Birth and Death

At about 360 million years ago, this is the Late Devonian Epoch. The area that will become the American state of Ohio is now a marine environment. The continental arrangement is remarkably unfamiliar. To the south pole, there is the supercontinent Gondwana, which stretches into both the Western and Eastern Hemispheres. Moving north, one can find the continents Euramerica (the fusion of Laurentia and Baltica) and, in even higher latitudes, Siberia. It is close to the southern tip of Euramerica where this tale takes place, in a stretch of water not far from the immense Gondwana. Since our last stop in the Cambrian, lifeforms have changed in very significant ways. Terrestrial habitats have been effectively colonized, with the presence of expansive forests and many arthropods. Even vertebrates, in the form of early tetrapods (the modern members of this superclass are the amphibians, the reptiles, which include birds, and the mammals), have timidly begun venturing out onto land, but are still largely restricted to aquatic habitats. Underwater, fishes have diversified greatly, sharing ecosystems with ammonoids, a group of cephalopod mollusks, corals, and trilobites just to name a few. Simultaneously, the Devonian biota has been ravaged by ongoing extinction pulses for more than 20 million years, which will continue beyond our stay, resulting in the disappearance of many organisms. 

This phenomenon, the Late Devonian extinction, as it is called, is traditionally considered one of the five mass extinctions from the Phanerozoic Eon. Although some have appointed this event not as an extinction per se, but rather a biodiversity crisis, in which the natural loss of species remains the same, but the amount of emerging species dwindles, habitats did indeed change, and a substantial number of lifeforms, as just brought up, did truly meet their fate during this epoch, signifying clear environmental disturbances, perhaps not as severe as the ones from other mass extinctions, but still present either way. Several causes have thus been appointed as potential “culprits”, with them likely acting in conjunction. Some are extraterrestrial, these being a collision with an asteroid/comet or a supernova, the amazingly bright explosion that results from the death of a massive star or from the absorption of mass by a white dwarf (remnant of a star), which would have compromised the ozone layer due to powerful radiation, allowing, in turn, more harmful radiation to reach the surface of the planet. Volcanism probably was also at play, releasing greenhouse gases, lava, and other substances (like mercury, a highly toxic element that could have bioaccumulated in food webs) which may have had opposing effects on climate (either increasing or decreasing temperatures by means of various ways), with these volcanic activities perhaps being another mechanism responsible for the aforementioned damage to the ozone layer. 

The many land plants maybe also contributed to the loss of life through a familiar phenomenon by now: the absorption of carbon dioxide, one of the just cited greenhouse gases, promoted cooling (related to a very long-lasting ice age that has just started and, when in combination with the volcanic warming, probably led to climatic instabilities that proved even more disruptive to the biota, as will be exemplified later). Additionally, it is a possibility the physical and chemical breakdown of rocks by the embryophytes' roots (especially by trees like Archaeopteris, which had large root systems and were likely widespread across the globe, potentializing their effects) could have generated a large input of nutrients into the bodies of water, fueling the uncontrolled growth of aquatic phototrophs, such as algae and cyanobacteria, and giving rise to widespread eutrophication, which would ultimately result in the formation of anoxic areas, incapable of supporting aerobic creatures.

Now, let us focus on our surroundings. Immediately, an animal catches all the attention. With its mouth wide open, it gracefully cruises through the water, capturing many smaller beings as it moves. Bearing some resemblance to a whale shark, it belongs to the genus Titanichthys and, more specifically, to the species T. clarki. A little bigger than 4 meters long, this gentle giant is one of the largest placoderms, a group of fishes characterized by their articulated dermal armor. It is also part of the smaller grouping Arthrodira, containing other large representatives, with all displaying (apart from the bony, armored head) a cartilaginous skeleton. Unlike closer relatives though, which specialize in consuming hard prey, either with shells or tough exoskeletons, Titanichthys has evolved to be a filter feeder, its ancestors likely taking advantage of the higher amount of available food due to the above-stated eutrophication, responsible, at least initially, for an increase in numbers of plankton. Regardless, the observed individual is a female and she is pregnant, having two offspring growing inside her at this moment. Like some other placoderms, she is viviparous. Even so, her advanced age, a condition made visible by the bruised and battered body she sports, likely means that this will be her last pregnancy.

