A section from a Callixylon log in the Kentucky Geological Survey paleontological collection. View along long axis and short axis. Transverse, short axis view is expanded to show details of internal petrified log.
This month’s fossil is a plant fossil found in Devonian rocks, called Callixylon. It is part of one of the oldest fossil trees on Earth and represents some of the oldest petrified wood known. Callixylon also is part of a tree that formed some of the first widespread forests in the world, which changed our world forever.
Description. Callixylon is the fossil wood from logs, branches, and roots of a fossil tree called Archaeopteris. Whole logs may be tens of feet in length, and tens of inches, to several feet in diameter. Some Callixylon fossils are obviously fossil wood, preserving wood grain and shape. Many, however, do not look like wood at first glance. Callixylon fossils are usually elongate but can have irregular shapes. Irregular shapes can result from originally rotten wood, burial processes, compaction, and fossilization processes.
Example of a Callixylon log more than 14 feet long and 20 inches in diameter from the New Albany Shale in Marion County, Kentucky (from Gooding and others, 2016).
Preserved wood grain in permineralized and petrified Callixylon log from Marion County, Kentucky. Right image is detail from area outlined.
Long fossil Callixylon logs may be cut by narrow transverse ridges or indents with sunken or depressed surfaces between. Transverse indentations are shrinkage cracks. Raised transverse ridges represent infilled shrinkage cracks. Cracks in the wood were sometimes filled with sediment that did not compact as much as the surrounding wood during burial and fossilization resulting in a raised ridge (Cross and others, 1996). Circular, oval, and “star-burst” patterns on the outside of the wood represent limb scars, where a smaller branch broke off the original log or large branch.
Infilled shrinkage cracks (arrows, dashes) in Callixylon fossil wood. In some cases, ridges form a somewhat rectangular patterns on the outer surface of the fossil logs.
When broken, cross sections of Callixylon may preserve concentric growth rings. Voids in the wood (pores, borings, rotted patches) may be filled with solid quartz, or quartz crystals, like geodes. Starburst patterns on the inside of logs, when viewed at right angles to the long axis of the log, represent cracks radiating out from the axis.
Star-shaped pattern (white dashes) inside Callixylon formed from shrinkage cracks. Fossil log from top of the page in Kentucky Geological Survey paleontological collection. Specimen from Marion County.
Permineralization and petrification. Callixylon logs found in Kentucky are permineralized and many are completely or partly petrified. Permineralization occurs when minerals in groundwater fill pore spaces in the material being fossilized. If the original solid organic material between pores is replaced with minerals, the term petrification is used. Wood is commonly permineralized and petrified because of its porous texture. In fact, some fossil Callixylon logs preserve the original cellular structure of the ancient Archaeopteris wood, which is similar to (but different than) modern conifer trees (e.g., Beck and others, 1982).
Microscopic views of fossil Callixylon wood, showing cell structures of the woody tissue at 500 X magnification. Scale is in nanometers. Quartz has replaced tissues (cell walls) and filled pore spaces (cell lumens) at the cellular level, but some organic material remains. Photographs from Cortland Eble (KGS). Views are from a specimen found in the New Albany Shale in Marion County.
Petrified Callixylon logs represent some of the oldest petrified wood on Earth. Kentucky’s Callixylon logs are 360 to 385 million years old! They tend to be replaced with dark gray to black quartz (silica), so lack the colorful banding many people associate with other kinds of petrified wood, although alternating black and gray quartz may highlight growth rings inside the logs. Callixylon can also be preserved (1) as fossil charcoal, called “fusinization,” from ancient fires, (2) as coalified vitrinite, (3) as pyritized fossils, and (4) in mineral nodules, including calcium phosphate and iron carbonate nodules (Cross and Hoskins, 1951; Beck, 1964; Beck and others, 1982; Cross and others, 1996).
What did the ancient tree look like? Callixylon are the woody parts (trunks, branches, roots) of the Late Devonian age fossil tree, Archaeopteris. Archaeopteris belonged to an extinct division of plants called the progymnosperms. These were the ancestors of modern gymnosperms, which include conifers (like pines), cycads, and ginkos. Archaeopteris were some of the first tall trees, possibly growing to heights of 50 to 60 feet or more (Beck, 1962a, 1964; Algeo and Scheckler, 1998). They may have looked similar to a modern conifer, with a vertical trunk and horizontal limbs branching off the trunk. Unlike modern conifers, however, Archaeopteris had fern-like leaves (Beck, 1962a; Meyer-Berthaud and others, 1999; Murphy, 2005) and reproduced with spores rather than pollen. Although Callixylon logs of the Archaeopteris tree are found in Kentucky, leaves of the tree (also given the genus name Archaeopteris) are not.
