Purgatorius may be from Hell Creek, but it's currently in Limbo:  Why it is so difficult to place early members of a group on the Tree of Life?

Recent discussions on a Facebook Group, Creationism, has centered around a series of yellow memes produced by Creationist and retired pastor Luke Lefebvre.  I've reproduced the latest with his permission, which summarizes Luke's point that since scientists can't exactly agree on whether Purgatorius is a primate, a near-primate or even a true (eutherian) mammal, we can't trust anything those scientists have to say about evolution.

So is this true? And if it is true, why should there be such confusion among paleontologists and paleoanthropologists over the nature of this little critter?  As is so often the case, the Young Earth Creationists like Luke seize upon the usual, healthy give and take of the scientific method as a sign of weakness, and use it to bolster their fear and loathing of science.

Background

First, a little background.  Purgatorius was first described by Leigh Van Valen and Robert E. Sloan in 1965 on the basis of teeth from Purgatory Hill, and placed in two species, Purgatorius unio and Purgatorius ceratops.  P. unio was assigned to the early Paleocene, while P ceratops, because it was a single eroded and weathered tooth, to the underlying Cretaceous part of the Hell Creek Formation.  Both teeth were from a small channel deposit which has a mixture of earliest Paleocene and latest Cretaceous fossils in what is known as a "time-averaged" assemblage. Most of the fossils represent animals that lived at the time the channel was formed, but a few, like the Purgatorius tooth, were eroded out of the older deposit into which the channel was cut.  Most securely dated occurrences of Purgatorius are limited to the second and third phase of the Puercan North American Land Mammal Age (Puercan 2 and Puercan 3), dating between about 64.75 and 64.11 MYA (million years ago), with only a few known from the Puercan 1 phase.

Purgatorius was thought by Van Valen and Sloan to be an early Paramomyid primate, but discussions between 1965 up to today have called Purgatorius a stem primate, or placed it outside the primates; one study has even suggested that it may not be a placental mammal (Eutherian) at all, but should be placed with the Metatheria, which includes the living marsupials as well as a host of extinct groups like the multituberculates.  But why should there be such confusion?  We don't need to get into all the details of dental morphology, although that is where the problem arises.  The answer is really much simpler than that.

The Facebook Meme

Above is Luke Lefebvre's meme entitled Lucy vs. Purgatorius: Who to Believe?  He quotes Don Johanson, then goes on to say "Purgatorius is thought to be an early form of primate (That means that's where you come from) although its exact place in the evolutionary history of primates is much debated.  This is mostly due to the fact that the only fossils we have of this animal are its teeth and jaws. Lefebvre concludes "Ask any evolutionist and wait to get different answers."

Luke is correct in these points:  Purgatorius is thought (by most paleontologists) to be an early primate.  Its exact place on the mammal tree is debated on the basis of different analyses of different data sets. A few paleontologists think it is a proto-primate, or even outside the placental mammals altogether, but the general consensus, using the total evidence available, is that it is a primate.  It is true that the fossil remains of Purgatorius are mostly teeth and jaws, but in 2015 7 astragali and 9 calcanea (both ankle bones) were described by Chester et al.  These were important in being the first postcranial bones attributable to Purgatorius, and in indicating the first Paleocene mammals which were at least partially arboreal.  Note that the tarsal bones were not directly associated in skeletons, but were isolated bones in faunas which had many teeth and jaws of Purgatorius.  How Chester et al. identified each of the particular species of Paleocene mammals is another story, but it is well supported by the evidence.

So why is there any uncertainty?

