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A whale dies.
Her great teal-skinned carcass plummets to the depths of the ocean.
She strikes ground three thousand meters below the water surface. No sunlight ventures here, but life continues to hum along (albeit with somewhat reduced vigor) in this pitch-black benthic universe, where water pressures can run up to an astonishing 300 atmospheres (that is, 300 times the pressure exerted by the overlying column of air at sea level) and photosynthesis is impossible. Adult whales attain weights that range between 30 to 160 tonnes and, upon their death and fall, represent a significant input of biomass for the carbon-starved lower reaches of the ocean. In fact, the dead and decaying body of a whale can sustain a mini-ecosystem of opportunistic deep-sea scavengers over a period of decades. Within a month, the body will be swarmed over by hundreds of slimy, superficially eel-like scavengers called hagfish. The hagfish has no jaws and is instead equipped with a protrusible “tongue-like” structure that bears “teeth” (or, really, two serrated tooth-plates that are designed to grasp and draw meat into the mouth). The animal forces its head against the flesh, ties itself into a knot against the skin of the dead whale and proceeds to rasp away at the muscle tissue. Often times it only eats away as much muscle as is necessary to bore a hole into the body cavity – where it can then feast on the soft internal organs. Some hagfish seek the easy route to the viscera by entering the mouth or anus.
One of the most chilling scenes in Attenborough’s masterful documentary on the Earth’s Oceans, the Blue Planet, is deep-sea footage of wormy hagfish, “thick as an arm”, chowing down on the pale remains of a whale.
Well, that’s a pleasant image – but what do hagfish have to do with vertebrate evolution?
Hagfish are unusual creatures. For starters, they aren’t really vertebrates, let alone fish – they aren’t endowed with a vertebral column. They lack jaws. They have multiple hearts. They can absorb organic matter through their skins, not unlike several lower invertebrates. They bear two simple eye spots which can detect differences in light levels but cannot, as far as we know, construct detailed images. A lot of the vertebrate innovations we take for granted are nowhere to be found in this curious organism.They do, however, posses a skull made of cartilage – and this property grants them a place within the clade Craniata, which we shall define and discuss shortly. Hagfish are one of two possible remnants of a very ancient group of so-called “jawless fish” (the latter word being used loosely in this context) that first made its appearance some 400 million years ago. Jawless fish were probably the first backboned animals to evolve cellular bone, paired fins and complex sensory systems to detect sound waves and monitor pitch, yaw and roll. They therefore represent a crucial evolutionary stage in the history of vertebrate life.
Vertebrates? Jawless fish? Craniata? What’s all this about?
It can be difficult keeping the various taxonomic hurdles that separate tunicates (sessile filter-feeding animals we discussed in a previous post) from vertebrates in mind as we move forward. But there is a logic to the madness. Let’s play God and try to construct a vertebrate from scratch.
Let’s start with a small eel-shaped organism with a series of paired muscle blocks running down the length of the body.
We need a bundle of nerve cells (and a collection of attendant supporting cells) to run along the back to carry electrochemical signals to and from each of these muscle blocks – let this be the nerve cord. We also need a rigid supporting rod to run along the back for muscle attachment and to prevent the body from telescoping when the muscle blocks contract. However, it should not be so rigid as to prohibit the body from twisting or curving at all (throwing the body into a series of lateral undulations or curves is precisely how the muscle blocks achieve propulsion). Cartilage performs this task adequately. We have just constructed the Notochord – a rod of cartilaginous tissue that runs below the nerve cord.
Since we’ve established that the earliest vertebrates were jawless creatures, our organism will have to be dependent on some kind of suction filter-feeding. We can imagine food and water being drawn into a collection sac. We shall call this the pharynx. The water can be expelled through ‘gill slits’ along the sides of the pharynx – while the food is conveyed to the gut.
In fish, it should be noted, the gill slits are richly provided with blood vessels which can extract oxygen from the departing water. The gills are also supported by a series of catilaginous gill arches.
Pretty much any organism that displays these characteristics – nerve cord, notochord, pharynx, gill slits- at some point in its life cycle (humans display these during embryonic development, many tunicates in the larval stage and lanclets in the adult stage) may be identified as a chordate. So we’re past the first hurdle. Pikaia (530 mya) is the among the earliest examples of a primitive chordate.
So we’re done with chordates, but what are craniates?