As she swims and moves into new areas of the sea, many other animals also make an appearance. Conodonts are abundant, swimming away agitatedly when the massive filter feeder passes by. On the substrate, a few corals endure. They are suffering enormously from the current extinction event: the incredibly abundant reefs once formed by them, stromatoporoid sponges, and other lifeforms are mostly, if not completely, absent, in large part due to the cooling temperatures and the associated drop in sea levels, both of which promoted the decline of the tropical, shallow seas where these animals once thrived (even though warmer temperatures perhaps also contributed to coral extinction as will soon be brought up). Microbial reefs have, though, remained. The widely seen stromatolites are one such example, being layered structures built by the accumulation of sediment trapped between mats of bacteria (such mats were discussed in more detail in our trip to the Cambrian). 

Most reef-builder corals were the rugose and tabulate corals, the two of them still alive, but in way fewer numbers. The rugose corals are either solitary, forming a horn-like layout (which can vary significantly in size and overall shape, being twisted or straight, for instance), or live together in large groupings, forming quite distinct and peculiar structures. In contrast, tabulate corals are colonial, creating curiously shaped arrangements, some branched, others in the form of mounds, etc. These two coral types have something in common however: their elaborate constructions are made by the secretion of calcium carbonate extracted from the water by their occupiers: polyps that catch small food particles with their stinging tentacles (a trait of their phylum, Cnidaria, as whole). 

Many tabulates in particular used to associate symbiotically with algae that took refuge inside their soft bodies. As a result of this, the algae were protected and received nutrients in the form of their hosts' waste products, while most of the corals' food was the carbohydrates synthesized by the algae. The photosynthesizers also helped significantly in the calcification process, besides many other interactions among the cooperating organisms. This likely ended during prolonged periods of excessive temperature rise millions of years ago (probably as an outcome of the aforementioned volcanic influences on climate, either warming or cooling, in this case, warming), prompting the expulsion of algal symbionts and the bleaching of the corals, further contributing to their decline. Since then and with the current decrease in temperatures, tabulates from deeper waters, lacking the symbiotic algae, have become more widespread following the extinction of their photosymbiotic relatives and are even on their way to associating with the algae again.

As several days pass, the still unborn pair of Titanichthys become more developed and, soon, they will be ready to go into the world. They have undergone what can only be described as an amazing process, going from single cells to fully-fledged, extremely complex, multicellular beings. Such transformation is intrinsically related to gene regulation. The constant turning on and off of specific genes is essential for cellular differentiation and specialization, as well as for determining how many divisions the cell is supposed to execute. The extensive eukaryotic genome is part of what makes this process possible. Some of the numerous non-coding sequences, for example, are responsible for the production of various types of RNAs, which control other events in various ways. 

Additionally, eukaryotic DNA is highly compacted, associated with proteins known as histones (most non-eukaryotic Archaea also utilize them, but, as eukaryotes, not only them). This, apart from the nucleus, the non-coding sequences, and several other proteins with variable action pathways (like transcription factors), provides another layer of regulation, visible also in the embryonic development of red algae, brown algae, and land plants, all independently evolved, but sharing regulatory mechanisms as a similarity among many distinctions. During embryogenesis (and for the rest of the life of the organism), all of these elements are highly coordinated not only between themselves but also with environmental stimuli, like chemical and mechanical signals originating, directly or indirectly, from neighboring cells for example. Unintended changes, even if small, can lead to anomalies. All these regulatory factors are known as epigenetic mechanisms, responsible for drastic variations without modification of the genetic material itself or, in other words, without modifying the nucleotide sequences. It is this that allows for the cells in a multicellular organism like Titanichthys to be clones and still function in amazingly contrasting ways: their difference resides in how their genetic material is processed. Also as amazing, epigenetic modifications can, under some circumstances, be passed down through generations, constituting another evolutionary drive beside the random mutations traditionally implicated in natural selection, being, as a matter of fact, a drive not as random as the mutations, since such modifications are, as said before, dependent, for instance, on external stimuli, with a few thus having the potential to function as powerful adaptative tools.

Meanwhile, their mother continues reasonably strong, even though the gestation is surely taking a significant toll on her capabilities. An example is the speed of her swimming, which has decreased slowly but constantly. This does not go unnoticed by predators. Nevertheless, most still keep a reasonable distance, intimidated by her large size. Such dimensions and muscular body allow for quite a powerful tail swipe and she, apart from the bony plates covering her head, has very thick skin, like the extant whale shark (Rhincodon typus), which follows a very similar, filter-feeding lifestyle. This thick and rubberlike covering provides extra material for predators to chew on without severely damaging the more valuable areas of the fish. Eventually, the young are about to be delivered. Accordingly, the female starts the birthing process, staying quite close to the surface in a very shallow part of the sea, an ideal habitat for a newborn Titanichthys, where, hopefully, it will be safe from most menaces.  However, as indicated by a shoal of young T. agassizi (a species with which T. clarki might actually be synonymous) that suddenly switched its direction, this is not the case.

