Tuesday, March 30, 2021

Assiduous Collector, But Can Be Nasty Sometimes

In a recent biography of paleontologist John Bell Hatcher, I found reference to an interesting method of paleontological fieldwork.  This post explores the story behind this collecting technique which features Hatcher, harvester ants, and mammal fossils.

John Bell Hatcher (1861-1904)

John Bell Hatcher, a member of my pantheon of paleontological heroes, is widely acclaimed to be one of the greatest fossil collectors who ever lived.  (Hatcher is the subject of a previous post on this blog.)  Paleontologist Lowell Dingus titled his book King of the Dinosaur Hunters:  The Life of John Bell Hatcher and the Discoveries That Shaped Paleontology (2018).  Paleontologist Charles Schuchert, who at one time worked with Hatcher, described him as the “king of the collectors” (O.C. Marsh:  Pioneer in Paleontology, 1940).  These titles of royalty are well deserved, though, in truth, Hatcher was a much more accomplished and well-rounded paleontologist than he is often given credit for.  (This image is taken from Charles Schuchert’s memorial for  John Bell Hatcher appearing in The American Geologist, Volume XXXV, Number 3, March, 1905.)

Hatcher spent much of two decades collecting for paleontologist Othniel Charles Marsh (1831-1899) of the Yale Peabody Museum of Natural History.  Later, he joined Princeton University and then the Carnegie Museum of Natural History.

Admittedly, Hatcher was a prickly character, known to snap at employers and colleagues.  The root causes of his often abrasive personality probably included a chronic illness that, at times, caused serious joint pain (certainly a challenge for hunting fossils).  Further, the hierarchical structure of paleontology in this period grated on the man.  Wealthy men dominated the museums that were amassing fossils, while they also controlled a mostly uneducated corps of men out in the field collecting.  The wealthy Marsh was an exacting boss and loath to share credit for the work his collectors did.  So, working for Marsh was certainly not ideal for the Yale educated Hatcher.  (Ronald Rainger, Collectors and Entrepreneurs:  Hatcher, Wortman, and the Structure of American Vertebrate Paleontology Circa 1900, Earth Sciences History, Volume 9, Number 1, 1990.)


Western harvester ants (Pogonomyrmex occidentalis) have a starring role in this story though it’s possible they should share the spotlight with other ant taxa.  Here is a picture of P. occidentalis at Alamosa, Colorado.  (This photograph is in the public domain under Creative Commons Attribution-Share Alike 4.0 International license.  It was taken by Robert Webster and is available at Wikimedia Commons.)

Harvester ants are collectors of seeds and tiny rocks, the former to eat, the latter to insulate their nests.  They create massive nest mounds of these collected rocks, mounds that may extend up to four feet in diameter and between two and ten inches tall.  (University of Colorado Museum of Natural History, Bio Lounge, Tiny Collectors:  Harvester Ants.

Pictured below is a P. occidentalis mound at Hallelujah Junction, California.  (This image is in the public domain under Creative Commons CC0 1.0 Universal Public Domain Dedication.  It was taken by Alex Wild and can be found at Wikimedia Commons.)

Mammal Fossils

Now, the fossils in this tale are those of mammals living in the Mesozoic Era, and, to be more specific, those at the end of the reign of dinosaurs in the Late Cretaceous Period (some 100 to 66 million years ago).  Mammals surviving in the dinosaur-dominated landscape were, by necessity, slight creatures (think on the order of rats), probably nocturnal for the most part.  Paleontologist Donald Prothero has noted, “They remained as tiny, nocturnal animals under the feet of the dinosaurs, or in the trees above them, through about two-thirds of their history (the entire Jurassic and Cretaceous, spanning over 120 million years).”  Significantly (for this post), he added, “Consequently, Mesozoic mammal fossils are also tiny, and tend to be fragmentary and hard to find.”  (Bringing Fossils to Life:  An Introduction to Paleobiology,  1998, p. 384.)

Though Marsh was fixated on large fossils and is known for his rivalry with fellow paleontologist Edward Drinker Cope (1840-1897) in a frantic quest for dinosaur fossils (the so-called Bone Wars), he had an abiding interest in mammals living during the age of dinosaurs.  Hatcher was pressed by Marsh for more and more mammal fossils.  They were not easily collected, particularly not for a man who suffered from what he called “rheumatism.”  Tiny teeth that could be gathered only by crawling across the ground on hands and knees for hours on end under a hot western sun were probably not high on Hatcher’s list of desirable objects.

