What might a Late Devonian floating log
community have looked like? 
Some of the types of organisms that might have been part of floating log habitats in the Late Devonian. Organisms with black text above the logs have been found with Devonian plant debris in marine black shales. Organisms identified with gray text (and circles) above the logs may have been associated, but fossils are not directly found with plant debris. Swimming organisms shown beneath the logs are organisms that lived in the Late Devonian seas at the same time that Callixylon logs and driftwood existed.
Most of the organisms found on floating logs and algae today did not exist in the Late Devonian seas, although ancient ancestors of these modern organisms did. With a little detective work and supposition, we can infer what types of creatures could have lived on or around a floating log in the Late Devonian. Direct evidence is provided by fossils of other organisms sometimes found on Callixylon logs. Indirect evidence is provided by fossils found in the Devonian black shales, or with other plant debris in the shales, but not directly with fossil logs. The types of organisms that could be preserved in Devonian shales, however, are biased. Fossils of organisms are more likely to be preserved when an organism has hard parts. Many of the plants and organisms found on modern fossil logs have only soft parts. Let’s look at what may have been possible in the Late Devonian.
Terrestrial organisms are sometimes attached to modern floating logs. Fungi, insects, even lizards, have been found on modern logs (Thiel and Gutow, 2005). Although insects and terrestrial vertebrates have not been found with Callixylon, some Callixylon logs preserve microscopic fungal filaments (called hyphae) (Arnold, 1931; Stubblefield and Taylor, 1988). Fossil fungi might be expected to be more common in logs which had partly rotted on land before being transported into the sea.
Marine algae are common on modern logs and driftwood. Algae has been around for a long time, and certainly was abundant in the Late Devonian seas. Microscopic algal spores, called Tasmanites, are found on Callixylon fossils and are common in the surrounding shales. They may have been deposited on the log after it sank, or while it was still floating, but it seems likely that algae would have grown on any log that was at sea for any length of time, just as in the present.
Diverse types of arthropods are common on modern floating logs and floating algae (Thiel and Gutow, 2005). Barnacles are probably the most common arthropod attached to floating wood today. Barnacles were not attached to Devonian Callixylon logs, because they did not exist in the Devonian. Smaller arthropods like amphipods and isopods are also common today, and had ancestors in the Devonian, but are rare fossils, and not specifically associated with Callixylon. Ostracods and shrimp are sometimes found swimming near floating logs and algae today (Thiel and Gutow, 2005). No fossil ostracods or other crustaceans have been found attached to a Callixylon log, but they are found in the Devonian shales (e.g., Barron and Ettensohn, 1981), and might have hovered around a floating log in the Late Devonian seas.
Several types of echinoderms (starfish, brittle stars, and crinoids) have been found on floating plant debris in modern oceans (Thiel and Gutow, 2005). Callixylon logs have been found with attached crinoids in Indiana and Ohio (Wells, 1939, 1941, 1947; McIntosh, 1978; Barron and Ettensohn, 1981). Crinoids are a type of echinoderm that looks like an underwater flower. They have a crown with arms attached to a stem that ends in a holdfast. Today crinoids are rare, but they were much more common in the past. Crinoids have been found directly attached to inferred floating logs in other younger geologic periods (Seilacher and Hauff, 2004; Hunter and others, 2020).
Modern floating logs sometimes host small bivalves (clams, oysters, etc.) and gastropods (snails). Both have been found on fossil logs in other deposits (Wignall and Simms, 1990), and both are known from the Late Devonian. To date, however, neither have been found on (or with) Callixylon logs. One particular bivalve that is common on floating wood (and man-made constructions made of wood at sea) today are “shipworms.” These creatures look like worms but are actually bivalves. They bore into floating wood (and ships, which is where they get their name). Some possible infilled borings have been found in Callixylon wood, but not with the abundance or shape of those commonly made by shipworms. Perhaps, since trees had just evolved in the Devonian, it took a little longer for boring types of bivalves to evolve that would take advantage of this particular kind of floating substrate.
Another type of seashell, brachiopods, may have attached to floating Devonian logs. Brachiopods are invertebrates with two shell valves, similar, but different than bivalves. Fossil inarticulate brachiopods (Lingula melie, Orbiculoidea herzeri) have been found attached to fossil plant debris in Late Devonian shales (Barron and Ettensohn, 1981; Pashin and Ettensohn, 1995), although not directly to Callixylon. Similarly, the articualte brachiopods Leiorhynchus and Barroisella have been found in black Middle Devonian shales in New York with plant debris, but not Callixylon. These brachiopods may have been hitchhikers on floating plants that fell off in deeper water (Thayer, 1974; Barron and Ettensohn, 1981; Pashin and Ettensohn, 1995). Lingulids are commonly found in shallow-water sediment today, and in shallow-water strata in the past. Lingulids are also found in the Devonian black shales, which are interpreted to have been deposited in deeper water than lingulids are found in today. Hence, it’s possible that some fossil lingulid brachiopods found in Devonian black shales may actually have attached as larvae or juveniles to floating logs, and hitch-hiked from shallower water to where they fell off the logs, sank, and were buried (Rudnick, 1965; Barron and Ettensohn, 1981). However, these associations are not always straight forward. Different lingulid species may have lived in shallow and deep water. Pasin and Ettensohn, (1995) inferred that while the species, Lingula melie, may have attached to floating plants, another species, Lingula meeki, is found in Late Devonian marine gray shales not attached to, or with fossil plant debris. It lived on the sea floor in apparently deeper water than Lingula today. Likewise, Leiorhynchus may have lived on the sea floor in deeper water, rather than being transported from floating logs (Thompson and Newton, 1987).