Think about evolution as the branching pattern often depicted as a phylogentetic tree.  The further back in time you go, the closer you get to the common ancestor, in this case the common ancestor shared by all eutherian mammals.  But remember, evolution isn't a single line from the past to the present. "Decent with modification', Darwin's wonderful phrase, predicts that all the diversity we see in  eutherian mammals traces back to the common ancestor, each modern mammal (as well as all the extinct ones) traces its ancestry back to that same spot on the tree, each by a different route.  Carnivores, proboscideans (elephants), rodents, perissodactyls (horses, rhinos and tapirs), artiodactyls (deer, sheep, cows, giraffes, etc.) all can trace their ancestry back to that same point.  While all those animals are wildly diverse today. as we trace the lines backward toward the common ancestor, they get more and more alike, less different and would be much harder to tell apart if we could see them as living creatures.  Their morphology - teeth, bone structure and all the details that are the basis for understanding the fossil record, also get more and more alike as you move backwards in time and down the tree.  In fact, when you get to the early Paleocene, 61-65 million years ago, all these eutherian mammals, just beginning their evolutionary radiation, are nearly impossible to tell apart.  If we didn't have their descendants identified in some detail, we'd lump them into one group as most closely related to each other.  It is only because we know what they would become later in time, that we can separate them from the other early "primitive" mammals and place them in the proper group known from their descendants.

So, most of the confusion - all of the confusion! - comes from disagreements between researchers as to the legitimate place of Purgatorius on the tree.  It isn't resolved now, and likely won't be until better evidence is available, such as a complete skull and skeleton of Purgatorius and some of the other Paleocene mammals.

In order to illustrate better what I'm talking about, I've taken a slide from a PowerPoint presentation by the Joint Experimental Molecular Unit and the Royal Museum for Central Africa entitled "Introductory seminar on the use of molecular tools in natural history collections" dated November 6-7, 2007.  On a part of this slide I have added more information, as follows:

Each red dot in a red circle indicates a possible positions that Purgatorius could be placed in, with the present hypothesis most favored by the evidence being a stem or near-stem primate.  Another possible position is at the base of the unresolved archontan trichotomy - which just means that the three groups, primates, tree shrews (Scandentia) and the colugo (Dermoptera) are closely related, but the evidence doesn't clearly tell us which two of the three are more closely related to each other than either one is to the third.  Yet another possibility is that Purgatorius represents a common ancestor of all the Euarchontoglires - a group composed of the colugo, tree shrews, primates, rabbits and rodents.  A final possibility which no one has suggested and which has no evidence is that Purgatorius could stand at or near the common ancestry of the rodents and rabbits.

The diagram also has some dashed green lines between the living taxa (blue dot to blue dot) which show that the distance between those dots indicates roughly the morphological "distance" or divergence between the evolving lineage leading up to those dots.  Remembering that each of those blue lines represents a long series of ancestor / descendant species, genera and families, we can also draw those dashed lines between the blue lineages at any time point along the line, demonstrating that as we move back in time - towards any of the common ancestors, the morphological difference between them gets to be less and less as shown by the green dashed lines between the blue Primate line and the blue Dermoptera line.  By the time we reach the early Paleocene - the time of Purgatorius - an early tree shrew is going to be morphologically quite close to an early relative of the colugo, and both will be very similar to an early primate.

The final figure provides yet a different illustration of the problem.  In this diagram, the three living orders, Primates, The Colugo and the Tree Shrews, are shown across the top.  The blue line under each one represents the evolutionary history of that group, called a "clade".  The red oval includes a cloud of fossil species - some are on the direct line back to the earliest member of each clade; others are either just a little off that mainline ("first cousins") or aberrant members, dead ends with no surviving descendants.  One such fossil species is represented for each clade in the diagram by a red dot.  

Now, as is often the case for the most primitive members of the major clades, the earliest members look very much like the earliest members of other related clades.  In fact, we often can't tell to which surviving clade they belong because they lack the specialized characters which help us define the clade.  So all these primitive species get lumped together in a group called a "grade"  It's much like a small dense cloud of species which we can't differentiate and connect to their eventual descendants until sufficient material is collected, a careful analysis is done, and unique shared characters are identified which will connect a given "red-cloud" species to a particular clade.

Summary

So Luke Lefebvre is correct - you can get different hypothses when you read different papers.  But you have to read them carefully, mindful of when each was written, and look at what new evidence each brings to bear on the problem.  That's what science is about.  Science would be no fun at all if there weren't new discoveries to be made, new connections to be drawn and new hypotheses to be proposed - and then tested.