Several carnivores begin appearing. The first is a trio of Cladoselache, shark-like cartilaginous fishes around 1.5 meters in length (though ones with larger dimensions do exist), which despite the resemblance, are likely more closely related to the chimaeras of current times than to sharks. They approach cautiously, waiting for the incoming baby. Adults such as them are more accustomed to prey nearing this size, being no strangers to cannibalizing younger individuals of their own genus, with such younglings generally going after way smaller victims, conodonts being one of those food sources. A much bigger animal, able to grow almost as large as T. clarki, the infamous arthrodire placoderm Dunkleosteus does not need to wait, as it can attack the mother herself thanks to its large, impressive teeth: bony plates that act as if they were guillotines, accompanied by powerful musculature that confers it an incredibly strong bite, capable of penetrating even the Titanichthys' armored segments. Another creature very similar to the latter is Gorgonichthys, also an arthrodire and equipped with the same shearing teeth. It, however, like the members of its family (Selenosteidae), is more lightly built than the very robust Dunkleosteus, being an active, fast swimmer.

Note that teeth, despite their location on the mouth of those that possess them, probably developed in ancestral gnathostomes (jawed vertebrates) from hard skeletal units composed of dentine (a major constituent of teeth) present all over their bodies, with these then differentiating themselves on the basis of those covering the oral cavity and those covering the external surfaces. Even to this day, cartilaginous fishes have a covering of tooth-like scales also composed of dentine, showcasing this ancient connection. Placoderms differentiate themselves by having less integration between tooth and jaw development when compared to other jawed fishes, in which both processes are more intimately associated. 

Returning to the tense scene, one baby has been rapidly expelled from the mother’s body. The Cladoselache head for it, but are temporarily scared off by the Gorgonichthys, which seems to have taken an interest in the small chondrichthyans, pursuing them away. This is enough of a distraction for the infant to escape, which will wander on its own for a while, later seeking shoals of T. agassizi, with which it will mingle, until it becomes large enough to fend for itself. On the other hand, neither its sibling nor its mother will be as fortunate. The Dunkleosteus goes directly for the Titanichthys, biting her underside and tearing a very big chunk of flesh. Blood stains the water red and the other predators soon join the feeding frenzy. After just one hour, the body is completely dilacerated and mangled. With most of the larger hunters going away, less conspicuous scavengers are beginning to take their share. Overall, this colossal carcass, this death, will nourish innumerable lifeforms, both macroscopic and microscopic, for a considerable amount of time, until none of it is left anymore, its components recycled and finding themselves on other beings.

Here is where the second tale ends. In terms of geological time, it would not be long until the end of the Devonian and, with it, the extinction of placoderms, victims of a final pulse of the already drawn-out event. But would they really cease to exist? Well, in a certain way, not quite. Placoderms are still very much alive and, dear reader, both of us are placoderms ourselves! This deserves some elaboration, so let me explain. In phylogenetics, the study of the evolutionary relationships between lifeforms, there is something called a monophyletic group, which consists of a particular most recent common ancestor and all its descendants, with this configuration being the most objective and trustworthy way of assembling organisms. In order to turn Placodermi into a monophyletic group, both cartilaginous and bony fishes (which are still alive and thriving in the present) immediately get turned into placoderms, since some animals normally referred to as placoderms actually shared a more recent common ancestor with the latter two than with other, "normal" placoderms. To illustrate the last point, the genera Entelognathus and Qilinyu were transitional forms between "normal" placoderms and bony fishes for example. One may still be asking how that makes us truly placoderms and the answer is simple: since we are bony fishes and bony fishes are placoderms, we are placoderms. The reason for us being bony fishes is also due to monophyly and the procedure of including all the descendants of the most recent common ancestor (and the most recent common ancestor as well for that matter) in one big batch. With this being revealed, the only thing left is for you to embrace your inner fish!

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1-Corals

2-Cladoselache 

3-Stromatolites

4-Bacterial mats

5-Conodonts

6-Ammonoids

7-Gorgonichthys 

8-Dunkleosteus 

9-Titanichthys agassizi

10-Titanichthys clarki