlers are heavier than water. If neutral buoyancy had been important for these animals they would have been forced to build addi- tional chambers and to shorten the body chamber. Such a behaviour could not be found. The shell parameters did not differ from other shells without settlers. This fact made S. Rein arrive at a the conviction that neutral buoy- ancy was of no importance for ceratites as well as for Germanonautilus.

Large areas covered by settling mussels on specimens of Leioceras opa- linum from Heiningen (Baden-Wuerttemberg) were also found by W. Riegraf (kind information by letter).

Further evidence for a life habit on the ground was given by healed shell injuries. An injury would disturb the animal`s normal beha- viour. Therefore, a change of the shell growth should be visible. For example great disturbances of the normal growth can be obser- ved in Nautilus if it is moved from its natural environment to an aquarium. No such features occurred in ceratites. Shell injuries were eliminated unspectacularly, although such injuries would have had great influence on the neutral buoyanvy.

A rather frequent injury happened to the chambered part of a shell. By such an injury the animal could be seriously hampered in its ability to build new chambers with a normal distance from each other. Instead, the soft body slid forward in a big step corresponding to a distance of many septa until it succeeded to fix the soft body to the wall again and to build a normal septum. However, this process had no impact on the shell secretion near the aperture; it is normally grown in all observed cases. If these ceratites had been swimmers such a heavy accident would have had strong consequences for the animal. By the way, the septa following such an accident present quite a different inclination compared to the earlier ones. All these observation have contributed to S. Rein`s and to my own conviction that only a crawling lifestyle in ammonites can be true.

New evidence has recently been provided: Demersal habitat of Late Cretaceous ammonoids: Evidence from oxygen isotopes for the Campanian (Late Cretaceous) northwestern Pacific thermal structure - Geology, Volume: 31 Issue: 2 Pages: 167-170 Authors: Moriya, Kazuyoshi, Nishi, Hiroshi, Kawahata, Hodaka, Tanabe, Kazushige, Takayanagi, Yokichi Abstract: Comparison of oxygen isotope data for exceptionally well preserved co-occurring plankton and benthos from the Cam- panian of Hokkaido, Japan, with nine species of ammonoids clearly indicates the demersal (nektobenthic) habitat of ammonoids; unlike Nautilus, the ammonoids studied did not engage in significant short-term vertical migrations in the water column. The new forami- niferal isotopic data suggest that seasurface and sea-bottom temperatures were 26°C and 18°C, respectively, at 40°N in the Cam- panian northwestern Pacific. The temperatures were significantly warmer than those in the modern northwest Pacific. This finding provides the first reliable evidence for the warm Late Cretaceous mid-latitude North Pacific. Isotopic analyses of ammonoids show that the average calcification temperature of all ammo- noid shells analyzed was 19°C, comparable to those of co-occurring benthos. None of these ammonoids display calcifi- cation temperatures equivalent to those of planktonic foraminifers.

4. A continuously active pull in ammonites explains why the shell shape differs from Nautilus

The starting point of my considerations were not the calculations and results presented above, but the insight that a pulling force had to be active in ammonites. The strong differences between the modern Nautilus and ammonites require different conditions during shell growth between the two types of shells. Otherwise ammonites had to be more similar to Nautilus. The shell should be smooth, roundish without edges. However, this does hardly apply to any ammonite. For this reason I found the idea very attractive from the beginning. The pulling force can essentially always be made responsible for features differing from Nautilus. Since Nautilus floats in the water column, there are only two continuously acting forces, namely the weight and the buoyancy. These forces act vertically, have the same magnitude, but opposed directions. Only during horizontal locomotion there are two additional shortly acting forces, the thrust and the hydrodynamic drag. They cannot produce a visible influence on the shell shape. Indeed, this pulling force causes the appearance of several features of the ammonte shell. Former workers tried to find a connection of almost all these features and the swimming abilities of ammonites, even if they were contradictory. Much would have been so simple with the eyes open! But the first step must be the insight that ammonites were bottom-dwellers.