References

Chester, Stephen G. B., Jonathan I. Bloch, Doug M. Boyer, William A. Clemens, 2015, "Oldest known euarchontan tarsals and affinities of Paleocene Purgatorius and Primates" Proceedings of the Academy of Science, U.S.A. Volume 112 (5): 1487-1492

Van Valen, Leigh and Robert Sloan, 1965, "The earliest primates". Science. 150 (3697): 743–745. <>

I thank Luke Lefebvre for allowing me to use his "Yellow Meme" for the anchor point of this blog entry.

Modern Bird Species in the Cretaceous?  No!

 Introduction:

Don Batten, a writer associated with Creation Ministries International, has been the most vocal advocate of the claim that modern species of birds have been found in rock layers with dinosaurs.  He usually quotes Dr. Carl Werner.  Two examples of this claim are:

"Dr Carl Werner’s book and DVD, Living Fossils, reveals that fossil researchers have found many modern bird remains with dinosaurs..."

"Most people are surprised to learn that many modern bird species have been discovered buried with dinosaur remains: “parrots, owls, penguins, ducks, loons, albatross, cormorants, sandpipers, avocets, etc.” (Batten, Don, “Living Fossils: a Powerful Argument for Creation,” Creation 33 (2), 2011.) “This symphysis appears to represent the oldest known parrot and is, to my knowledge, the first known fossil of a ‘terrestrial’ modern bird group from the Cretaceous. The existence of this fossil supports the hypothesis, based on molecular divergence data that most or all of the major modern bird groups were present in the Cretaceous.” (Stidham, Thomas A., Nature 396, 29-30, November 5, 1998.)" [quoted from the webside "Genesis Park"]

What is important to notice here is that Dr. Werner never says that modern species of birds are found buried with the dinosaurs.  He says "modern birds" or "modern groups of birds".  Werner is hoping that his audience won't know that there is a difference between a modern group of birds, and a modern species of bird.  A modern species of bird is one which is living today, such as the Red-crowned Amazon Parrot, (Amazona viridigenalis), native to Mexico.  The modern group is the Order Psittaciformes, which has almost 400 species in 92 genera living today, but includes all the fossil relatives of the Order, going back perhaps to the Late Cretaceous 70 million years ago, if Stidham's fossil is confirmed to be a parrot.  Modern species are limited to the present day, or perhaps a few million years into the past.  No modern species is known from the Oligocene, Eocene or Paleocene, and certainly not from the Cretaceous.  But most of the modern orders of birds  are well known back to the beginning of the Paleocene, some 65 million years ago, meaning that they evolved in the Late Cretaceous, or earlier.

Analysis of Werner's claims:

We can take Werner's claims and check the facts.  We have to make some assumptions about what he meant, since the "modern birds" he mentions are not all of equivalent rank in the scientific classification, but I've tried to do so to mirror what I think was Werner's intention.  I've given examples of modern species which would fit within Werner's group name, and indicated the family or order to which they belong.  Next is the age of the earliest known fossil from that family or order, and the fossil species upon which it is based.

Werner's name	Living example common name	Living species name	Order or Family	Age of earliest known fossil	Fossil species
parrot	Amazon Parrot	Amazona viridigenalis	Psittaciformes	Late Cretaceous	Unnamed but described
owl	Great Horned Owl	Bubo virginianus	Strigiformes	Paleocene	Berruornis, Ogygoptynx Supposed Cretaceous owls are non-avialian dinosaurs
penguin	Magellan's Penguin	Spheniscus magellanicus	Sphenisciformes	Early Paleocene	Waimanu manneringi
duck	Mallard	Anas platyrhynchos	Anatidae	Late Eocene	Romanvillia sp.
loon	Common Loon	Gavia immer	Gaviformes	Late Eocene / Early Oligocene	Colymboides minutus
albatross	Wandering Albatross	Diomedia exulans	Diomedeidae	Middle Eocene	Murunkus subitus
cormorant	Double Crested Cormorant	Phalacrocorax auritus	Palacrocoracidae	Late Cretaceous	Unnamed species in Asia and North America
sandpiper	Sandpipers, curlews and snipe	various	Scolopacidae	Early Eocene	Paractitis bardi
avocet	American Avocet	Recurvirostra americana	Recurvirostridae	Late Eocene / Early Oligocene	Recurvirostra sanctaeneboulae

So where does Werner and the Creationists who parrot him go wrong?  They make three mistakes:

1.  The  Cretaceous and early Tertiary fossil birds are not members of any species living today.  They are not members of any living genus.  Recurvirostra is the sole exception, and one suspects that the very fragmentary fossil, a partial coracoid, is insufficient to support such a generic assignment.  They may be members of families which have modern survivors.

