Go back here to read about Mid-Devonian Plant Life
Written by T.K. Sivgin
The Devonian is commonly referred to as the age of fish, but while not as successful as in the preceding periods, many of the invertebrate groups were still at top of their game and represented by forms entirely extinct today. We unfortunately cannot account for every group, as animals such as annelid worms are mostly or entirely soft-bodied and thus only left behind a very spotty fossil record.
Sponges
Sponges are the first major branch on the tree of animal life to evolve and it should come as no surprise that the main classes of sponges, Demospongaea, Calcarea and Hexactinellida, were already established by now, though it gets difficult to say for Homoscleromorpha. Freshwater sponges did not exist yet, as they would curiously not evolve until the Miocene.
A unique group that died out at the end of the Devonian were the stromatopores. These were sponges of still mysterious affinity that often grew into large mat-colonies that could be up to two meters thick and were very common reef-builders during this period. Their internal anatomy was made up of horizontal layers called laminae and vertical canals called pilae, which likely acted as filtering chambers. Between these elements the stromatopores built hollow chambers. The whole construct was held up by a calcareous skeleton that may have been made of aragonite or very magnesium-rich calcite. Some stromatopores may have lived in endosymbiosis with algae.
Ctenophora
Comb jellies as they are called were often grouped together with the true jellyfish and other cnidarians as Coelenterata, but most work has shown that ctenophores were a far older animal group than the cnidarians and bilaterians (in some genetic studies even resolving as being older than sponges). Ctenophores only outwardly resemble jellyfish. For example, they have no stinging cells (some living forms on Earth have actually been observed stealing the stinging cells of jellyfish they've eaten to use them for their own hunts), but are in turn the largest lifeforms that still use cilia to swim. While most are radially symmetrical, some have a pseudo-bilateral (often called biradial) symmetry and even a proto-anus. Thanks to fossils such as Archaeocydippida hunsrueckiana and Paleoctenophora brasseli, both from the German Hunsrückschiefer, we know that comb jellies of essentially modern type and the probable crown-group had already evolved by the Early Devonian.
In the Cambrian there existed a special group of comb jellies, which lacked tentacles but unlike all later forms actually built internal skeletons. Among this group, the Scleroctenophora, existed forms such as Thaumactena ensis, which had long, streamlined bodies and may have moved similarly to modern arrow worms. These bony comb shellies are thought to have already gone extinct by the end of the Cambrian (we cannot say for sure because comb jellies rarely leave fossils behind), but have become relevant for us again, as a bioluminescent worm-like animal has been recently captured in the lower decks that eerily resembles Thaumactena.
Cnidarians
The two main orders of corals during this time were the Tabulata and Rugosa, which went completely extinct either during or shortly after the End-Permian mass extinction. Our modern stone corals do not descend from either group but independently evolved after the extinction out of the anemones.
Tabulate corals built their septae out of calcite and were exclusively colonial, always growing in hexagonal shapes that resemble honeycombs in the fossils.
Rugose corals were also calcitic but they came both in solitary and colonial forms. The nickname horn-corals comes from members such as Caninia, which grew in curved cone-shapes. Some of these horn-corals could grow up to a meter in length. Some rugose corals also lived in endosymbiosis with stromatopores, joining their matrixes together to form one large reef.
Bryozoans and Brachiopods
As this ancient document shows, brachiopods were pretty rad and back in the Devonian they were at their raddest, being probably the most important reef-builders. All main orders, the Inarticulata, Strophomenida, Pentamerida, Rhynchonellida and Terebratulida, were already present but the most dominant ones were the Spiriferida, who looked like your regular articulate brachiopods, but had ridiculously coiled internal arm-skeletons.
Close relatives of the brachiopods were the phoronids and bryozoans. While the "shell" of the phoronids is made of a chitinous cuticle and they are thus not often preserved as fossils, bryozoans live a lot like corals, growing exclusively together as zooid-colonies that build up mineralized shells/skeletons and we have thus a good idea of their history. All of their four main orders had already evolved by this time.
Apart from these three, another group of lophophorates still alive during this time were the hyoliths, which were strange, cone-shaped brachiopod-like animals that were able to elevate themselves off the ground using curved, mobile stilts.
Molluscs
Molluscs had widely radiated by this point, with most classes being present in some way, except for the Helcionelloida, which went extinct in the Ordovician, and maybe the Aplacophora, which are worm-like forms without skeletal elements and so usually do not leave fossils behind.