4.1. Shell orientation

In order to be able to carry the shell benthic ammonites must have had a shell orientation different from Nautilus. A crawling lifestyle requires an apertural orientation as it is present in benthic snails. An argument for this orientation follows from the evolutionary history of shelled cephalopods. Since these are traced back to monoplacophores the ancestors have certainly been benthic.

Fig.8. Plectronoceras is considered as the first ancestor of shelled cephalopods. Although chambers separated by septa are already present this form has always been regarded as benthic.

The early forms such as Plectronoceras were regarded as negatively buoyant, since the gas in the nar- rowly spaced chambers appeared not sufficient to allow the animal to float in the water column. Conse- quently they were reconstructed as crawlers. However, the following nautiloids and later the ammonites should be neutrally buoyant, maybe by witchcraft. It should be taken into account that a drastic change from a life on the ground to swimming in the water column would mean an enormous change of lifestyle. I cannot recognize the advantage of such a change, and which attractive food source should have cau- sed it? The time of lifestyle change in the modern Nautilus is completely unknown, and he finds his food also nowadays on the ground. An advantage may be seen in the improved motility. There is a large gap in the evolutionary history of Nautilus from the Tertiary to modern times. There are several indications, as mentioned above, that fossil nautiloids also were bottom-dwellers. Fig.9 presents an ammonite in life position. The shell is carried in the same fashion as in gastropods. The pull (red) that runs through the soft body from its attachment at septa and outer shell to foot area and ground keeps the shell in its upright position. Without this pull which in an analogous manner is performed in gastropods by the spindle muscle the shell would roll down.

Fig 9. Ammonite in life position with schematic acting forces. The pull (red) is acting in the soft body from its attachment at septa and outer shell to foot area and ground. The pull keeps the shell in its upright position. Of course, the outlined soft body is hypothetical. As I will demonstrate lateron, it was presumably markedly larger.

Unfortunately, there is no direct evidence of this shell position, unless one day somewhere in the deep sea an ammonite as a living fossil will be discovered. But I am sceptical as to this chance. Nevertheless, I am deeply convinced that the upright position is the correct one. A different position is hardly conceivable for such bottom-dwellers. Epizoans might serve as indicators, however, they cannot present unequivocal evidence, as mentioned above. In my view a strong argument for this shell position follows from the fact that all shell features can easily be explained on the basis of this position. Presumably, the pull is not acting on a single line, the line of action, as indicated in in Fig.9 in a simplifying manner, but is distributed to several or many muscle fibres. The size of the pull is dependent on the size of the overweight itself. The heavier a shell is the greater a pull is necessary to keep in its position. This work must be done by muscles, which only can transmit pulling forces.

4.2. Spiralization

The weight force is thought ideally to act in the centre of gravity, the buoyancy in the centre of buoyancy. The resulting overweight (yellow in fig.9) has its centre between these centres. The bearing force (white) has to counteract two forces, namely the overweight on the one side and the pull on the other. These continuously present forces have the effect that the growing shell is rolled up. This process may happen more or less continuously, in certain cases also step by step, extremely well to observe in Palaeozoic clyme- niids. This process finds its reflection in sculptural features. The size of forces and the involution of the shell are dependent on each other, in other words the degree of involution is a consequence of the size of the acting forces. The shape of a normally coiled ammonite indicates how heavy it was compared to others. Furthermore it means, that without any doubt every coiled ammonite was heavier than water, all normally coiled ammonites had to be bottom-dwellers! - Former suppostions were merely based on fantasy.

Fig. 10. Ammonites with a different degree of involution, overweight decreasing from left to right. All these forms were heavier than water.

This result is in harmony with those achieved by calculations. However, even slightly coiled heteromorphs must have been heavier than water, since their shell is curved. Only in straight shells the decision becomes a bit uncertain, if spiralization is used as the only criterion.

Fortunately, there is another way to arrive at a definitive statement. The overweight effects a stress of the adhesive musculature. Position and shape of this musculature which connects soft body and shell is unfortunately unknown. However, there are certain indications which can be derived from various shell features.