2.  They cherry-pick paleontologists who make statements about a fossil species like "This fossil would have looked very similar to a modern duck".  That does not mean it is from a living species of duck, or that it is even in the same family (Anatidae) or order (Anseriformes) as living ducks.

3.  They use common names with vague definitions rather than give the name of the fossil so that anyone can check their claims.

Conclusion:

  

Cretaceous dinosaurs did not co-exist with any modern bird species.

The Fallacy of the Horizontal Transitional Species

Otherwise known as the fallacy of jumping from bough to bough in the tree of life without climbing down one branch and climbing up the other, the Horizontal Transitional Species Fallacy is one of the most frequently repeated errors in much of Creationism's internet presence.  How many times do we see demands from Creationists to see "one kind giving birth to another kind".  They demand to see a dog turn into a cat, or a cow turn into a whale, or even a crocodile turn into a duck - Kirk Cameron and Ray Comfort's famous Crocoduck, which is what they imagined we would have to produce if evolution was a valid theory.  So bizarre was this claim that famed Internet YouTuber Potholer54 (Peter Hadfield), actually hosted the Golden Crocoduck Awards (2008 - 2013) for the most outlandish Creationist claims of what evolution is about.

In spite of the efforts of almost every supporter of evolutionary biology who participates in various internet venues like Facebook and YouTube, Creationists continue to push this fallacy, for two reasons, I believe.  First is that it is an easy shot - "Show me a cat giving birth to a dog!" - just a few short words, and their base is energized.  Second, it makes those of us on the side of rational thought stop, shake our heads and ask ourselves "Do we really want to go through this AGAIN?". 

There are no transitionals between living species.  Evolution does not predict that there are.  In fact, were a cat to give birth to a dog, evolution would be dealt a serious blow, if not killed outright.  Remembering that modern species are each the product of a lineage of species going back in time, we can use the Tree of Life as a metaphor to demonstrate the irrationality of what Creationists are asking for.  I've attached a graphic, adapted from a slide in Peter Chen's set of graphics for Biology 1151 Principles of Biological Science (found here: bio1151b.nicerweb.net) which illustrates the difference between what Creationists seem to think a transition should be, and what evolution say it actually is.

In this example, a Creationist might ask to see an otter giving birth to a dog, or a dog giving birth to an otter.  This is illustrated on the left hand side of the graphic.  Notice that there are two limbs of the tree involved - the limb we call the family Mustelidae, which consists of the skunks, weasels, otters, badgers and other similar carnivores, and the limb we call the Canidae - the wolf, jackal, fox and others.  Creationists want us to jump from the otter directly to the domestic dog. 

But what evolutionary theory really says is that these two modern animals have a common ancestor that they share with no other living animal.  That common ancestor was a species of the genus Miacis, which lived in the Early Eocene, between 45 and 60 million years ago.  The actual common ancestor may not be a known species or even genus, but Miacis is the closest thing to the common ancestor which we have found, identified and described.  There is a whole series of transitional species between the domestic dog and that species of Miacis which is, or stands in for, the common ancestor with otters.  There is also a whole series of transitional species between the otter and that same species of Miacis.  Thus, as illustrated on the right side of the graphic, to get from the otter to the dog, one must climb down the bough that is the Mustelidae until you get to the common ancestor, and then climb all the way back up a different bough, the Canidae, to get to the domestic dog.

Evolution is a branching pattern, a set of nestled hierarchies.  There are no straight line  horizontal connections between living animals on separate branches.  There are two sets of transitionals, one from the otter to the common ancestor, and one from the domestic dog to the common ancestor.

