Monday, May 18, 2015

"That's Huge!" ~ Exploring the Glen Rose Limestone, Part 1

Home is where one starts from.  As we grow older
The world becomes stranger, the pattern more complicated
Of dead and living.
                       ~ T.S. Eliot, East Coker, Four Quartets

This is an illustration of . . . something stored in a warehouse in Roswell, New Mexico, . . .  no, wait, it’s a schematic drawing of an Egyptian pyramid with a particularly complicated interior, . . . or, better still, this is a 1950s look at the future of apartment buildings.

Actually, it is a drawing showing the interior structures of a shell (also called a test) created roughly 100 million years ago by a foraminifera in the extinct genus Orbitolina.  Forams, those single-celled marine protists, have oozed their way across this blog several times (here is a link to one such instance).  They are amoeba-like organisms that live inside a shell, extending filaments of protoplasm (pseudopods) into their environments.  (The drawing of the shell interior is taken from Raymond C. Douglass’ The Foraminiferal Genus Orbitolina in North America, Geological Survey Professional Paper 333, U.S. Geological Survey, 1960, p. 19.)

All three of the off-the-wall possibilities suggested at the outset are, in some ways, at least partly true.

Alien life form?  As life forms, there’s something decidedly different about forams and their brethren in their separate Kingdom Protista, a much debated catch-all category.  Eukaryotes (organisms with nucleated cells) are divided into four kingdoms: Animalia, Plantae, Fungi, and Protista.  Paleontologist Steven Stanley called that last kingdom a “hodgepodge” (Earth System History, 2nd edition, 2005, p. 54) which it really is, containing many members that are less closely related to each other than to members of some of the other kingdoms.  Quite possibly, the protists should be divided into several separate kingdoms of organisms.  I do like how the University of California’s Museum of Paleontology puts it on its website, “We retain the word ‘protist’ as a convenient term to mean ‘eukaryote that isn't a plant, animal, or fungus.’”  Savor how strange that is.  A life form defined by what it’s not.

Egyptian pyramid?  There is an actual connection through the limestone used in pyramids' construction.
Foraminifers were first recorded in the literature in the 5th century B.C. by Herodotus, who noted the nummulites [a kind of foram] in the rocks of which the Egyptian pyramids were constructed, but not until nearly 2000 years later were they recognized as the being the fossil remains of organisms.  (Alfred R. Loeblich, Jr. and Helen Tappan, Treatise on Invertebrate Paleontology:  Part C, Protista 2, Volume 1, 1964, p. 55.)
Apartment building?  Yes, this shell was an abode and, though a single foram occupied much of the interior spaces of the chambers, each of which was added as the organism grew older and larger, members of the Orbitolinidae family of forams are believed to have shared their tests with algae in a symbiotic relationship, the algae residing in the shell’s marginal rim.  (Gianluca Frijia, et al., An Extraordinary Single-Celled Architect:  A Multi-Technique Study of the Agglutinated Shell of the Larger Foraminifer Mesorbitolina From the Lower Cretaceous of South Italy, Marine Micropaleontology, Vol. 90-91, 2012.)

My first encounter with foraminifera of the genus Orbitolina was through the gift from a friend of a sample of matrix from the Glen Rose limestone formation in Texas.  This formation, famous for dinosaur tracks (subject of a previous post on this blog), dates from the Lower Cretaceous Period and is one of several stratigraphic units that constitute the Trinity Group.  According to Loeblich and Tappan, the extinct Orbitolinidae family lived from the Lower Cretaceous into the Eocene Epoch, while Orbitolina genus was confined to the Cretaceous.  The Glen Rose limestone was built up during the Cretaceous in a warm, shallow sea, an environment probably similar to that in which the extant large foraminifera are currently found.  (Raymond C. Douglass, Revision of the Family Orbitolinidae, Micropaleontology, Vol. 6, No. 3, July 1960.)