Practically no invertebrate head displays the degree of functional and structural complexity seen in the vertebrate head. Although a “front end” can certainly be identified in lancelets and tunicate larvae, there is no “head”, so to speak. The notochord extends right up to where we’d expect the head to be. The creation of the head, then, is the next step we ought to take. The anterior (or front) end of the nerve cord must be expanded into a complex brain which can wield centralized command over the nervous system. New sensory apparati and cranial nerves must be built to detect oncoming sources of food or possible predators. The earliest traces of this process can be seen in Pikaia. At some point, a cartilaginous brain case became necessary to house and protect this new equipment. We have now constructed a “craniate” – a chordate with a highly specialized head. This group contains both true vertebrates and hagfishes.
This transition was made possible by the duplication of clusters of “Hox genes” over evolutionary time by mutations. The end result of this was the creation of a special zone of tissue above the nerve cord in the embryo, the neural crest, from which cells migrate away during development to create various features important to the form and function of the brain. The origins of the neural crest – which is also associated with other “typically vertebrate” tissues like bone – is an area of active research.
And what are vertebrates?
A series of cartilaginous arches called neural arches running along the length of the back evolved to provide for further muscle attachment and protection of the nerve cord. A complementary series of inverted arches called centra was added to this design. A neural arch plus a centrum constitutes a vertebra. The vertebral column consists of a series of articulating vertebrae. In vertebrates, this segmented backbone replaces the notochord during embryonic development.
We’ve been speaking of cartilage thus far, but what of bone?
The next great evolutionary leap in this story is the appearance of a hard mineralized tissue called bone. Many complicated explanations for the emergence of this hugely important verebrate characteristic have been formulated. It is worth remembering that many invertebrate groups have evolved hard mineralized tissues – consider, for example, the armor of crustaceans or the calcium carbonate shells of mollusks. The formation of bone involves the secretion of a matrix of long chain sugars and fibrous protein by certain cells (called Osteoblasts) and the consequent deposition and crystallization of calcium phosphate in this matrix, hardening it. The general pattern of this process is roughly mirrored in invertebrates that build exoskeletons and shells, but with different cells, matrix components and minerals. In a sense, vertebrates have two skeletons: an outer dermal (exo)skeleton and an endoskeleton constructed from either cartilage or bone. Of the former, little remains in humans outside of the skull. Dermal bones are formed within the skin and are derived from the dermis, a skin layer beneath the epidermis.
The earliest bones were dermal bones. A cartilaginous endoskeleton was probably already in place by the time bone made a real appearance in craniate anatomy. Cartilage does not usually make it into the fossil record unless it is particularly dense and thus our knowledge of the skeleton that supported the tail and trunk of the earliest fishes is wanting. The dermal bones we will deal with in the next section, on the other hand, are far better preserved and shaped out some of the most extraordinary heads in all of vertebrate history.
The endoskeleton in most extant vertebrates is composed of non-dermal bone derived from cartilage. The beginnings of this type of bone, called endochondral bone, can also be seen in later jawless fish.
Even in primitive Jawless fish, bone comes in two flavors: cellular and acellular. These types are essentially the same in terms of matrix composition, but differ in that cellular bone has cell-spaces for bone forming cells. The dermal bones of these early fishes consists of a lower compact and closely layered base of acellular bone, a middle layer of spongy cellular bone and an overlying later of dentine projections.
So why did fish evolve bones?
1) Bone might have initially served as a storehouse for phosphate and calcium. Calcium plays a massive role in cell signalling and in maintaining osmotic pressure in the body. Phosphate plays a similarly major role in cell metabolism and the generation of energy. By maintaining steady levels of these ions, marine jawless fish might have been able to venture into brackish or fresh waters with lower calcium or phosphate levels without experiencing any physiological distress. They could make withdrawals from or deposits in their calcium phosphate “banks” as necessary.
2) Bone can serve as a protective casing for delicate sensory apparati or viscera. One rather picturesque suggestion from some workers in the field is that bone evolved in early fishes in response to predation from giant sea-scorpions called Eurypterids in the Ordovician seas. Early fish are often described as “armored” for good reason.
Other suggestions point to bones as being beneficial for swimming in some way and/or for the functioning/insulation of a set of electrosense receptors.
When and where did the first fish evolve?
The earliest fossil fish (which can incontrovertibly be identified as such) dates to about 430 million years ago – a period in the earth’s geologic history known as the Ordovician.
Astraspis was a jawless fish with two far-set eyes and a bony “head shield” made up of a number of solid plates. It is a little strange to think of a vertebrate with no movable bony mouth parts to speak of – jaws are, after all, central for the feeding behavior of most backboned animals, from sharks to elephants. The trunk of the body was covered in small overlapping scales. A large number of gill openings may be found on the sides of the head shield. This structure bears a few other holes – two for the eyes, one for the nostril and one for a third eye or pineal organ. The pineal organ was sensitive to light levels but used a mechanism of photoreception that was different from that of the other two eyes. The pineal organ is actually visible in some modern animals, including certain amphibians. The animal is roughly torpedo shaped, a design that limits drag.