Thanks to FB member Hal Ucigenia for the wonderful phrase "jumping from bough to bough in the tree of life without climbing down one branch and climbing up the other."

Meet Kolponomos: A Raccoon, or a Bear, or a Seal, or an Otter? 

Reconstruction below by Roman Uchytel

Or to put it more scientifically, is Kolponomos a procyonid, an ursid, a mustelid or a pinniped?

[Second in a series of blog posts entitled "I must go down to the sea again, to the lonely sea and the sky..."]

Kolponomos was first described in 1960 by Rueben A. Stirton, from the Museum of Paleontology and the University of California, Berkeley. He named and described a new genus and species, Kolponomos clallamensis, found in
the sea cliffs of Clallam Bay, Washington, and dating back about 20 million years, to the early Miocene epoch. The fossil was the front part of a skull, with only the roots of three teeth preserved - the left upper third incisor and both upper first molars. (see photo right)

On this meager evidence, Stirton speculated that Kolponomos might belong with the Procyonidae, a family of carnivores which includes the raccoon, coati, and other living and fossil species. Others considered it not surely allied with any modern carnivore family; one paleontologist, Clayton Ray suggested in 1985 that it was related to Enalicarctos, the earliest pinniped.

For more than 30 years Kolponomos was in limbo. Because some scientists had considered it an arctoid, the group containing bears, pinnipeds and the extinct amphicyonidontidae, it became popularly thought of as a bear, and many reconstructions showed it as very similar to a living brown bear.

In 1994, several complete or nearly complete skulls of Kolponomus were described, including one of K. clallmatensis and one of a new species, K. newportensis. [see photos] This more complete material enabled Richard Tedford, from the American Museum of Natural History, Larry Barnes from the Los Angeles County Museum of Natural History, and Clayton Ray, from the National Museum of Natural History (Smithsonian) to propose a new phylogenetic hypothesis: Kolponomos was in fact an arctoid, placed in the family Amphicyonidontidae, but was the sister taxon to all of the pinnipeds (seals, walruses, etc). Kolponomos is close to the ancestry of seals (and all other Pinnipeds), much as Ray had suggested in 1985.

But that isn't the end of the story. A marvelous paper was published in the Proceedings of the Royal Society B in March of 2016, by Jack Tseng, Camile Grohe and John Flynn: "A Unique feeding strategy of the extinct marine mammal Kolponomos: convergence on sabretooths and sea otters". In this paper, they used sophisticated computer modeling of the stress in the jaws of different carnivores, including Kolponomos. They concluded that "Based on mandibular symphyseal morphology shared by Kolponomos and sabre-toothed carnivores, we hypothesize a sabretooth-like mechanism for Kolponomos prey-capture, whereby the mandible functioned as an anchor. Torque generated from jaw closure and head flexion was used to dislodge prey [shellfish] by prying, with prey then crushed using cheek teeth." So while not closely related to either the sabretooth or to the sea-otter, it combines feeding strategies used by each, but combining them in a unique way.

We've got a lot yet to learn about Kolponomos. Almost none of the postcranial skeleton is known, so we don't know what, if any adaptations there are to swimming. It is only a matter of time until the skeleton becomes known. I wonder what surprises await us?

[Thanks to Bobby Boesennecker who provide the color photos of the skull and jaws of Kolponomos.]

The Sea Sloth

"I must go down to the sea again, to the lonely sea and the sky......." (John Masefield)

We all are familiar with the development of whales from terrestrial ancestors, whether or not you accept the theory of evolution. But are you aware that many different mammals made that same transition from a terrestrial to semi-aquatic or fully aquatic way of life? Sirenians (manatees and dugongs), Pinnipeds (seals and walruses), sloths (yes, sloths!) Mustelids (the sea otter and the sea mink), Ursids (the polar bear) and a number of fossil forms which have no direct living descendants (such as the amphicynodont Kolponomos)  all returned to the sea..

This will be the first of a series of blog posts which will introduce you to these creatures, and present what we know of their evolutionary history.