My sample was collected from an eroded hillside across the street from the house where my friend’s sister lives and, as luck would have it (we should all be so lucky), the Glen Rose outcrops there.

The following picture captures the most amazing aspect of this exposure, but it takes a close look to see it.

Yes, the picture above shows the internal mold of a gastropod (top arrow) and an echinoid (sea urchin), probably Heteraster (Enallaster) obliquatus (bottom arrow).  But, if one studies the background of this image, it may come clear that the surface’s not just littered with a few gray disks, it’s awash in them.  And that’s huge, because each of those disks is the fossil shell of an Orbitolina texana foraminifera.  Three of these specimens are pictured below.  In each picture, the same specimen is presented in views of the dorsal (top) side, edge, and ventral (bottom) side.  To help with orientation, the dorsal side is identified in each edge view.

Diameters of these specimens are:  approximately 9 mm for the top specimen, slightly less than 8 mm for the middle one, and under 5 mm for the bottom one.  To get the specimens to lie flat, I photographed each on clay which, with my failure to smooth things over for each shot, sacrificed a uniform background.  Think of it as similar to sitting in the front row of a movie theater watching a Wallace and Gromit movie, which I once did.  Finger prints and other impressions in the clay do show up.

In terms of identification, I’ve concluded that these particular forams from the Glen Rose limestone are O. texana based on their general morphology and where they were found (they are the hallmark of the lower sections of the Glen Rose limestone), believing that the differences in size and shape apparent in the pictures of these three specimens reflect natural variation in the species (due, for example, to age or stage in the reproductive cycle).  I do have to point out that conclusive identification of Orbitolina species rests on a much closer analysis of foram specimens, including cutting through specimens to expose differences in the structure of the chambers inside the shells.

As is evident by the labels on these pictures, I have thrown in with school of thought whose members believe these large disk-shaped forams oriented themselves in the substrate on the sea bottom with their convex side up (labeled dorsal in these images).  More on the structure of these shells in a moment.

The sheer abundance of these forams in this exposure is almost beyond my comprehension.  Such dominated carbonate sediments are sometimes referred to as Orbitolina facies because the characteristics of the rock formation, including the presence of the forams, may reflect particular, localized environmental conditions.  (Lorenzo Vilas, et al., Orbitolina Episodes in Carbonate Platform Evolution:  The Early Aptian Model from SE Spain, Palaeogeography, Palaeoclimatolgy, Palaeoecology, Vol. 119, Issues 1-2, December 1995.)  Frankly, this was the first time I’d worked with so-called microfossils where, if one or two bounced out of my picking tray and disappeared into the detritus on the floor, I didn’t curse and feel my heart skip a beat.  I just have too many of them.

Of course, escapees from the picking tray were relatively easy to recover since many of them challenge my operating definition of microfossils – microfossils, I would argue, require the use of some form of magnification for the full range of their analysis.  For true microfossils, my tools include fine brushes; instead, with the larger versions of these O. texana, tweezers frequently come into play.  Douglass called O. texana “medium sized” which, at first, really threw me, since every one of them is, to me, decidedly “jumbo-sized” when compared to typical microfossils falling in the 0.5 mm and below range.  Still, Douglass was right in his characterization since another Orbitolina species, O. concava, had a diameter of some 30 mm, a size that merits a label of “large.”

This leads me to consideration of the incredible complexity of the internal structures of these shells, complexity which belies the simplicity of the organisms that created them.  I use that word – created – advisedly, because the Orbitolina were agglutinated foraminifera, that is, they didn’t just secrete material to fashion their shells, they, as with all other agglutinated forams, actually had a “hand” (well, pseudopod composed of their protoplasm, given the organism’s amoeba-like reality) in selecting and gathering the materials from their environment which they cemented together to build their homes.  (Agglutinated forams are the subject of a previous post.)