And so Hatcher seized the opportunity for help when it presented itself.

Hatcher Enlisted Invertebrate Collaborators

In a letter to Marsh written on July 6, 1889, from Hat Creek, Wyoming, Hatcher wrote to announce some impressive success in the pursuit of mammal fossils.

I send you today by registered mail three packages containing over 500 mammal teeth besides many bones.  You will have when all get there at least 800 teeth of Laramie mammals, abundant material for two more papers.  I broke the record yesterday by finding 87 mammal teeth in one day.

Then Hatcher added the kicker:

We sifted all the anthills in the two best localities & were rewarded a hundred fold.

(The Yale Peabody Museum of Natural History has digitized the O.C. Marsh correspondence.  There are multiple PDFs of Hatcher’s letters to Marsh.  Hatcher wrote in a very legible hand and the letters make for fascinating reading.  Dingus’ biography offers many excerpts from them.)

Sifted the anthills!  Why am I not surprised that this paleontologist who pioneered grid-based mapping of collecting sites and the jacketing of fossils in the field would have solved his problem so creatively?

This fieldwork technique capitalized on the harvester ants’ intrinsic behavior.  Though Hatcher doesn’t identify the kind of ant with whom he collaborated, it is accepted they were harvester ants.  From a widespread area around their nests, these insects gather small rocks (as noted earlier) to create their distinctive mounds, but it turns out they are not picky about what constitutes “small rocks.”  Indeed, the mix of “things” in the dirt of the mounds often turns out to be rich, ranging from seeds to beads to fossils.  And the fossils “include tiny mammal, dinosaur, and fish teeth, small bone fragments, chucks of amber, and small snails.”  (University Colorado Museum of Natural History, Bio Lounge, Tiny Collectors, see link above.)

Several years later, in an article about collecting mammal and ceratopsian fossils in Wyoming, Hatcher wrote a bit more about this collecting approach.  (He was not consistent as to whether he thought it was “anthill” (the 1889 letter) or “ant hill” (in this article).)   

In such places the ant hills, which in this region are quite numerous, should be carefully inspected as they will almost always yield a goodly number of mammal teeth.  It is well to be provided with a small flour sifter with which to sift the sand contained in these ant hills, thus freeing it of the finer materials and subjecting the coarser material remaining in the sieve to a thorough inspection for mammals.  By this method the writer has frequently secured from 200 to 300 teeth and jaws from one ant hill.

But it wasn’t inevitable that immediate sites likely to contain mammal fossils also supported these ants.  So Hatcher gave nature a hand.

In localities where these ants have not yet established themselves, but where mammals are found to be fairly abundant it is well to bring a few shovels full of sand with ants from other ant hills which are sure to be found in the vicinity, and plant them on the mammal locality.  They will at once establish new colonies and, if visited in succeeding years, will be found to have done efficient service in collecting mammal teeth and other small fossils, together with small gravels, all used in the construction of their future homes. (Some Localities for Laramie Mammals and Horned Dinosaurs, The American Naturalist, February 1896, p. 119.)

I suspect the technique has been improved and made less destructive by successive generations of paleontologists and archaeologists.  The Bio Lounge piece on harvester ants as fossil collectors (cited earlier) notes that scientists looking for fossils or artifacts in the ant’s mounds try to limit the damage to the mound and “usually take only the outer layer, preferably while the ants are hibernating underground in winter.  Harvester ants rebuild their nests every spring, so this doesn’t damage their nests unduly.”

I hasten to stress that harvester ants aren't friendly colleagues; they pack a nasty bite, which is a good reason to raid the mounds while they are hibernating.  If that’s not possible, there are other approaches.  Paleontologist Natasha Vitek, whose research includes analyzing the variation in the dentition of small mammals across the Paleocene-Eocene Thermal Maximum (some 56 million years ago), has said,

Sometimes, if a paleontologist wants to be able to spend more time at an ant hill, they try to put part of their lunch out, hoping the ant will spend more time collecting the food than attacking the paleontologist. . . .  It sometimes works and sometimes not.  It’s not always a guarantee.”  (As quoted by Jerald Pinson, Harvester Ants:  The First Solar Engineers, Fossil Hunters, The Austin-American Statesman, July 2, 2019.)

The anthill method still remains part of the paleontologist’s arsenal of tools for collecting small fossils.