It must be noted here that Hagfish lack bony parts and therefore represent a condition that is even more primitive than this.
These early fish – called ostracoderms – appear to have dwelt in warm, shallow seas on the margins of the continents (the supercontinent of Gondawanaland is sometimes posited as the cradle of ostracoderm evolution). Deeper sediments yield numerous invertebrate fossils, but no examples of ostracoderms – suggesting that they were incapable of swimming across stretches of open ocean. In terms of ecological niche, many ostracoderms appear to have been somewhat closer to the lowly tribolite than to modern carp or tuna. The position of the mouth on the underside of the body (as well as various details of the skull anatomy) seems to indicate that they were bottom-feeders, sucking up detritus and other organic matter and burrowing into the sediment. The true patricians of this era were eurypterid scorpions and giant squid-like nautiloids. BBC’s Walking with Monsters had a decent scene depicting an encounter between a school of Cephelaspis and a mighty eurypterid.
How diverse were Ostracoderms?
As in other vertebrates, the structure of the skull can be used to divvy up Jawless Fish (ostracoderms) into a number of taxonomic categories.
One group is the Heterostracans. Their head bones include one bony plate on the top, the bottom and on each set of gill slits. The water isn’t expelled through individual slits, but through two vents at the back of the skull. These were the first fishes to invade freshwater.
During the Ordovician, the earth experienced a massive glacial episode that locked up large amounts of water in ice-sheets, reducing global sea levels. New lanes of shallow water opened up between the continents – and the ostracoderms may have taken advantage of that to distribute themselves across the world’s oceans.
Among the heterostracans, which were at their height in the following Silurian period, we see numerous experiments with horn-like structures that may have helped stabilize the organism, anchor it to the sea bed or stir up food particles. They did not have paired fins behind their heads and were probably clumsy swimmers in comparison to most modern fish.
While the tail and posterior half of the body were covered in small scales, granting the body some measure of flexibility and the tail the ability to undulate and generate propulsion, the weight and dimensions of the head shield probably made stable directional swimming a difficult affair (think of how difficult it is to push a heavy cart from behind and keep it on course!). This would not have been a serious impediment anyway, given that many of these creatures probably spent most of their lives wriggling about on the sea floor.
Some jawless-fish, like Hemicyclaspsis, a member of the Osteostraci (yet another group of jawless fishes), were optimized for bottom feeding in terms of body shape. It had two closely-set eyes and a large roof of solid bone covering the head (with a fairly low slope from the rim of the head shield to the eyes). The flatter the organism, the lower the pressure differential between the top and bottom. This reduces lift – meaning that the organism has to expend less energy trying to stay close to the bed.
Hemicyclapsis also sports two paired fins and many complex bits of sensory hardware – a proto-nose, a proto-ear and the rudiments of a lateral-line system for detecting movement and vibration in the surrounding water. A line of sense receptors (probably chemosensory or electrosensory) is located on the head shield. Water was forced into the pharynx by the act of swimming – this allows for water to pass through the gills. This sort of respiration, called “Ram respiration”, means that the fish would asphyxiate if it ceased to move for a sufficient time period.
The structure of the brains of some jawless fish can be inferred from studying the thin outlines of cartilage-derived bone that enveloped many of the soft organs of the head. Reconstructions reveal a primitive brain similar to that of lampreys and hagfish. It can be partitioned into a fore, mid and hindbrain.
Not all jawless fish were bottom dwellers. Anaspsids, for example, were scaly streamlined jawless fish with paired fins that could feed on suspended food particles – possibly algae. The skull is not weighty – there is no head-shield – and anaspids may have been active swimmers. They may be ancestral to the modern lamprey – which is not covered in scales, but in skin. There are traces of a circular cartilage with surrounds the mouth – a condition similar to what is observed in lampreys. The lamprey and the hagfish are the only two surviving jawless fish.
Illustrations of some jawless fish resemble surrealist works of arts. Some of these fish have heads shaped like flying saucers, screwdrivers and vacuum cleaners. Many of these adaptations have no modern analogue and their precise functional significance may remain unknown forever. Some of them had rod like bony processes that could be used to suck up food particles. Others had huge dorsal crests or eyes set apart like headlights. The diversity of forms is massive and one gets the sense that we’re trying to reconstruct a lost opera from just a few notes. What is clear is that the jawless fish were successful in terms of both distribution and longevity – after all, they lasted into the geological epoch during which the first fish were making that giant leap onto land.
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