4.3. Feather stripes and stripe formation

Occasionally, a sculptural feature is found in slender and tightly coiled forms, which is called feather stripes, because it is in a way reminiscent of a feather (fig. 11). These stripes clearly indicate that there was a stress in the musculature pulling it towards the aper- ture. However, it should be mentioned that these stripes were not generated simultaneously, but two by two after another, corres- ponding to the advancement of the soft body in the shell, thereby indicating that the attachment of the soft body to shell was different from Nautilus.

Fig . 11. Feather stripes in a broken specimen of Oppelia sp.. These stripes are not part of the shell sculpture, which is for comparison shown in the lower picture in a similar specimen, but were generated in an inner shell layer. In my opinion, these stripes mark the attachment of muscle fibres and are a direct indication of the acting pull.

It is significant that feather stripes do occur predominantly in slen- der involute shells which as explained above have the highest over- weight. In such shells apparently the pull is concentrated mainly in the middle area of the flanks. A similar feature can sometimes be found in other forms such as Amaltheus, however, these stripes are part of the sculpture of the external shell. Presumably, these stripes also can be traced back to pulling stress. They are an individual feature and not typical for certain genders. But it seems that such stripes do only occur in involute and thereby heavy shells. All these feature make clear that the musculature transmits the stress from the shell to soft body and ground in different manners. There seems to be no generally valid scheme.

4.4. Shape of the aperture during ontogeny

In addition, the shape of the aperture presents indications of the acting pull respectively of the number of muscle strings involved in the transmission of forces through the soft body to the ground. There are differences between genders, but presumably these do not represent as many tentacles as found in Nautilus. There are no definite indications at all that ammonites possessed tentacles, not to mention how many.

Fig. 12. Different shapes of the apertural area due to stagnation of growth

Left: Lytoceras sp., in the middle and right: Phylloceras sp.

In many forms more or less regularly a stagnation of growth can be found, particularly often in Lytoceras and Phylloceras. This leads to an extension of the shell or to a thickening of the apertural margin. These stages of stagnation preserve the shape of the mantle near the aperture. They are reminiscent of corresponding features in certain marine gastropods such as Murex, which builds three rows of spines per whorl. Obviously, regular varices are an expression of the animal’s special behaviour. In lytoceratids a so-called collar is formed, which in large specimen can reach a width of several centimetres. The collar shows that here soft parts protruded from the aperture which supported the shell. Presumably, the soft body widened substantially outside the shell and passed on pulling forces to the ground. There are no indications that ammonites could completely withdraw into their shell as Nautilus can do. As outlined lateron the soft body of heteromorphs seems to have been much larger than former suppositions would suggest. There is much reason to believe in a relatively low overweight in lytoceratids because of their roundish cross-section and low invo- lution. In phylloceratids during stages of growth stagnation exhibits rather different shapes. In all cases the apertural margin is mar- kedly thickened, which becomes evident on steinkerns. In the ammonite in fig.12 on the right side the middle of the flanks is mar- kedly drawn forward. Presumably the pull was concentrated in this area. It presents a certain similarity to the feather stripes of fig.11. On the contrary, the area of main stress is not clearly marked in the other phylloceratid of fig.12. All phylloceratids indicate a considerable overweight by their high involution.