In closing, I present a couple of the mammal teeth that Hatcher found in Wyoming in 1889.  I assume that both were actually collected first by harvester ants.  The two are from multituberculates, an extinct line of “squirrel-like” mammals (Prothero), so named because their molars have multiple cusps or tubercules.  They went extinct in the Eocene, unable to compete with rodents, but they had a long run of 180 million years beginning in the Late Jurassic.

Both of these images are from the collection of the Smithsonian’s National Museum of Natural History.  Much of what Hatcher collected for Marsh ended up at the National Museum.  The first image is of an upper molar from an Allacodon rarus (it can be found on the National Museum’s website here).  

The second fossil is from a Meniscoessus sp. which was found by Hatcher in August of 1889 (on the museum website here).

No scale bar in either photograph, I'm afraid, but, rest assured, each of these is miniscule.

Friday, February 26, 2021

Tuatara, One of a Kind Survivor

The biological and paleontological history of New Zealand’s tuatara troubles me.  Might the fate of a few thousand tuatara speak to our own in this period of the sixth mass extinction?

I am easily distracted, a propensity that has been exacerbated by the pandemic.  Certainly giving in to a new, random impulse requires less energy than seeing something through to its conclusion.  This post was born of a distraction that proved so irresistible and interesting that it brooked few further distractions and, as a result, I may have come to a sort of exhausted closure.  All of this spins off a postage stamp.  As I have noted once or twice, I not only carry the “burden” of collecting fossils, I am also a stamp collector.  These interests sometimes intersect.  (See, for example, the post titled Where Worlds Meet or Perhaps Collide.)  They do at the outset of this post as well.

I have a fondness for the postage stamps of New Zealand, specifically those issued from the 19th century up till about 1950.  The commemorative stamps often draw on the uniqueness of New Zealand, portraying its history, cultures, flora, fauna, and geography in striking designs that are typically superbly engraved.  In a series of pictorial stamps (so-called Second Pictorial) that had its original release in 1935, the 8 pence stamp stands out.  It features the image of a tuatara, a reptile endemic only to New Zealand.

This engraved image was designed by New Zealander Leonard Cornwall Mitchell (1901-1971) who was responsible for some 90 stamps for the post office department over the course of 40 years.  (Design – Victorian Design to Modernism, 1980s to 1940s, Te Ara – The Encyclopedia of New Zealand; see also, Mitchell, Leonard Cornwall, 1901-1971, New Zealand’s National Library.)  (The story of this 8 pence stamp is for a different time and venue, though it resonates with the theme of this post.  I will simply note that the individual stamp shown above was printed in England during the Blitz, the German bombing campaign against England in 1940 – 1941.  As such, it is a survivor of the awful vicissitudes of war.)

The species of tuatara on this stamp is Sphenodon punctatus Gray, belonging to the taxonomic order Rhynchocephalia (also called Sphenodontia).  Its common name, tuatara, means “peaks on the back” in Māori.  The reptile, weighing as much as three pounds and being up to nearly three feet in length, is presently found in small numbers in the wild on only a handful of offshore New Zealand islands.  The tuatara was driven out of the New Zealand mainland by the arrival of humans, and the dogs and rats that accompanied them.  The animal is long lived (upwards of 80 years in the wild), matures late (at roughly 14 years of age) and reproduces exceedingly slowly (females lay small clutches of eggs only every 2 to 5 years).  It survives in relatively cold temperatures.  Long-lived, for sure.  The tuatara named Henry living in captivity, shown below in a picture from 2007, sired offspring in 2012, when he was 111 years old.

(This image, downloaded from Wikimedia Commons, is in the public domain and licensed under the Creative Commons Attribution 3.0 Unported license.)

Despite the superficial similarities, the tuatara is not a lizard, having evolved separately from the order Squamata (lizards and snakes), beginning roughly 250 million years ago.  An initial signal to scientists that this was not a lizard was its dual rows of upper teeth, seen in the interior skull view (upper right) in this illustration of a tuatara that appeared in The Zoology of the Voyage of H.M.S. Erebus & Terror 1839 – 1843 (Volume 2, Reptiles, Fishes, Crustacea, Insects, Mollusca, 1845).  (This was one of many publications that resulted from the scientific exploration lead by James Clark Ross in command of the H.M.S. Erebus and Terror.  This expedition explored the flora, fauna, and geography of the Antarctic region, including New Zealand.)

The specimen in this illustration is identified in the publication as Hatteria punctata and was found on New Zealand’s North Island.  This image was downloaded from the Museum of New Zealand/Te Papa Tongarewa.

