Welcome to the next issue of the Seaweed for Sceptics series. The first post of the series explored the rough origin of the red algae, showing the genetic impariment that these seaweed suffer as a result of their extreme origins. In this second post we shall focus on one of these weird connections that crop up between branches of the tree of life: seaweed and ammonoids.
Now presenting: the ammonoids
Ammonoids are popular fossil cephalopods, originating in the lowermost Devonian and dying out at the base of the Paleocene (Danian). The typical ammonoid sported a monomorph shell shape, i.e., had a spiral shell, elegantly coiling in a single plane, with all whorls in tangential or overlapping contact.
Interesting enough, a number of ammonites depart from the arquetypical image of a single coil; with puzzling coiling patterns that involve elbows, uncoiling, twisting in many planes… The ammonites that depart from the monomorph shape are called heteromorphs. You can find pictures exploring the picture galleries from here, here or here.
Monomorphy and heteromorphy refer exclusively to shell shape, they have no phylogenetic meaning. Heteromorphs don’t form a natural group (neither do monomoprhs). Heteromorph forms have evolved repeatedly and independently from monomorphs in separate lineages.
In this super-brief introduction about ammonoids I’ve focused on the shell for a reason: it’s the only thing we have. They left almost no fossil aside from their shell, the soft-tissue anatomy remains largely unkown. Thus, the taxonomy, classification and ecological interpretations rely (almost) solely upon the structure and form of the shells.
Ammonoid ecology: many speculations (and seaweed?)
Inferring the ecology, behaviour or habitat of an ammonoid using the shell alone is no easy task. There has been a considerable amount of debate, and ecological interpretations remain quite speculative. You know what they say: In the absence of proper data, speculate wildly.
Given the wide diversity of coiling patters, a common life style seems unlikely. Most monomorphic ammonoids are considered to have been pelagic (roughly equally divided between drifters, swimmers and vertical migrants), with a few bottom-feeders.
There is room for seaweed in this ecological conundrum. Floating and benthic algal mats have been proposed as an habitat for both monomorphs and heteromorphs, as well as possible spawning places. Association with algal mats may have reduced vulnerability to predation.
In 2014, a certainly creative link was proposed between T-C heteromorphs* (belonging to the suborder Ancyloceratina) and seaweed (namely the Phaeophyta): adults were stationary, hooked or clipped to algal stipes and branches.
* Terminal-countdown heteromorphy (T-C for short) is most often represented in ammonoids by an U-shaped body chamber. T-C heteromorphs combine determinate growth with the development of novel form.
Branches are an ammonoid’s best friend
Various features of the adult shell are consistent with the stationary adult hypothesis: a weaker (sometimes worn) ribbing pattern on the inner surface of the hook, asymmetry of adult habitation chamber, lack of epizoans (parasitic animals glued to the external surface of the shell) and variability in adult shell shape.
This T-C morphology has been shown to be evolutionary convergent, yet temporally constrained essentially to the Cretaceous. The convergent evolution of this fancy coiling by a number of lineages simultaneously and independently of one another lends support to the idea that there was an ecological shift going on at that time.
This shift could be an adaptative response to radiations of algal macrophytes (forming kelp-like forests), following the adoption of a new reproductive strategy, with a sedentary adult female phase in which the eggs were brooded while the shell was hooked.
An stationary brooding phase has some advantages. Females would have been able to channel more resources into egg production (since less energy is spent in active swimming), the seaweed provided concealment from pelagic predators, and as seaweed inhabit warm and productive areas, egg development would have been faster, and hatchlings would have had more resources (compared to offshore habitats).
The right type of seaweed
What kind of branches were these fancy heteromorphs attached to? The best candidates seem to be our good friends, the seaweed. Branching colonial animals, such as bryozoans, crinoids and corals were abundant in the Late Cretaceous oceans… but the calcified branches would have caused greater wearing of the ribbing pattern on the inner side of the hook. Following a similar reasoning, calcified seaweed can be ruled out (Rhodophyta and some green algae).
We are left with the brown algae* (Phaeophyta). The Phaeophyta are a distinct group of eukaryotic algae with approximately 1.800 modern species. They exhibit quite a variety of life cycles and reproductive strategies. Most of them are found at sea (just 4 genera with freshwater species), with a worldwide distribution, yet they are predominant only in cold waters. Some of these algae are well known as kelp (most of them belong to the order Laminariales), big, fleshy macroalgae with robust stems and stipes. Supposedly the stipes were able to hold the curved shell of the ammonoid.
* Calcification of the thallus is known for some species of Padina, nevertheless, the fan-shaped structure of this elegant algae makes it unsuitable as a candidate.
The fossil record of the brown algae is quite poor (alas, no calcification means small chances of fossilization). The exact timing of the origin, radiation and diversification of the main brown algae lineages is not known. Various lines of evidence (genetic, geological, palaeo-oceanographic) suggest that brown macroalgae radiated during the Early Cretaceous, becoming more diversified during the Late Cretaceous. These two events of proposed diversification of brown algae coincide with two peals in diversity of T-C heteromorphs, during the Early and Late Cretaceous.
Personally, I find this hypothesis to be creative, bold, and highly speculative (which is something I tend to like), perhaps even bordering on the ludicrous side of speculation. But nevertheless it is thought-provoking and worthy our time to examine. Even it ends up being wrong, it will still have value. All the effort that is put into debunking wrong hypothesis might bring forward better ideas (even if that was not the primary goal).
A few problems can be pointed out. Some heteromorphs were covered in spikes. What use could the spikes serve in the middle of a kelp forest? They make more sense in a planktonic environment (as a way to avoid predation and/or sedimentation). Kelp tends to dominate the intertidal zone, meaning that they remain emerged for some time (they have a special wall composition to prevent dehydration). If the same is true for Cretaceous kelp, would it really have been a suitable habitat for ammonoids?
I can visualize an ammonite getting hooked onto a branch, but I really can’t see it remaining that way. For some reason, I imagine the shells sliding away, drifting away from the stems. The radiation of heteromorph forms could have been fueled by a greater abundance of food, since heteromorphs would have been able to feed in spite of their non-hydrodynamic or inefficient shell shapes (Juárez, J. & Matamales, R., personal communication, 2016)
UPDATE (27/05/2016): Well, the original 2014 article was indeed thought-provoking (told you so!). There has been a reply (here) againt the hypothesis, and a counter-reply defending it (here). Be sure to check the comment section in this post for further discussion on this matter!
If you would like to share any ideas or suggestions, let us now in the comment section below!
Arkhipkin, A.I., 2014. Getting hooked: the role of a U-shaped body chamber in the shell of adult heteromorph ammonites. Journal of Molluscan Studies, 80(4), pp.354–364.
Kaplan, P., 2002. Biomechanics as a Test of Functional Plausibility: Testing the Adaptive Value of Terminal-Countdown Heteromorphy in Cretaceous Ammonoids. Abhandlungen der Geologischen Bundesanstalt, 57, pp.181–187.
Lee, R.E., 2008. Phycology 4th ed.
Lukeneder, A., 2015. Ammonoid Habitats and Life History. In C. Klug et al., eds. Ammonoid paleobiology. Topics in Geobiology. Dordrecht: Springer Netherlands, pp. 697–800.
Westermann, G.E.G., 1996. Ammonoid life and habitat. In N. H. Landmann, K. Tanabe, & R. A. Davis, eds. Ammonoid paleobiology. Topics in Geobiology. Springer, pp. 607–707.