Probably everybody is familiar with the living tree sloths, of which there are 6 species in two genera.  Many of you will know that the fossil relatives of the modern tree sloths were the very large, sometimes gigantic, ground sloths of South, Central and North America. The group evolved in South America, with representatives of two lineages making their way into Central and North America, probably by rafting, in the Miocene, but most of them arriving during the Pliocene and Pleistocene, beginning about 2-4 million years ago. Huge, slow moving, lumbering giants which are hard to imagine as an animal likely to take to the sea.

In the late 1980s and early 1990s fossils of sloths began to be found in the near-shore marine Pisco formation in Peru. At first they were considered to be rare individuals of ground sloths which had died and been washed into the sea. But as field work progressed, it turned out that numerous skulls and complete skeletons were found.

In 1995, in an important paper published in the prestigious journal Nature, authors Christian de Muizon, from the Institut Francais d'Etudes Andines and H. Gregory McDonald, then a paleontologist at Hagerman Fossil Beds National Monument in Idaho, described the first species of this new sloth, which they named Thalassocnos natans, meaning "the swimming sea-sloth". From several lines of evidence, including their abundance in near-shore marine deposits and the absence of any other terrestrial mammals in the deposits, as well as the fact that the coast of Peru was a desert during the Pliocene , they concluded that Thalassocnos lived along the shore and entered the sea to feed on vegetation, most likely the abundant sea-grasses and sea weeds, which are represented by fossil remains.
The sea sloth is by far the most abundant mammal in the deposits exceeding the number of either seals or dolphins, both of which are very common. Between the original description in 1995 and 2005, four more species of Thalassocnos were described, making up what is most likely a single lineage of 5 ancestor - descendant species. The swimming sea-sloths are the most closely dated series of fossil species documenting the re-invasion of the sea by terrestrial mammals currently known.

The skulls and jaws underwent a progression of changes as the muzzle, both upper and lower jaw, became better adapted to grazing on sea-grasses and seaweeds.

Subsequent research focused on the postcranial skeleton has revealed evolutionary changes in the forelimb which better adapt the sea-sloth to a power stroke for swimming, as well as changes in bone density of all the bones to give the animal neutral buoyancy.

References:

Muizon, C. de and H. G. McDonald, 1995 An Aquatic Sloth from the Pliocene of Peru, Nature, Volume 375:224-227.
McDonald, H. G. and C. de Muizon, 2002, The cranial anatomy of Thalassocnos natans (Xenarthra, Mammalia), a derived nothrothere from he Neogene of he Pisco Formation, Peru. Journal of Vertebrate Paleontology, 22(2)340-365.
Muizon, C. de, H. G. McDonald, R. Salas and M. Urbina, 2003. A new early species of the aquatic sloth Thalassocnos (Mammalia, Xenarthra) from the Late Miocene of Peru. Journal of Vertebrate Paleontology, 23 (4): 886-894.
Muizon, C. de, H. G. McDonald, R. Salas and M. Urbina, 2004. The youngest species of the aquatic sloth Thalassocnos and a reassesment of the relationships of the nothrothere sloths (Mammalia, Xenarthra).
Journal of Vertebrate Paleontology, 24(2):387-397.
Muizon, C. de, H. G. McDonald, R. Salas and M. Urbina, 2004. The evolution of feeding adaptations of the aquatic sloth Thalassocnos. Journal of Vertebrate Paleontology, 24(2):398-410.
Canto J, R. Salas-Gismondi, M. Cozzuol, J. Yáñez, 2008. The aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the late Miocene of north-central Chile: biogeographic and ecological implications. Journal of Vertebrate Paleontology, 28:918–922.
Amson, E., C. Argot, H.G. McDonald and C. de Muizon, 2014. Osteology and functional forphology of the forelimb of the marine sloth Thalassocnos (Mammalia, Tardigrada). Journal of Mammalian Evolution 22(2):169-242.
Amson E, C. de Muizon, M. Laurin, C. Argot, V. de Buffrénil, 2014. Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru. Proc R Soc B 281:20140192.

The Sea Mink: We hardly knew ye. 5,000 BP to 1860/1890

[Third in a series of blog posts entitled "I must go down to the sea again, to the lonely sea and the sky..."]