Species. At least five species of Callixylon are known from eastern North America: C. arnoldi, C. brownie, C. clevelandensis, C. erianum, and C. newberyi. Species are defined based on microscopic variations in woody structures, so most specimens are not identified to species level. Callixylon newberyi has been recorded in Kentucky (e.g., Cross and Hoskins, 1951) and is probably the most common species. C. arnoldii has also been found here and was originally defined by Beck (1962b) for small specimens preserved in the old Sanderson Formation near Junction City, and Boston, Kentucky. Those specimens are in the University of Michigan paleontological collections.
Range. Archeaopteris first evolved in the Middle Devonian (early Givetian stage), became widespread forests in the Late Devonian (Famennian stage), and went extinct by the Early Mississippian (Kinderhookian stage) (Beck, 1962b; Berry and Fairon-Dermaret, 2001). Callixylon clevelandensis, C. erianum, and C. newberyi are Late Devonian species. C. browni and C. arnoldi are Early Mississippian species (Beck and others, 1982; Cross and others, 1996). In Kentucky, Callixylon is mostly found in Upper Devonian black shales, but also occurs in Lower Mississippian shales. Upper Devonian and Lower Mississippian shales are exposed at the surface in a belt that corresponds to the Knobs Region in Kentucky. In the eastern Knobs, the Devonian black shale is called the Ohio Shale. In south-central Kentucky, the black shale is called the Chattanooga Shale. In west-central and parts of central Kentucky the shale is called the New Albany Shale. Mississippian shales containing Callixylon logs belong to the upper New Albany Shale (Falling Run Member). Fossil Callixylon logs have been found in all of these shales, in at least 13 Kentucky counties.
Callixylon logs on display in Kentucky and surrounding areas. (A) Log with wood grain on display at the Department of Earth and Environmental Sciences at the University of Kentucky; (B) Log with less obvious grain and some infilled cracks on display at the Falls of the Ohio in Clarksville, Indiana, across from Louisville; and (C) elongate but irregular shaped log with many infilled cracks at Holy Cross grotto in Loretto, Kentucky. Scales are in centimeters.
Paleoecology. The Ohio, New Albany, and Chattanooga black shales were deposited as organic-rich muds in a Late Devonian sea that covered much of what is now the southern and central United States. The black shales are not abundantly fossiliferous but contain marine fossils including marine algae, conodonts, inarticulate brachiopods, crinoids, and fish (Thayer, 1974; Barron and Ettensohn, 1981). The shales also contain Callixylon logs of the tree, Archaeopteris. How did fossil trees end up in marine muds?
None of the Kentucky Callixylon logs are found in a vertical position or rooted into an ancient soil horizon (called a paleosol). All of the fossil logs are found horizontally in layered marine shale, which means they were transported to what is now Kentucky from somewhere else. Vertically-oriented Archaeopteris stumps are found in eastern Pennsylvania and New York. These fossils are preserved upright above paleosols with common fossil leaf litter. They represent in-place fossil forests. Surrounding strata were deposited in the floodplains of an ancient delta, called the Catskill delta (Driese and others, 1997; Stein and others, 2020). The Late Devonian shoreline in North America extended from Canada through west-central New York, southward to eastern West Virginia, into western North Carolina (e.g., Scotese, 2002). West of that shoreline, a sea covered Kentucky, Illinois, Indiana, Ohio, Michigan, and Tennessee. Hence, fossil logs in the Chattanooga, New Albany, and Ohio shales of Kentucky are 300 miles or more from the Late Devonian shoreline. How did the logs get to Kentucky?
How Callixylon logs were transported from Late Devonian land into the seas that covered Kentucky.
Today, plant debris, including large logs can wash into streams and rivers. Large logs can float for great distances. Most plant debris in rivers ends up in the river or along riverbanks. Some debris, however, is carried into the sea or ocean. When felled trees are transported in water, their branches break off from abrasion. With time, bark and outer tissues break or slough off in a process called “decortication.” Eventually, only the woody tissue of the trunk remains. With time, floating logs become waterlogged, and sink, where they can be buried and ultimately become fossils.
Rafted paleocommunities. In our modern seas, floating debris is called “driftwood,” “flotsam” or “rafting debris.” Floating debris can be colonized by a host of microscopic and macroscopic organisms. More than 1,200 different species representing most of the major phyla on earth have been recorded on floating debris in the world’s oceans (Thiel and Gutow, 2005). Some floating logs have diverse species on them, while others are dominated by a few species. The variety of life on and around a floating log depends on the size of the log, type of wood, the amount of time the log has been afloat, and the waters through which it has traveled, among other factors (Thiel and Gutow, 2004, 2005). Algae and barnacles are common on floating logs. Algae on floating logs are food for other tiny invertebrate organisms. Many invertebrates require a stable surface to attach to for part of their growth cycle. Larvae may attach in one area, grow on the floating log, and leave or fall off the log at another growth stage. The tiny invertebrates and algae are both food for larger invertebrates and smaller fish. Some floating logs may provide protection and act as traveling nurseries for fish. Schools of small fish are common around floating logs. In fact, floating logs and man-made, floating, artificial devices (FADs) are used by commercial and sports fisherman as fish attractors, because schools of smaller fish attract larger fish like tuna or even sharks, creating a floating, traveling community (Hunter and Mitchell, 1967; Gooding and Magnuson, 1967; Caddy and Majkowski, 1996; Thiel and Gutow, 2004, 2005; Boyce, 2012).