Today’s floating plant debris also host a diverse variety of soft-bodied organisms like hydroids, various worms, and infant and juvenile phases of other invertebrate organisms (Thiel and Gutow, 2005). Many of the marine worms and invertebrates found on floating plant debris today had ancient ancestors in the Devonian, but such organisms are not likely to be preserved as fossils.
An organism with mostly soft parts which is partly preserved in the Devonian shales are conodonts. Conodonts were small (1 to 15 in), eel-like organisms. Their bodies are rarely fossilized, but they had an assortment of mineralized (phosphate) teeth-like features (called conodont “elements”), which are commonly fossilized. Conodont elements are microscopic, but are preserved in Devonian black shales (e.g., Barron and Ettensohn, 1981). Perhaps, some shallow-water conodont species would have hovered around floating logs for protection, similar to tiny fish beneath floating logs today.
Fish commonly congregate around floating logs today. Because fish are free-swimming, however, they would not likely be fossilized with a log when it sank. Hence, we can’t tell for sure which fish may have congregated around Devonian floating logs. We can only tell which fish are found in the same shales as fossil logs are preserved.
The Devonian is sometimes called the “Age of Fishes” because a wide diversity of fish evolved and spread across the globe during this time. Devonian fish were different than the fish that live today. Small fish in the Late Devonian black shales of Kentucky and surrounding states included early ray-finned fish (actinopterygians, palaeoniscoids). One example is Kentuckia, which was 4 inches or less in length. Perhaps small fish like these would have schooled around a floating Callixylon log for the same reason small fish congregate beneath modern floating logs.
Today, a host of medium-sized fish and larger game fish are attracted to floating debris (e.g., Thiel and Gutow, 2005). The most common medium- to large-sized fish in Devonian seas were called arthrodires. Arthrodires had thick, bony-armored heads. Rather than separate teeth rooted in jaws, they had solid jaws with sharpened edges. There were many types and sizes. Most appear to have been predatory. Small-sized arthrodires from the Ohio Shale include Coccosteus, Gymnotrachelus and Paramylostoma. Medium-sized arthrodires include Diplognathus, Heintzichthys, Mylostoma, and Stenosteus. Larger arthrodires included Dunkleosteus, Gorgonichthyes, Holtdenius, and Titanichthyes (e.g., Carr and Jackson, 2008). A few pieces of fossil evidence provide direct information about arthrodire diets. A specimen of the arthrodire, Holdenius, was found near Cleveland, Ohio, which has shark fin spines stuck in its jaws, suggesting its last meal was a spine-finned shark (Hlavin, 1990). Holdenius is estimated to have been 10 feet long. Dunkleosteus terrelli was an even larger arthrodire, perhaps 15 to 20 ft in long, with a bite force similar to that of a modern Great White shark (Anderson and Westneat, 2009; Carr, 2010). Orodus shark teeth have been found near the bones of a Dunkleosteus in the Cleveland Shale Member of the Ohio Shale, suggesting the teeth may be the remains of a digested shark. We don’t know for sure if any of these arthrodires would have been attracted to a floating log, but if there were smaller fish around the log, larger arthrodires may have followed.
Sharks are also common around floating logs today (Thiel and Gutow, 2005). The oldest sharks evolved in the Devonian. Devonian sharks looked a little different from modern sharks but appear to have had similar lifestyles. Cladoselache and Ctenocanthus are the most common shark fossils in the Cleveland Shale (Carr and Jackson, 2008). Cladoselache was 2 to 5 ft long, and Ctenocanthus was as much as 6 ft in length. Shark’s skeletons are composed of cartilage, so are not commonly preserved as fossils, but rare soft-part fossils from Devonian black shales near Cleveland, Ohio, provide glimpses of the marine food web of the time. Cladoselache shark fossils have been preserved with the small paleoniscoid fish Kentuckia hlavini, conodont elements, and the crustacean Concavicaris in their stomachs. One specimen of Ctenocanthus contains the partial remains of the arthrodire, Stenosteus gaber in its gut (Williams, 1990). Both of these sharks are similar in size to the types of sharks which visit large floating logs today; perhaps they would have been attracted to smaller fish around a floating Callixylon log!
Text and images by Stephen Greb (unless otherwise noted)