The tuatara is a taxon in the amniote order Rhynchocephalia whose members are distinguished not only by the “enlarged palatine tooth row near parallel to the maxillary row” (i.e., that upper dual rows of teeth), but also by having teeth fused to the jaw (so-called acrodont teeth).  (Marc E.H. Jones, et al., A Sphenodontine (Rhynchocephalia) From the Miocene of New Zealand and Palaeobiogeography of the Tuatara (Sphenodon), Proceedings of the Royal Society B, January 20, 2009.)  It occupies a unique niche among the six “terminal taxa” of the amniotes shown in the tree below.  (Amniotes are those animals whose fetuses develop within a fetal tissue membrane called the amnion.)

Evolutionary biologist Marc E.H. Jones makes the tuatara’s unique status quite clear:

The animal group known as “amniote vertebrates” includes more than 30,000 species divided between six major radiations:  mammals (5,416 species), turtles (341), crocodylians (25), birds (at least 15, 845), lizards and snakes (10,078) and (tuatara).  (Not a Lizard, Nor a Dinosaur, Tuatara Is the Sole Survivor of a Once-Widespread Reptile Group, The Conversation, May 11, 2017.)

Sphenodon punctatus is not only the sole species of tuatara in existence, it is the single extant representative of an entire taxonomic order, the Rhynchocephalia.  This uniqueness means that the tuatara is

an extremely important component of extant biodiversity.  The species has played a key role in phylogenetic analyses investigating the origins of turtles and estimating the divergence dates of major amniote clades.  (Marc E.H. Jones and Alison Cree, Tuatara, Current Biology, Volume 22, Number 23, December 4, 2012.)

But the extreme paucity of species in the order Rhynchocephalia belies the diverse fossil record in the Mesozoic Era for this order, encompassing more than 40 different species.  The Paleobiology Database records the earliest known fossil of this order at some 247 million years ago (in the Triassic Period), and lists different fossils through to the end of the Cretaceous Period.  Following the Cretaceous, rhynchocephalian representatives are scarce.  Overall, these fossils appear in 67 collections and were found in many locations, including Triassic formations in the United Kingdom and the United States; Jurassic Period formations in Germany and France; and Cretaceous Period formations in Argentina.  After that, the database cites only finds in New Zealand – 2 in the Miocene Epoch and 13 in the Quaternary Period (2.58 to 0 years ago).

This record includes taxa with a variety of morphological configurations, including a Jurassic genus described by Jones and Cree as “small and gracile,” and another genus as “long-bodied and aquatic, plus a Cretaceous genus from Argentina that was “a large, heavily built herbivore.”

Why does only a single species survive from this long line of Rhynchocephalia?

Jones and Cree suggest some possible explanations.  Perhaps members of this taxon were unable to compete with lizards and mammals.  Perhaps climate change worked to shrink this order to a single species.  I would observe that, clearly, the tuatara is ill-equipped to withstand many challenges to its environment, be those related to the climate or to predators, because its reproduction rate is so dramatically slow it cannot easily recover from losses.  Its inability to withstand rising temperatures also puts it at risk.  Perhaps the other Rhynchocephalia taxa that have gone extinct were similar in these attributes.  Further, given that the fossil record of this order nearly disappears after the Cretaceous, one must also consider a role for the extinction event at the end of that period in setting the order on its downward trajectory.

The geologic history of New Zealand adds a further element of mystery to the history of this survivor.  New Zealand is known to be part of Zealandia, “a large submerged continental crustal fragment that rifted from Gondwanaland in Late Cretaceous times.”  (Steven A. Trewick, Hello New Zealand, Journal of Biogeography, Volume 34, 2007.)  The tuatara is considered by some scientists to be part of an ancient flora and fauna surviving from Gondwanaland and one whose distinctiveness is the result of prolonged isolation.  Evolutionary ecologist Steven Trewick and his colleagues observe that this supposition conflates the geological connection to Gondwanaland with a Gondwanalandian origin of the New Zealand flora and fauna.  An alternative scenario would see all or nearly all of the terrestrial portions of Zealandia that split from Gondwanaland having been completely submerged by the end of the Oligocene Epoch, thereby requiring that the present New Zealand flora and fauna be of a relatively recent origin.  They do note,

At present, there is insufficient geological evidence to compellingly demonstrate permanent land or total immersion.  Therefore both perspectives must be considered as real possibilities.