4.5. Whorl cross-section

A very strong argument for a pulling force being active is given by the cross-section of ammonite shells. The reason is that forces are required to realize for example a tetragonal cross-section. In Nautilus which grows free of the ground the cross-section is always oval up to roundish, without any edges. He is suspended on his shell in the umbilical area. There are no continuously acting forces besi- des weight and buoyancy. The shell growth takes place in small steps, marked by fine stripes of the shell. In this case the cross- section must become roundish to oval, that is, abrupt changes of the shape cannot occur. Also other shell-bearing molluscs such as planctic gastropods have also smooth shells without sculpture. However, in fossil nautiloids which are the precursors of the Recent animal, angular forms with strong sculptures are widespread. An abundant representative in the Muschelkalk is Germanonautilus, which nearly has a square cross-section. Probably, these nautiloids still were mere bottom-dwellers. Only later successors became free of the ground and learned to hover. The point of time for this change is unknown. To achieve such a cross-section there must be forces to transform a roundish to an angular shape. It is probable that this change was not achieved by tentacles protruding from the soft body but by the soft body itself. Its extension outside the shell caused this distortion. In my opinion, the cross-section of the soft body near the aperture which only is responsible for the resulting cross-section is connected with the animal’s activity during locomotion on the ground and feeding. It is important to keep in mind that a steady connection with the ground or a solid matter is absolutely necessary to generate a pulling force; a pull always needs a connection between to solid points to be active. In ammonites the cross-sections are very variable (Fig. 13). They vary between broad and depressed (cadicone) and slender and acute (oxycone). Also in ammonites the cross-section near the aperture is determined by the activity of the crawling animal. There are cross-sections with a differing number of edges and corners, which even may change in number and position during growth. In each case the question is where forces of which relative size and direction have to be placed in order to transform a roundish cross- section to the real one. This can easily be tested by distorting a rubber band from its roundish shape to an angular one.

Fig. 13. Cross-sections in three ammonites and outlined forces in the apertural plane that are required to gene- rate a certain cross-section, emanating from a roundish shape.

The shape of the cross-section is not primarily dependent on the animal’s overweight. Involute as well as evolute forms with the same cross-section can occur. Even within a well defined species remarkable differences are possible. This is not to be wondered at. Presumably, these diffe- rences follow from the animal’s individual behaviour in con- nection with its food source which makes the one individual take up food moving slowly and covering a broad strip on the ground, the other one faster feeding on a narrower strip. However, the food source of ammonites as well as many other unresolved questions is unknown. Indeed, the enor- mous number of individuals compared to Nautilus can be used as evidence that they were a low member in the food chain feeding on a even lower and abundantly occurring food source. It is conceivable that they grazed algae or fil- tered the substrate.

Any ammonite preserves its individual behaviour during its entire growth in its shell. Contrary to diverging statements in the literature the shape of the shell is not genetically determined, but variable within certain limits. Insofar, the definition of many species appears too narrow. Only a huge number of individuals is suitable to clear the intraspecific variability and thereby the real range of a species. Apparently, the size of the pulling force is individually different. However, the location where it is active is very constant within a species.

Frequently, these locations are marked by spines, kinks or splitting ribs. Obviously these spots were connected with protrusions of soft parts, in particular where spines were situated on the shell, which later were covered by new shell layers and then withdrew, to be pushed out again after the advancement of the soft body. Comparable sculptural features occur in marine gastropds. Rows of spines are present in a variable number and position. Usually they are associated with an edging. Obviously, at these edges the soft body suffered a more or less strong pull near the aperture. A statement as to the effect can only be made for the apertural plane, the true direction remains uncertain. Presumably, the pull direction was obliquely forward, since it had to be directed towards the ground.

5. The meaning of the suture

The sutures are a particularly noteworthy feature of the ammonite shell. It is well known that it marks the line of attachment of a septum to the shell wall. It can therefore only be visible on steinkerns after the dissolution of the shell. From an aesthetic viewpoint sutures appear very decorative on many steinkerns. Meaning and function of the suture were a matter of a long debate, and many functions have been proposed without the presentation of a really doubtless explanation.

The most obvious task of a septum consists in shutting the body chamber behind the soft body after advancing to a new position, respectively in covering the apex with shell material. A corresponding process can be observed in marine gastropods in which hollow spaces from which the soft body has withdrawn are covered by a septum-like wall. In Nautilus as well as in ammonites a further task consists in the formation of a gas-filled phragmocone.

Since previously all ammonites were regarded as capable of swimming and floating a function of the suture in this context was pre- sumed. Indeed, in the course of the ammonitic evolution a change of the suture can be stated. There is a general increase of compli- cation. However, there were already very complicated sutures in Triassic forms. On the one hand, the highest degree of complication is found in the extremely acute Pinacoceras (fig.15) from the Triassic of the former Thetis sea. On the other hand, in certain decoi- led heteromorphs from the Cretaceous the degree of complication is retrogressive, and there are very simple sutures in some ortho- cone heteromorphs. Obviously, there is a coincidence between shell shape, cross-section and degree of suture complication, in addition, between overweight and shape of lobes.