Orbitolina exhibited a uniserial arrangements of its chambers, that is, connected chambers were added one on top of the other as the organism outgrew its old chambers.  That’s extremely hard to visualize from the illustration and photographs presented above.  Think of the chambers as a stack of something like pancakes being pinched and lifted in the middle.  The height of each chamber (pancake) is narrow at the center, expanding as one moves away from that center point.  In the crude drawing below, I tried to capture this uniserial arrangement.  It is based on illustrations in Douglass, The Foraminiferal Genus Orbitolina in North America, and Frijia et al., An Extraordinary Single-Celled Architect.

The apex of the shell is the embryonic apparatus – the initial structure containing the organism.  Given how complex the sexual life cycle of these and most other foraminifera was, and is, (involving sexual and asexual reproductive phases), I will gloss over the embryonic apparatus and its influence on the development of the foram.

Each chamber added to the shell was composed of three different zones.

The central complex was the area of each chamber under the embryonic apparatus and was, as Douglass described it, “relatively unorganized.”  Over time, the central complex typically became somewhat filled with debris and, apparently, not much of the foram’s protoplasm occupied that area given space limitation.

The radial zone, adjacent to the center complex, was subdivided into many passages by partitions that ran outward from the central complex, initially in a weaving pattern that became straighter as they reached the outer rim of the chamber.  The organism lived primarily in the radial zone.

On the rim, the marginal zone featured many horizontal and vertical partitions, including the partitions coming from the radial zone.  These various partitions created small “chamberlets.”

(For this description of the internal organization of the Orbitolina chambers, I have relied primarily on Douglass’ The Foraminiferal Genus Orbitolina in North America, and the article by Gianluca Frijia and colleagues, An Extraordinary Single-Celled Architect.)

Recent analysis suggests that these forams selected grains of material from the environment to incorporate in their shells based on shape, not on size.  And such “foreign” material was used in some parts of the chamber, but not others.  (See Frijia et al.)

I’m intrigued by the implications of the complexity of the Orbitolina shell.  Presumably members of this genus had to be long-lived, simply because it would have required a lot of time to build a structure of this size and complexity.  Regarding the related Palorbitolina lenticularis, Lorenzo Vilas and his colleagues posited that its large size suggested a lifespan of one to two years.  (Vilas, et al., Orbitolina Episodes in Carbonate Platform Evolution.)  Beyond the time needed to create the shell, one has to wonder what evolutionary advantages might have accrued from such characteristics as the sheer size of the members of the genus, the warren of passageways and rooms dividing up each chamber, and the mixture of secreted and “foreign” materials used in their construction.  I have ideas but no answers and, so far, haven’t found much discussion in the literature (with the exception of Frijia's article and he ultimately throws up his hands).  So, I’m left appreciating a very strange and complex home.

There’s another issue that puzzles me.  If this concentration of Orbitolina texana in the exposure reflects the actual faunal diversity at this location in the Early Cretaceous (and I don’t have reason to think it’s some artifact of post-mortem change in the environment), why was there such a concentration of this foram species in the first place, and what might it have meant for other potential substrate dwellers, like ostracodes (microscopic crustaceans)?  Perhaps the first step is to see what other microfossils show up this material.  (Consider this a segue to another post – in essence, it’s that ever-offensive “to be continued” message.)

Friday, May 1, 2015

Teeth and Denticles ~ A Loss of Clarity

In vertebrate paleontology, increasing knowledge leads to triumphant loss of clarity.
~ Paleontologist Alfred Sherwood Romer as quoted by evolutionary biologist Keith Thomson

The more I learn, the less I know for sure, and, yes, this can be a good thing.

I have been working through a sample of matrix from the Archer City Formation in Texas (early Permian Period, roughly 295 to 290 million years old).  This particular sample abounds in fossils, though without much apparent diversity.  This squares with my reading of Lower Permian Freshwater Sharks and Fishes of Texas and Oklahoma (2014) by Kieran Davis, a commercial collector (I think).  The predominant shark genus in the Archer City Formation, according to Davis, is the freshwater Orthacanthus of the Xenacanth family, whose teeth are fairly easy to identify, at least to the genus level, because the teeth have a strange and striking double-bladed arrangement.  These pictures show the teeth from the so-called lingual side (facing the middle of the mouth).