Floating logs on today’s seas provide a habitat for a multitude of organisms. Many of the organisms have soft bodies or are composed of soft parts that would not commonly fossilize. Others are shown here which have mineral shells or carapaces that might be preserved as fossils. What might a Late Devonian floating log community have looked like?
Ancient floating Callixylon logs also harbored small, attached invertebrate organisms. Callixylon logs and other terrestrial plant debris in the marine black shales have been found with attached crinoids and brachiopods in Indiana and Ohio (Wells, 1939, 1941, 1947; McIntosh, 1978; Barron and Ettensohn, 1981). Although many of the organisms found on or around today’s floating logs did not exist in the Devonian, different types of fish and sharks lived in those ancient seas. It seems likely that they also would have been attracted to large, floating logs for the same reasons that fish are attracted to logs today. If you find a Callixylon log in Kentucky, look to see if any other fossils are attached to it. It’s important not just to look on the log, but in the shale around the log as well.
By providing a means for organisms to hitch-hike across a sea or ocean, floating logs (and other debris) also may play a key role in dispersing some species across great distances (Thiel and Gutow, 2004; Thiel and Yaye, 2006).
Oxygen conditions in the black shales. Preservation of Callixylon logs in Devonian black shales may have been aided by low-oxygen conditions near the seafloor in the Late Devonian seas. The black color of the shales comes from preserved organic carbon. In normal, oxygenated sea water, aerobic bacteria and organisms on the seafloor consume much of the organic material deposited. This didn’t happen in the Late Devonian seas of Kentucky and surrounding areas. In the Late Devonian, the water column in these seas appears to have become density stratified. The upper part of the water column had normal oxygen levels and marine life flourished. At depth, a pycnocline developed. Pycnoclines are zones or layers of rapid change in density, temperature, salinity, and/or oxygen levels (e.g., Ettensohn and Elam, 1985). In the Late Devonian seas in which the Ohio, New Albany, and Chattanooga Shales were deposited, the water column below the pycnocline had less oxygen. Low-oxygen conditions are called “dysoxic or dysaerobic.” No oxygen conditions are called “anoxic or anaerobic.” Geochemical and palentotological data indicate the dark gray and black shales of Kentucky and surrounding states were deposited under fluctuating dysaerobic (dysoxic) to anaerobic (anoxic) conditions (Barron and Ettensohn, 1981; Ettensohn and others, 1988; Pashin and Ettensohn, 1992; Rimmer, 2004; Perkins and others, 2008). This is why the black shales don’t contain more abundant marine bottom-dwelling fossils (corals, bryozoans, etc.). It also is why sunken logs were preserved and some fish fossils preserve details of soft parts. Less bacteria are available in low oxygen conditions. Hence, on the Late Devonian sea floor, material like ancient logs, would not rot any further, providing time for them to be slowly buried in mud and ultimately preserved as fossils.
Deposition of Late Devonian black shales may have been caused by development of a pycnocline and a stratified water column. Dysaerobic to anaerobic conditions existed near or at the sea floor which may have aided preservation of sunken logs (modified from data in Barron and Ettensohn, 1981; Ettensohn and Elam, 1985; Pashin and Ettensohn, 1992).
A fossil log that represents global changes. Fossil Callixylon logs in Kentucky, are transported parts of the oldest widespread forests on earth. Today we recognize that trees are important parts of our planet’s carbon, oxygen, nitrogen, nutrient, and water cycles. Prior to the mid-Devonian, however, trees didn’t exist, and those cycles were much different then today. The spread of Archaeopteris trees (and their roots) changed all that (e.g., Greb and others, 2006). Rooting stabilized soils and stream banks (e.g., Gibling and Davies, 2012), and changed erosion and chemical weathering rates and processes (e.g., Berner, 1992). The spread of forests and trees created a new habitat for evolving animals on land. The spread of trees also dramatically influenced Late Devonian climate. Trees are major parts of the carbon cycle. Leaves on trees take in carbon dioxide and give off oxygen. In the late Devonian, the spread of trees (and other land plants) led to dramatic increases in carbon dioxide, global cooling, and a Late Devonian Ice Age (Berner, 1994; Algeo and others, 1995, 2001; Beerling and Berner, 2005; Hir and others, 2011; Stein and others, 2020).
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Text and images by Stephen Greb (unless otherwise noted)