The first Polyplacophorans started appearing the in the Devonian. These were slug-like animals, such as the modern chiton, whose backs were covered by multiple shell-plates similar to a trilobite. What makes these animals interesting is that these plates have primitive mineral-eyes spread across them, which some tests have confirmed all work together like a compound-eye to form an image. Fossils which have been identified as Monoplacophorans have been described from the Devonian, though true crown-group members are only known from as young as the Pleistocene. Monoplacophorans, as the name suggests, are similar to Polyplacophorans, except that they only have one shell-element. A group alive during this time that is now extinct were the Rostroconchia. Theses looked a lot like bivalves, but were unable to articulate their two-shell halves, as these were actually fused into a single shell at their hinge and so stayed semi-open permanently. These probably lived a burrowing lifestyle, using a muscular foot. Despite their looks, it is thought that the rostroconchs were most closely related and maybe even gave rise to the scaphopods, of which stem-group members are also known from the Devonian. Scaphopods are also named tusk-shells, because their shells are hollow tubes (which unlike the shells of cephalopods are open at each end) that outwardly resemble the tusk of an elephant. To this day these animals live by vertically burrowing themselves into the ground to feed on scum-matter in the soil, while using the upper end of their shell like a breating-tube.
The Devonian was the period where bivalves (clams, oysters, scallops and such animals) essentially started looking and acting like they do today, with siphons, filter-feeding gills and a burrowing lifestyle. They were however quite low in abundance and did not contribute significantly to reef-communities, as they faced stark competition from the way more successful brachiopods whom they superficially resembled.
Gastropods, better known as snails, fared much better and were represented by numerous, though still poorly understood groups, such as the genus Bellerophon above, whose shell was not coiled to the left or right but bilaterally symmetrical to the back. However, all of these gastropods would have been solely marine. The first fossils of true land snails only start appearing in the Carboniferous.
Cephalopods in the Devonian were still partially represented by snail-like creatures such as the Discosorida and Onocerida. Among them, the nautiloids were still at the peak of their diversity as they were in the Ordovician and Silurian, only going to decline in the Late Devonian. Found among them can be ones with loosely coiled shells such as the Rutoceratidae (seen above) and Tetragonoceratidae with tightly coiled shells resembling the modern Nautilus. Also still very common were the Orthocerida. The lifestyles of these animals are still debated. While sometimes represented as Kraken-like macropredators, most were likely simple filter-feeders, with thin, though numerous, spaghetti-like arms.
Likely evolving out of the Orthocerida were the Bactritida, which also first start appearing in the Early Devonian. The Bactritida were in turn likely the common ancestors of both the Ammonoids and the Coeloids. The ammonoid-orders present during this time would have been the Agoniatitida (seen above) and Goniatitida. The first clear members of Coeloidea (Octopodes, squid, cuttlefish and belemnites) are only recorded from the Carboniferous, though some possible (meaning highly disputed) stem-group members are known from the Devonian.
Arthropods and relatives
One of the oddest finds from the Hunsrück slate is the discovery of Schinderhannes bartelsi, a 10 cm long animal that was a hurdiid radiodont, a group also referred to as Anomalocarids. While Ordovician members such as Aegirocassis were already known, the fact that a miniature version of the famous Anomalocaris from the Burgess Shale survived all the way to the Early Devonian still came as a surprise. That these animals apparently survived into the Middle Devonian is evidenced by them being on the Rhynia and in Dome 2 even taking over the waters in lieu of fish. Other stem-arthropods were the "lobopodians", of which the modern tardigrades and velvet worms could be seen as surviving members. The fossil record of these animals, especially in the later Paleozoic, is very spotty though, so we cannot comment much on them. It is possible that velvet worms may have started colonizing the land during this time, though no fossils of them are known from the Devonian specifically.
Among the true arthropods we find the subphylum of the trilobites, still existing in their full glory and represented by some key-orders, the Harpetida, Proetida, Phacopida, Lichida and Corynexochida. Only the Late Devonian mass extinction would decimate this group down to the single order Proetida. Planktonic and swimming trilobites had already vanished in the End-Ordovician mass extinction but the Devonian forms were still diverse in body-forms and spiny defences. Some also had some quite experimental ways of feeding on detritus, such as the Harpetida, which had a perforated head-shield that acted as a sand-sieve.
Next up are the chelicerata, the group that today includes horseshoe crabs and the various arachnids. Despite their dubious status as "living fossils", the horseshoe crab is not found during the time, with the order Xiphosurida only appearing in the Late Devonian and the family Limulidae in the Middle Triassic. Some stem-group members likely were present however. Various familiar arachnid groups did live during this time and were among the first animals to colonize the land, such as scorpions, mites and ticks. With the Givetian Attercopus fimbriunguis there even was a stem-spider which seems to have possessed silk-producing spigots. Unlike their Carboniferous descendants most of these animals were still very small. The most famous Paleozoic chelicerates, the eurypterids, were still doing quite well, especially in the Early Devonian where we find over 2.5 meter long predators such as the Jaekelopterus seen above. The Middle Devonian was nonetheless a pivotal turning point, where their ecological role as marine top-predators was gradually being challenged by various new jawed fish. The true final blow to their dominance would however not come until the Late Devonian mass extinction.
