Middle-Devonian Life III: Vertebrates

Go back here to read about Mid-Devonian Invertebrates

Written by T.K. Sivgin

While many invertebrate groups were still growing to impressive sizes and diversity, there is a good reason why the Devonian is called the Age of Fish.

Jawless Fish

Jawless fish, often grouped together into the paraphyletic Agnatha, are quite rare today, only represented by the Cyclostomata, to which lampreys and hag fish belong. In the Middle Devonian this type of organism was however a lot more common and widespread and they would only be reduced towards the transition to the Late Devonian.

One very successful group of agnathans were the conodonts, which were actually more derived and closely related to us than the cyclostomatans. Conodonts are famous for their teeth being reliable trace-fossils from the Cambrian until the Late Triassic, so they of course would have lived during the Givetian as well. Conodonts were eel-like animals with a ridiculously complex mouth-apparatus. Instead of a jaw, their mouth was made up of dozens of saw-like tooth-blades that all articulated with each other to form an organ that functioned like a blender. What exactly they used this for is debatable, especially given the longevity and diversity of the group. Some conodonts likely used their blades to filter plankton out of the water, others were probably scavengers like the modern hagfish, while some may have been parasitically clinging to larger animals to feed on them like modern cookie-cutter sharks.

Out of the armored agnathans, the Astraspida and Arandaspida went already extinct in the Ordovician. Out of these arose however the Heterostraci, who had their heyday in the Silurian and early Devonian and only went extinct at the end of the period. Most likely to be found during this time would have been the Pteraspidiformes and Psammosteiformes. Pteraspids (top) had an armored front-half of the body, sometimes with a protrusion that looks like a long horn. The fan-shaped tail was only lightly armored and covered in scales similar to later fish, unlike the chainmail-armored tails of earlier Heterostraci. Psammosteiformes, like Drepanaspis from the Hunsrückschiefer, had the same arrangement of armor-plates, but these were fused into a single carapace and pressed into flat shapes. Both groups had no fins whatsover and likely lived close to the seafloor, living off detritus.

Anaspida were already extinct by the Early Devonian, though their close relatives, the Thelodonti, seem to have survived until the Late Devonian. These are named for their abundantly found scales, which were composed of bone and dentine, but lacked enamel. Full-bodied specimens are rare, but what they show are animals that were already close to the "fish-shape", as we imagine it, with paired pectoral fins, and the beginnings of dorsal and anal fins, as well as a heterocercal tail. At least some like Furcacauda above could evolve into some quite aberrant forms however. Unlike the heterostraci, most thelodonts were likely active swimmers, though what they ate is debatable.

The most complex among the jawless fish were the Osteostraci, especially their Devonian members. These were the first fish to evolve paired pectoral fins and their heads were surrounded by a massive head-shield that could assume many forms, from torpedo- to UFO-shaped. Close relatives of these were the Galeaspida and Pituriaspida, whose head-shields were bizarre, ranging from side-wings to flattened sword-noses. Conspicuous is an opening at the front top of the head-shields, which is likely where the nose sat. The mouth was at the bottom of the skull, meaning that these animals also probably sucked on detritus and algae

Placoderms

Placoderms were the first jawed fish and if transitional fossils such as Entelognathus are to be believed, also directly gave rise to all other gnathostomatans, including us. This either makes the placoderms a paraphyletic group or us placoderms. 

The most numerous group of placoderms were the Arthrodira. These are named after the big neck-gap in their armor which gave their skull greater freedom. Arthrodirans came in many shapes and sizes and occupied most of the same roles we expect today from modern fish. There were detritus-feeding bottom-dwellers, small reef-inhabitants, active predators of many sizes, including absolute apex-predators such as the famous Dunkleosteus and even giant, whale-shark-like planktivores like Titanichthys (though note both of the latter ones come from the Late Devonian, so would not have been indexed for the Rhynia). In their dominance, arthrodirans were to all other fish what dinosaurs were to mammals, in that their complete extinction at the end of the period is what allowed the great diversification of ray-finned fish and sharks in the first place.

The second-most successful group of placoderms were the Antiarchi. These were more strongly armored than the Arthrodira, to the point that, unlike all other vertebrates, even their pectoral fins became enveloped in armor-plates to form jointed limbs similar to those of arthropods. They are thus maybe best described as fish trying to be crustaceans. Living during the Middle Devonian would have been the quite well-known genus Bothriolepis, whose jointed arms were long and scimitar-shaped. What these limbs were used for is not known, though the most common assumption is that they helped lift the heavy animal off the seafloor, perhaps even to walk across the ground. In addition to this, genera like Bothriolepis and Pterichthyodes also possessed a pair of sac-shaped organs extending from their gut. This has led some to hypothesize that these might have been a form of early lungs and that, with their strong arms, these placoderms may have been able to crawl onto land and breathe there like lungfish, a few millions of years ahead of tetrapodomorphs. Due to the ambiguity of the evidence this hypothesis never gained much attention and was eventually forgotten. The many terrestrial antiarchs found on the Rhynia have brought it back into discussion however, to say it lightly.

