I have a hand-sized piece of gray rock (limestone, possibly) in my collection from a Devonian Period formation in western Ukraine. (This post goes deep in time and, so, avoids that country’s horrific present and uncertain future.) The rock is covered with many small fossils, all from the same animal, an animal about which paleontologists have a surfeit of unanswered questions. Of this particular chunk of rock, I not only share the paleontologists’ questions, but, at the end of the post, admit to a fundamental one of just my own.
In his excellent introduction to fossils, paleontologist Richard Fortey wrote of paleontological “enigmas:”
Every now and then the fossil record throws up fossils which are palaeontological puzzles. They are obviously the remains of some kind of animal, but the problem is to decide what kind. They tend to be rather rare and preserved in a special way. Like so many palaeontological matters, they stir up arguments between specialists who think they have a way of solving the enigmas. (Fossils: The History of Life, 2009 edition, p. 156)
Well, in this case, yes and no. The subject of this post is among those puzzling fossils that don’t fit the general case Fortey outlined. This particular one is an enigma that is actually incredibly well represented in the fossil record, so much so, one would think any questions about it would have been answered long ago. Further, I don’t think there’s anything special about the ways in which it was preserved (I return to this at the end of the post). Still, the basic problem – deciding what kind of animal it was – remains.
(I should note that I don’t believe just because something is commonplace that it necessarily comes without questions, only that the opportunities to study it would be manifestly greater than those attending something truly rare.)
The fossils on my rock are the remains of a genus of marine animals, Tentaculites, that lived within calcareous shells, and are considered members of a class of animals known as the Tentaculitoidea. This class existed probably from the beginning of the Ordovician Period (some 485 million years ago) through until possibly the end of the Carboniferous Period (perhaps as late as 299 mya). In many ways, these fossils are so common as to be ordinary. Hervey Woodburn Shimer and Robert Rakes Shrock in Index Fossils of North America (1944) characterized the presence of Tentaculites as “extremely abundant” in Silurian and Devonian strata. (p. 526) These fossils are found worldwide and, indeed, have served as index fossils (useful for the identification and dating of different geologic formations).
As Paul D. Taylor and David N. Lewis observed (Fossil Invertebrates, 2005), tentaculitid shells are typically straight and small, ranging in size from 4 to 30 mm in length, tapering from a close end to an open end (typically 1 mm in diameter).
The outer surface is marked by transverse annulations, comprising prominent rings with gentler ridges called annulets in between, as well as a more subdued ornament of longitudinal ridges called lirae. (p. 133)
Closeups below of some of the Tentaculites specimens in the Ukrainian rock reveal some of the features Taylor and Lewis described.
Familiarity from their sheer abundance has not bred contempt for these animals. Rather, they have been the source of endless speculation and subject of many questions with no consensus answers as yet. The title of an oft-quoted piece on these organisms by paleontologist D.W. Fisher nicely captures what was known, or, rather, not known, 60 years ago: “Small Conoidal Shells of Uncertain Affinities.” (Appears in Treatise on Invertebrate Paleontology, Part W, R.C. Moore, ed., 1962.) The intervening years since Fisher’s work haven’t cleared things up very much. Paleontologist Eberhard Schindler delineated the breadth and depth of our continuing ignorance about the tentaculitids in a recent publication. I summarize below several of the open questions and unknowns about this life form which he described:
- its taxonomic position in the animal kingdom is unclear with no agreement even as to its common name, much less as to which animals should be grouped with it;
- there are many hypotheses about the kind of animal tentaculitids were, including crinoids or parts of crinoids (a filter-feeding marine invertebrate that’s been around for hundred of millions of years, often called “sea lilies”), parts of trilobites, mollusks, or lophophorates (any of a broad group of marine worms with a spiral feeding structure on its head, the group has included brachiopods);
- despite being used as index fossils, there is ambiguity about precisely when the tentaculitids originated and, even more in doubt is when they actually went completely extinct; and
- the fundamental questions of where these animals lived, what their lifecycle was, and how they secured food remain unanswered, with some members of the group likely to have lived in the water column while others probably lived rooted to the ocean floor.
Schindler’s work is a chapter aptly titled Tentaculitoids – An Enigmatic Group of Palaeozoic Fossils in Earth and Life (edited by John A. Talent, 2012). If another appropriate adjective for these creatures is needed, geologist Mark Wilson on the Wooster Geologists blog (March 3, 2017) labeled them “mysterious.”
Perhaps the holy grail for addressing some of these fundamental questions is the discovery of preserved soft tissue. Until then, the questions remain and the debate continues.
One Last Uncertainty
Perhaps the first order question I had to deal with regarding the death assemblage of Tentaculites pictured at the outset of this post, and one for which I have a tentative answer, is: how were these specific fossils created? That is, what do I have in hand? External casts of Tentaculites where all of the original material dissolved, creating voids that were infilled with new minerals? Or, instead, products of chemical change as the original material was infiltrated with precipitating new minerals (permineralization)? Or the results of neither of those processes and of something else?
I am currently inclined to believe permineralization accounts for the fossils on this piece of rock. My reasoning isn’t very deep and focuses on the absence of evidence for casting and molding as the process by which these fossils were preserved. Were these external casts, so my thinking goes, there should also be some molds, that is, depressions retaining the patterns of the external surfaces of the shells, on this rock. I don’t think one (casts) comes without the other (molds).
Clearly, there is little clear about these fossils.
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