Among the Mandibulata the most common groups would have been the crustaceans and myriapoda. Of the crustaceans almost all orders were already present, but their numbers and forms were mostly limited to Triops-like forms. Familiar animals such as shrimps, crabs and lobsters would not appear until the Mesozoic. Myriapods are animals such as the millipedes and centipedes and these creatures were already crawling about on land as far back as the Silurian, in form of Pneumodesmus above. In addition to possibly being the oldest land-dwellers, forms such as millipedes could also be called the oldest terrestrial herbivores, as they fed on the detritus of various of the earliest land plants. Like with the arachnids, myriapods were still small during this time, with giants such as Arthropleura only appearing in the Carboniferous.
The perhaps most alien aspect about the Devonian was the general lack of insects, at least as we know them. Wingless insects such as springtails would have already existed, yes, but the oldest winged insect fossils so far only come from the Carboniferous. The genus Rhyniognatha from the Early Devonian is sometimes theorized to have been the first winged insect, but it is only known from a head and the presence of wings is only inferred from shaky phylogenetic bracketing. So far no animal resembling a winged insect has been found on the Rhynia, instead most niches we usually attribute to insects are occupied by various other arthropods and sometimes even vertebrates.
Echinoderms
Many groups of familiar echinoderms can be encountered during this time, such as starfish (Asteroidea), brittle stars (Ophiuroidea), sea urchins (Echinoidea) and sea cucumbers (Holothuroidea), though not as numerous or diverse as they are today. Echinoidea for example would have lacked sand dollars and heart urchins, as these would only appear in the Mesozoic or later.
Truly dominating during this time however were the crinoids, also called sea lilies, who were the largest reef-builders together with the brachiopods. These were joined by a now extinct group called Blastoidea, who looked similar to crinoids but were built differently.
Speaking of extinct groups, there was one which deserves special mention, due to its weirdness and relevance. Found in Paleozoic strata from the Cambrian until the Carboniferous is the group Homalozoa, sometimes also referred to as carpoids. A sub-group of these surviving at least into the Early Devonian were the Soluta, such as Syringocrinus above, who had a single small arm on the front next to the "presumed" mouth and a large fish-like "tail" growing out of the calyx. How these lived and if they could indeed swim a bit, like the tail suggests, is still debated.
Another sub-group of Homalozoa were the Stylophora, once also referred to as Calcichordata. This group can be further divided into Mitrata and Cornuta. Mitrata, such as the one above, were similar to solutes, but were approaching a nearly bilateral symmetry and also had what appears to be pharyngeal slits, unlike all other echinoderms but like hemichordates and early chordates. This has led some to hypothesize that the chordates actually directly descend from echinoderms such as this, but this is generally not accepted. Trace fossils of these animals show that what looks like a tail was actually used as an arm to drag the animal forward. If the hole in the center was a mouth, an anus or both is also not known.
In complete contrast, the cornutan stylophorans, such as Cothurnocystis above, completely lacked any sense of symmetry. The highly asymmetrical, roughly crescent-shaped body lied flat on the sea-floor like a sand dollar and what at first looks like a tail later turned out to be a starfish-like arm that the animal used to drag itself across the ground or bring food to the mouth right at the arm's base.
Various other extinct echinoderm groups existed during this time, such as the blastoid-like Rhombifera, the starfish-like Somasteroidea, and the sea-urchin-like Cyclocystoidea and Edriosateroidea, but these were thankfully not as strange.
Hemichordates and such
Closest among the invertebrates to us are basal chordates, such as lanceletes and tunicates, which have a notochord at some point in their lives but lack the vertebrae which give us our name. These therefore leave little fossils behind, so we cannot say much except that they werre present during this time.
Next closest might be the Hemichordates, though most work suggests that despite their name they are actually more closely related to the echinoderms. Modern forms of these animals are creatures such as the acorn worm and pterobranchs. The latter is closely related to the extinct graptolites. Graptolites essentially lived like large versions of bryozoans, being worm-like, tentacled creatures that grew together into coral-like colonies. What made them unique however (and cosmopolitan index fossils) is their floating lifestyle. Graptolite colonies lived by either attaching themselves to floating driftwood or some even had specially evolved buoyancy organs. This way they could float freely around the whole ocean surface and leave their fossils behind in every marine environment. By the Middle Devonian the graptolites had become greatly reduced in number, only being represented by bush-like form such as Dictyonema above. Their decline may be due to them being easy targets for early fish.
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