Another, less well-known order of the placoderms were the Phyllolepida. These were somewhat catfish-shaped animals with large, flattened armor-plates that lived close to the seafloor similar to stargazer fish, however, unlike these, they had either very small eyes or were completely blind.

Another order were the Ptyctodontida, who had heavily reduced armor, large heads, large eyes and a long, rat-like tail, which all combined to give them a form very similar to the modern cartilaginous fish Chimaera, though these similarities are convergent. These are the only placoderms from which sexual dimorphism is known, with only the males having claspers. From the famous ptyctodontid Materpiscis attenboroughi we know that at least this group practised internal fertilisation and gave birth to live young. It is assumed that this probably applied to many other placoderms.

The order Rhenanida is similar to the Phyllolepida in their flat shape and bottom-dwelling lifestyle, though they were the sister-group to the Antiarchi and their armor was not made up of large plates, but instead a mosaic of many smaller plates and tubercles. More importantly, their fins were flat and expanded to such a degree that they looked remarkably similar to modern rays. Very similar orders of placoderms were the Petalichthyida.

Cartilaginous Fish and Acanthodii

Acanthodii like the one below are a group of early jawed fish that lived from the Silurian until the Permian. They are considered a paraphyletic grade that led up to the cartilaginous fish as we know them, though in some respects they also resembled bony fish, especially with their ossified pectoral fins. Three of their four main orders would have lived during this time and they came in both marine and freshwater forms. 

Chondrichthyans are today best known from sharks, rays, sawfish and ratfish. Like placoderms, their skeleton is mostly made up of cartilage, but because they lack the armor-plates this means that full-bodied fossils of them are quite rare and instead most of what we have are teeth and their dentine scales. What we can say is that by the Devonian the group had already split up into Holocephali, to which ratfish like Chimaera belong, and Elasmobranchii, which includes all other cartilaginous fish. From the latter group there probably were early members of Symmoriida, Xenacanthiformes or Hybodontiformes around, but none of those groups was of particular note or prominence in this time. The first successful chondrichthyan taxon, Cladoselache, would not appear until the latest Devonian. While early chondrichthyans such as Cladoselache, Hybodus and even some of the acanthodians are often referred to as sharks, it should be noted that actual sharks of the clade Selachimorpha would not appear until the Triassic (along with the clade Batoidea which includes rays and sawfish).

Ray-finned Fish

Despite today being the most numerous vertebrate-group on Earth, the Actinopterygii were also not of great importance in the Devonian, being largely marginalized by the placoderms and lobe-finned fish. Paleonisciformes such as Moythomasia above would have been the only common types, along with early relatives of bichirs and sturgeons and that was pretty much it.

Lobe-finned Fish

Sarcopterygii were perhaps the second-most successful Devonian fish-group after the placoderms. In the Early Devonian they had already split up into the two main groups. On one side you have Actinistia, who are better known as coelacanths. These looked broadly similar to the modern Latimeria, though their diversity in shapes and sizes was far greater. On the other side you have Rhipidistia, which consists of lungfish and tetrapodomorphs. The former would again not have looked too dissimilar from modern members. 

The latter however is more difficult to assess. The direct record of fossil bones shows us that all tetrapodomorphs during the Givetian would have still been rather fishy and almost purely aquatic, mainly being comprised of the evolutionary grade Osteolepiformes, consisting at the time of the families Osteolepidae and Tristichopteridae. Prominent members of the latter group would have been famous Eusthenopteron above and large Eusthenodon. While commonly depicted as amphibious, Eusthenopteron was a fish living in the open ocean and likely did not crawl onto land. Rhizodontida was also alive during this time. During the Carboniferous these had evolved giant members such as 7 meter long Rhizodus, which were true swamp monsters, but during the Givetian there would have only been smaller members like Sauripterus. 

More highly derived tetrapodomorphs, the elpistostegalians, such as Tiktaalik, Elpistostege or Panderichthys, and true tetrapods like Ichthyostega are not found from the Middle Devonian and only appear from the Frasnian onward. The closest we get are tristichopterids like Platycephalichthys above. At least that is what the body fossils tell us, though multiple controversial fossil tracks exist from the Middle Devonian as early as the Eifelian which seem to have been made by derived tetrapodomorphs on land and maybe even true tetrapods. The species which made these have yet to be found from these strata however and the possibility remains that they were perhaps made by other types of walking fish with convergent similarities to tetrapods. If these phantom-tetrapods truly existed in the Middle Devonian, they do not seem to have made it onto the Rhynia or at least did not leave a big impact. All tetrapodomorphs on the station seem to have directly descended from rhizodonts or osteolepiforms.