Orthacanthus teeth are often larger than those that I have turned up.  For a fuller treatment, see Gary D. Johnson’s Dentitions of Later Palaeozoic Orthacanthus Species and New Species of ?Xenacanthus (Chondrichthyes:  Xenacanthiformes) From North America, Acta Geological Polonica, Vol. 49, No. 3, 1999.

Along with the Orthacanthus teeth in this material are a great many other tooth-like fossils which, based on Davis’ guide, I initially identified as denticles from the Orthacanthus.  Two clusters of specimens representative of these finds are pictured below.

But then there are these specimens.

They don't resemble the denticles included in Davis' guide.  Is that because these specimens are broken, consisting of only crowns with no root element, or are they actually teeth but from some other fish taxa?  I should note that Davis observes that he found few fossil fish specimens (other than the Orthacanthus fossils) while collecting in the Archer Formation.

At that point, the question became:  What do I know about shark denticles and what they look(ed) like?  Not much,  So I delved into the literature.

Denticles, also called placoid scales, are teeth-like structures embedded in the skin of extinct and extant sharks, rays, and skates (the elasmobranchs) which differ by species.  Further, denticles on a single individual may also differ in shape and size depending upon their location on the skin.  Researchers believe that, depending upon species and dermal location, denticles may play roles in locomotion (reducing drag or turbulence), protection from predation, protection from abrasion, reproduction, or luminescence.  See, for example, M.J. Johns, et al., Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths from Northeastern British Columbia, Geological Survey of Canada, Bulletin 502, 1997, p. 17; and Reef Quest Centre for Shark Research, Skin of the Teeth.

The actual composition of shark teeth and denticles is basically the same.  According to Johns and her colleagues,
A typical elasmobranch tooth or scale consists of a crown with an outermost shiny layer of enameloid followed by dentine.  Dentine layers enclose the pulp cavity which is filled with blood and nerve tissues.  The crown sits on a base (tooth) or pedicle (scale) . . . .”  (Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths, p. 15.)
Ancient jawed vertebrates exhibited this “dermal skeleton,” but most groups lost it, but not the sharks, rays, and skates.  (Qingming Qu, et al., Scales and Tooth Whorls of Ancient Fishes Challenge Distinction Between External and Oral “Teeth,” PLOS ONE, August 2013, p. 1.)

My doubts about the identity of my Archer City “denticles” were not resolved as I considered the basic shapes of shark denticles depicted in the literature.  These mostly do not resemble those Davis identifies as Orthacanthus denticles.  Consider these drawings of denticles from common extant sharks that appeared in Lewis Radcliffe’s The Sharks and Rays of Beaufort, North Carolina (Bulletin of the United States Bureau of Fisheries, Vol. XXXIV, document issued April 6, 1916).  These drawings show (from top to bottom) denticles from Cacharhinus isodon (Finetooth Shark), Galeocerdo cuvier (Tiger Shark), and Cetorhinus maximus (Basking Shark).  Only the last from the Basking Shark favor at all those I found in the Archer City material.  (I've given the scientific names currently applied to these sharks, not those Radcliffe used for them.  Many more denticles from different shark species are pictured in Radcliffe's book.)

Additional descriptions and illustrations of denticles appear in many other sources, such as the study by paleontologist Marjorie Johns and her colleagues cited earlier (Taxonomy and Biostratigraphy of Middle and Late Triassic Elasmobranch Ichthyoliths from Northeastern British Columbia); some of the articles by paleontologist Wolf-Ernst Reif (such as Types of Morphogenesis of the Dermal Skeleton in Fossil Sharks (Paläontologische Zeitschrift, Volume 52, No. 1-2, June 1978); and the gallery of pictures of shark and ray denticles on the website of the Australian Museum.  With regard to this last source, the Bashing Shark denticles shown there do not bear much resemblance to my "denticles," despite Radcliffe's drawing suggesting otherwise.