But they suggest that the evidence in favor of the latter hypothesis (submersion and recent colonization of New Zealand) is strong, conforming with molecular data showing such a recent colonization and with the absence of different, specific taxa.

With a more recent emergence of New Zealand, the absence of mammals and snakes is easier to understand as they tend to be the last to colonize over significant water gaps.

Yet, the tuatara is at the heart of a key challenge to this argument:  If Zealandia was completely submerged, how did the tuatara and a few other specific taxa come to be in New Zealand when they had already gone extinct in the rest of the world?

Trewick and his colleagues suggest that the submersion of Zealandia might not have been complete, and these taxa might have survived from Gondwanaland; “perhaps these taxa are indeed descendants of the lucky few survivors from Zealandia that persisted through a period of small, ephemeral islands.”

That’s a troubling scenario:  a handful of Sphenodon punctatus retreating before rising sea levels, living on the meager exposed remnants of Zealandia.  Surviving there until tectonic action brings New Zealand into being at some point in the last 25 million years.  Populating that newly emergent mainland, only to be driven to extinction there by colonizing humans, and, once again, managing to survive only in meager (and threatened) numbers on small offshore islands.  A sad, disquieting story to say the least.

Despite its nearly depleted numbers, I find it hard to consider the tuatara an evolutionary failure, given a track record stretching back to the time of the first dinosaurs.  Still, it’s a cautionary tale, conveying hard truth about the dire challenges faced by species, no matter how resourceful and tenacious they might be.

Saturday, January 30, 2021

Unconformities - The Evidence of Absence

Another post that is neither one thing or another.  An absence of structure.  Fragments on the page.  My apologies.

The Book of Unconformities – Not A Review

This post was originally prompted by anthropologist Hugh Raffles’ new book The Book of Unconformities:  Speculations on Lost Time (2020), but that post remains mostly unwritten because I’m really not sure what to make of the book.  Here are some fragments that might have gone into that intended post.

The book is a challenging amalgam of personal, anthropological, and geological history.  All of these?  Not really any of them?

The snare for me was his central metaphor:  geological unconformities.  These fascinate me, but very little of the geology treated by Raffles has anything to do with unconformities.

He defines an unconformity as “a physical representation of a gap in the geological record, a material sign of a break in time, readily readable once you know here and how to look.”

Raffles takes this idea of a rift in time into the social and cultural histories he recounts.

The book consists of seven chapters, each grounded in a different kind of rock and each delineating disheartening and disquieting social and cultural dislocations (e.g., the Lenape Indians living on Manhattan brutally displaced by Europeans, or the Inughuit of Greenland who were exploited and dehumanized by Robert E. Peary).

The immediate context for his nearly quarter century exploration of these stories of rocks and people is a search for an “anchor” in an “unmoored world” that had a sharp rift caused by the unexpected and tragic deaths three months apart of two of his sisters in the mid 1990s.  He writes, “[A]lthough my sister’s deaths were only minor horrors in the history of the world, for those closest to them even minor horrors transform all that follows.”

In this light, is it significant that Raffles describes an unconformity as “a cleft that can’t be closed”?  Is this book an effort to heal that scar and, thus, doomed to failure from the start?

Raffles certainly marshals myriad words in seemingly endless sentences and paragraphs that engulf the reader in details.  It’s not stream of consciousness, it’s stream of research. 

Unconformities – Some Thoughts


The Facts on File Dictionary of Earth Science (2000) defines unconformity as:

A surface representing a period of nondeposition or erosion separating rocks of different ages.  Some unconformities show a marked angularity, the beds above and below the unconformity surface having different dips and strikes, where other unconformities can be detected only by paleontological means.  (p. 339.)

In essence, an unconformity is a gap in geologic time.  It is where layers of rock laid down in discontinuous time periods come into contact.  This definition identifies two possible causes of this gap:  the intervening layer was never deposited or it was deposited but subsequently eroded away.

Unconformities are a source of fascination for me because of the challenge they offer:  how to explain the absence of rock, those gaps in time that they exhibit.  In the history of geology, though, unconformities are possibly most significant for their claim on great periods of time, a death blow to young earth creationism.

Scotsman James Hutton (1726 - 1797), justifiably considered by some as the father of modern geology, saw in unconformities, confirmation of his hypothesis that cyclical geological processes on Earth occurred over vastly long periods.  He described geologic time as “of indefinite duration,” and “that we find no vestige of a beginning, no prospect of an end.”  (John McPhee, Basin and Range, 1980, p. 108.)