Since a thorough understanding of physics is missing in many palaeontologists, various authors have supposed that the increasing complication of sutures was caused by a reaction of the animal to a growing pressure difference between ambient pressure and pressure within the phragmocone. It is well known that the modern Nautilus performs vertical migrations of several hundred metres in the water column. This means a considerable change of ambient pressure, namely one atmosphere per 10 metres water depth. Nevertheless, in Nautilus the attachment line of a septum to the shell wall is as simple as possible. In the opinion of some alleged experts this should mean that ammonites were even better swimmers than Nautilus. The stronger complication and narrower space between septa should allow ammonites faster movements in the water column. The distribution of lobes on the flanks should lead to a uniform support of the walls against the water pressure. The same should apply to the folded septa as to the water pressure in the body chamber. Although this appears quite plaubible at first sight it is complete nonsense. Actually a better resistance against water pressure cannot be achieved in this manner. The pressure is a force per square unit. If a critical pressure is exceeded an implosion occurs which cannot be prevented by any means. As in a submarine there is a critical pressure or depth which must never be exceeded, otherwise a sudden break of the shell occurs. The shape of the suture has definitely nothing to do with a resistance against water pressure.J.Wiedmann (1969) and W.Blind (1975) had already mentioned plausible arguments in this sense. Neverthe- less, some experts such as G. Westermann and his followers retain their false ideas.

As the features mentioned above also the complicated sutures have their origin in the pulling force needed to carry and balance the ammonite shell. This pull normally originates from the end of the soft body at the septal wall. The great differences between the modern Nautilus and ammonites as to the shape of the septa are in a close connection with the different lifestyles.

In the freely floating Nautilus the mass distribution of the soft body in its body chamber during growth of the phragmocone is not en- tirely stable. With the soft body growing the last secreted septum advances continuously to an elevated position, since the apertural orientation shall remain unchanged. If the rear part of the mantle were not held in its position by the attachment musculature it would glide down to a stable position respectively stay on the apertural level. However, since the soft body is connected with the last sep- tum, a pressure gradient is generated between the last septum and the apertural level, with the elevated soft body portion pressing on the adoral part with a force K (fig. 14). I have estimated this force to be roughly 1 gram. Nautilus can utilize this small pressure differential for the advancement of the soft body without any stress.

Fig.14. Formation of the watchglass-shaped septa in Nautilus by a glide of the rear part of the soft body down to a new position

The maximum pressure occurs just when the attachment musculature starts to abandon its connection with the shell wall. In Nautilus the soft body is only connected to the shell near the aperture and at the septum. Between these two attachments the soft body is se- parated from the shell by the pallial fluid. After giving up the attachment to the shell the soft body starts gliding down to a new position. This movement comes to an end automatically when the pressure equals the ambient pressure. Maybe, during the advancement the soft body is also suspended on the siphonal tube, because this tube is straight between two septa. It serves the following emptying of a new chamber. This tube is located in the middle of a septum, thereby indicating a holding function. The hollow space between the last sep- tum and the new position of the rear part of the soft body is filled with a fluid, essentially water. The advancing soft body can already easily be stopped by stopping the water supply. At this moment the soft body can attach again to the shell wall, and a new septum can be secreted. Thus, the formation of septa is done without additional forces, merely by emploing a small pressure differential. The septum preserves the roundish shape of the apical end of the soft body. After the secretion of a new septum the newly built chamber can be drained by an osmotic process. The simple shape of the septum can finally be traced back to the fact that it is generated by very small and periodically acting forces and practically no holding forces by muscles being required.

However, the condition of low forces during soft body advancement cannot be demonstrated for the last known precursors of the Recent Nautilus, even less for earlier relatives. The sutures of such forms differ considerably from the living animal. Watchglass-like septa do not yet occur. On the contrary, the position of the siphonal tube outside the centre of septa as well as a strong folding of septa for example in Aturia indicate that also in these forms a pull was active which can also be used as evidence of a lifestyle on the ground. Not only cross-section and sculpture of such forms but also the sutures make likely that these precursors were not independent of the ground.