So, where am I on my “denticles”?  Well, learning more hasn't conquered the uncertainty, but a more informed exploration continues.  Davis may be right that most or some of those simple hooks I’m finding are Orthacanthus denticles.  Right now, I really don’t know.

There’s another aspect of the relationship between teeth and denticles that interests me.  The basic similarity between teeth and denticles has sparked a debate over, and uncertainty about, the evolutionary origins of teeth.  Two hypotheses are in contention:  (1) The outside-in hypothesis posits that dermal denticles (which can be found not only on the external skin and sometimes near the mouth, but also, indeed, inside the mouth) evolved into teeth as we know them.  (2) The inside-out hypothesis, as I understand it, argues for the independent development of these two kinds of structures (perhaps. it would be better labeled the inside and outside hypothesis).  For background on these hypotheses, see, for example, Peter S. Ungar, Teeth:  A Very Short Introduction, 2014, p. 48-50; Philip C.J. Donoghue and Martin Rücklin, The Ins and Outs of the Evolutionary Origin of Teeth, Evolution & Development, published online September 15, 2014; and Gareth J. Fraser, et al., The Odontode Explosion:  The Origin of Tooth-Like Structures in Vertebrates, Bioessays, Volume 32, No. 9, September, 2010.

For much of two decades, according to paleobiologists Donoghue and Rücklin, the outside-in hypothesis held sway until various pieces of evidence helped the inside-out thesis gain traction.  As a consequence, they state at the outset of their recent article,
The diversity of jawed vertebrates is predicated on the two formative evolutionary innovations of teeth and jaws, the origins of which appear to be becoming increasingly unclear.
The origins of which appear to be becoming increasingly unclear.  Initially, I understood this sentence to mean that the more the issue’s been studied, the less certain things have become.  That was the tale I assumed the authors were about to tell.  But, not so.  Their article, in fact, is a point by point refutation of the evidence mounted for the inside-out hypothesis.  They make their case and then conclude, with a garnish of scientific language:
This suggests that the traditional ‘outside-in’ hypothesis is the best explanation for the evolutionary origin of teeth through expansion of odontogenic competence from the external dermis to the internal epithelia.  [Translation:  Outside-in prevails because denticles in the skin evolved into teeth inside the mouth.]
They are not experiencing any loss of clarity, while other researchers haven’t ceded the field on this issue.  But, for me, uncertainty prevails.

Returning to the epigraph that opened this post.  I came upon it in an essay by evolutionary biologist Keith Thomson (subject of recent post) which is part of the collection titled The Common But Less Frequent Loon and Other Essays (1993).  After quoting his old teacher, paleontologist Alfred Sherwood Romer, Thomson undercuts the witty and thought-provoking statement by calling it Romer’s “little joke.”  What Romer meant, writes Thomson, is that, in vertebrate paleontology, the discovery of a truly transitional fossil lying between two taxa, say fishes and tetrapods, leads to a loss of clarity - is this fossil "a late fish or a very early tetrapod?"

Damn that constraining context.  I searched unsuccessfully for the document Thomson cites for Romer's sentence because I wanted so much to free it.  Then, very recently, I learned that Thomson uses it in another work, Morphogenesis and Evolution (1988), where he does the deed himself by largely jettisoning this limiting context and meaning.  All fields of science, he asserts, go through a repeating cycle, where, in the face of increasing knowledge, a synthesis that seems to tie the field together unravels (loss of clarity), later to be replaced by the emergence of another synthesis, and so on.  In light of that application of the Romer quotation, I think I'm allowed to liberate it still further, turning it into a maxim, thereby giving it the universality (and overstatement) it deserves:
Increasing knowledge leads to triumphant loss of clarity.
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