Probably the most famous unconformity is at Siccar Point, Scotland, on the North Sea.  In 1788, the phenomenon of unconformities came famously into stark relief (literally) for Hutton and colleagues mathematician John Playfair (1748-1819) and geologist James Hall (1761-1832).  They approached the point by boat and it’s an understatement to say they were blown away.  Playfair would later write,

On us who saw these phenomena for the first time, the impression made will not easily be forgotten. . . .  We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of a superincumbent ocean. . . .  The mind seemed to grow giddy by looking so far into the abyss of time.  (McPhee, p. 107-108.)

Pictured below is part of the unconformity at Siccar Point.  The vertically oriented sheets of rock are Silurian greywackes and shales, some 440 million years old.  The red rock resting on top of the vertical rock and sloping from right to left is Old Red Sandstone, 380 to 370 million years old.  The gap between them represents 70 million years of elapsed time.  The lowest portion of the red sandstone is a conglomerate of red sandstone with pieces of the gray Silurian rock that had been eroded during the time it was exposed.  (These dates are those cited by the British Geological Survey in its spectacular video titled Siccar Point:  Birthplace of Modern Geology.)

(This picture was taken by Dave Souza and is licensed under the Creative Commons Attribution-Share Alike 40 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.  It can be found on Wikimedia Commons.)

At Siccar Point, the orientation of the Silurian rock and unconformity between it and the Devonian sandstone is the product of multiple forces.  Hutton was among the first to make clear that these occurred in sequence over time, not in one fell swoop as though through some cataclysmic event.  Geologist Andrew Kerr describes these events as follows, noting that Playfair’s sense of teetering on the abyss of time was in recognition of what they meant:

The profound message of Siccar Point, where relationships show that one group of sedimentary rocks first must have been deposited, then twisted and uplifted and then eroded, before being submerged and buried by a second series of very different sedimentary rocks, and then together uplifted and tilted for a second time to be eroded anew, is most eloquently delivered by John Playfair.  (Classic Rock Tours 1.  Hutton’s Unconformity at Siccar Point, Scotland:  A Guide for Visiting the Shrine on the Abyss of Time.  Geoscience Canada, Volume 45, 2018, p. 39.)


Geologist John D. Glaser observes that considering unconformities on a broad geographic scale, such as a continent, fewer will be identified.  As the lens is applied to ever smaller areas, the gaps in the fossil record caused by the absence of sedimentary action or by erosion more readily emerge.  Traveling from Scotland to a place that I call home, Glaser notes that there are three broad ones in Maryland:

In Maryland, interruptions such as this occurred during the Permian and Jurassic periods, and during the Oligocene epoch.  These, then, are major unconformities in our local rock record.  (Collecting Fossils in Maryland, Educational Series No. 4, Maryland Geological Survey, revised 1995, p. 8.)

I suspect that a fossil collector in Maryland, particularly one who focuses on the state’s coastal plain which encompasses fossiliferous sites on the Potomac River and on the western side of the Chesapeake Bay (c'est moi), is most likely to become aware of the unconformity I consider the most striking and most obvious.  That is, the absence here of fossils from the Oligocene Epoch (33.9 to 23.03 million years ago).  Earth scientist Martin F. Schmidt, Jr. states categorically that “there are no Oligocene sediments in Maryland.”  (Maryland’s Geology, 1993, p. 109.)

 A geologic map of Maryland makes this abundantly clear, at least as far as exposed rock formations are concerned.  Consider this small portion of the key of the 1968 Geological Map of Maryland (Maryland Geological Survey):

This portion of the map provides the key for identifying exposed rock formations of Maryland's coastal plain by color and alpha code.  This is the only area in the state with Cenozoic Era formations.  The key jumps from the Eocene (56.0 to 33.9 million years ago) to the Miocene (23.03 to 5.33 million years ago).  More telling, perhaps, cross sections on Maryland geologic maps also show no Oligocene deposits.  [After the initial posting, this paragraph was edited to make clear that the geologic map key references rock formation exposures.]

Why no Oligocene sediments and, hence, no fossils from that epoch?  Geologist John Means, among others, explains this absence by sea level retreat which exposed this area, precluding deposits of Oligocene-aged sediment.  (Roadside Geology of Maryland, Delaware, and Washington, D.C., 2010, p 231.)  Further, he makes it clear that the coastal area of Maryland has experienced repeated sea level drops and rises.  It’s an ongoing process stretching back into deep time.  Hutton would have understood.

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