The Sea Mink of the coastal islands off Maine (and Canada) is an example of a mammal that appears to have invaded the sea sometime around 10,000 years ago to travel to islands of the coast of Maine. It flourished there until about 1860 when intensive hunting resulted in its extinction. No specimens were collected and placed in museums, and almost nothing is known about it.

The sea mink came to the attention of the scientific world in 1903, when Daniel Webster Prentiss published a short article in the Proceedings of the U. S. National museum entitled "Description of an Extinct Mink from the Shell-Heaps of the Maine Coast". In it he described a single specimen from a shell midden at Brooklin, Hancock County, Maine. It was very similar to the skull of the mink, Mustela vison, but significantly larger. Prentiss named it Lutreola macrodon. Prentiss did not speculate on its ecology.

Frederick B. Loomis, then at Amherst, described much more abundant remains of the giant mink in 1911 in the American Journal of Science. These were recovered from a shell heap on Flagg Island, Casco Bay, Maine. Loomis had the remains of at least 45 individuals available to him for study, including 34 lower jaws of males and 11 of females. Apparently unaware of the prior description of Lutreola macrodon by Prentiss 8 years earlier, he named the sea mink as a new subspecies of the mink  - Lutreola vison antiquus. Loomis also did not speculate of the biology of these animals beyond noting their large size, although his illustration [see below] does document the appreciable sexual dimorphism in this species.

The first extensive modern treatment of the sea mink was by Richard H. Manville, then Director of the Bird and Mammal Laboratories of the US Fish and Wildlife Service, and a research associate at the Smithsonian. In 1966, he published a description of all specimens known by then, which included many not available when Loomis published. He summarized data published by others, and provided previously unpublished historical data from hunters documenting how the sea mink was hunted. One hunter said that he had seen no specimens of the sea mink after about 1860; another said he had encountered them up until about 1880.

Manville speculated on the natural history of the sea mink, noting that "because of its large size, [it] was pursued avidly for its pelt. Its seashore habitat rendered it relatively easy to capture. Overly zealous hunting, and possibly other factors of which we are unaware, led to its diminution and, ultimately, to its complete replacement by other, smaller forms of mink..... All the evidence indicates to me that the sea mink is most realistically considered as a subspecies, now extinct, of the prevalent mink, Mustela vison, of today."

Jim Mead, then at Northern Arizona University, with two co-authors, published a review of the sea mink in 2000, validating it as a species, Mustela macrodon. Work published that same year, 2000, separated both Mustela vison and Mustela macrodon into a new genus, Neovison. The sea mink is now Neovison macrodon.

Some workers continued to question whether mere size difference was enough to justify a separate species for the sea mink, including Russell Graham in 2001. Rebecca A. Sealfon published a detailed analysis of the teeth of the sea mink in 2007, showing that there were significant differences in the shape of the teeth from the mink to support the sea mink's status as a separate species. Unfortunately, Sealfon did not include any specimens of Neovison vison ingens, the Alaskan form to which the sea mink is likely most closely related, in her analysis.

Even today, there has been very little published concerning the diet and behavior of the sea mink. It is said to have inhabited rock crevices on the islands and to have been very easy to hunt - and was done so to extinction. Several workers have suggested that it ate fish and marine invertebrates (shellfish). Sealfon in her 2007 study showed that the sea mink had smaller cutting/shearing ability, and greater crushing ability, in its teeth as compared to the American mink, supporting this hypothesis.

The only subspecies of the brown mink which is as large as the sea mink is Neovison vison ingens, from Alaska. Mead speculated that "The Sea Mink is a relict of a M. vison ingens, or ingens-like, form that existed along the fringes of the late Pleistocene continental glaciers, of North America. Post glacial climate has restricted this "larger than normal" form to Alaska-Yukon and led to a North Atlantic isolate that contrasts with other members of the genus."

A claimed taxidermy mount of the sea mink has been shown to be a large individual of the brown mink.

There is much we do not know about the sea mink. We may never know. But the sea mink was a terrestrial carnivore which successfully invaded the sea, and as such, can serve as a model of how animals such as seals and sloths may have begun their marine lives.