In ammonites the suture is however much more differentiated in the course of their evolution than in nautiloids. This fact can be explained with a special adaptation to the requirements of their permanent benthic lifestyle. While the Recent Nautilus is carnivore, the nutrition of ammonites is unknown. Obviously, their organization was different. If ever they had jaws, these differed from those in nautiloids. Also the shell cross-sections were very variable. In my opinion these features reflect a variable activity on the ground, maybe by grazing algae.

Fig. 15. Sutures in three ammonites with a dif- ferent degree of complication. Waagenoceras from the Middle Perm, Pinacoceras from the Upper Trias, Hoploscaphites from the Upper Cretaceous

The pulling forces resulting from balancing the shell can only be transmitted by the attachment musculature, since here is the strongest connec- tion between soft body and shell. Without any doubt the attachment to the shell was reliable in all fossil forms, since the occurring pull was not necessarily stronger than in the modern Nautilus. However, the pull was continuously active, not only periodically. That is an important difference. Thus, there was a steady stimulus to minimize this stress by differentiation of the attachment musculature. This process can be observed during the whole evolutionary history of ammonites. The purpose of such a differentiation can only be found in an improvement of the musculature. The simplifica- tion of sutures in straight forms of the Cretaceous is not in conflict with this statement, on the contrary the low weight and conse- quently low pull force can be regarded as an excellent confirmation.

The supposition of a pull as the cause of the increasing complication of sutures was not entirely new, although I had the idea inde- pendently and found the reason of the complication. A. Seilacher (1975) made simulations of suture generation by fixing a membra- ne in a tube at some points (lobes) and pulling at other points forward (saddles). He produced by mechanical simulation what he observed in ammonites, but he was unable to find the cause of the pull. W. Blind (1975), too, saw an adoral pulling stress. He traced this stress in his model back to the continuous advancement of his longitudinal musculature on the one hand and the momentary stay of his subepithelial musculature in the present position. His model still was lacking the reason of the adoral stress. Apart from this deficit Blind’s ideas are next to my own. However, some modifications are necessary. The pulling stress following from the craw- ler position of the shell delivers the reason for the differentiation of the sutures.

Fig 16 shows a compilation of suture formation in a Permian ammonite with a comparatively simple suture which is better suitable to serve as an example than a complicated one.

Fig.16. Model explaining the suture generation in ammonites by three stages of ad- vancement ot the soft body. The two stripes of musculature SM and LM have alterna- tely to perform the holding function in order to allow the soft body to advance by one septal spacing.

Green = pulling force, red = retaining force

a. After the advancement of the soft body by one septal spacing the subepithelial musculature SM is attached again in the lobe area and solely transmits the pull to the shell. In the saddle area it advances as well as the detached longitudinal muscu- lature LM due to an adoral pull.

b. The longitudinal musculature LM is attached again on its whole length and has taken the holding function completely. The subepithelial musculature is likewise attached again on the whole length. Because of a stretching the pulling stress in lobe area is still present. The saddle area is free of stress after the attachment of the longi- tudinal musculature LM and has slided back and shrinked a little before the definite attachment. The new septum now is secreted.

c. The longitudinal musculature LM remains attached on its whole length and per- forms solely the holding function. The subepithelial musculature SM has detached from the shell and advances utilizing the preserved stress.

a new cycle as in a follows

By the differentiation of the subepithelial musculature in areas which alternately carry out the holding function the replication of a lobe pattern is fixed and essentially invariable in an individual. Only the first sutures do not show the typical shape. They originate from the planctic stage of the ammonite egg with no forces being active. The following generation of the typical suture is accompa- nied by two further changes, namely the shift of the siphuncular tube to the venter and the transition from retro- to prosiphonate septa. These modifications can be interpreted as an adaptation to the pulling stress. They have their appearance with the start of the crawling lifestyle.