Flowering of the Dogwood

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Image credit: Curious Sengi.

Blooming of the dogwood trees is a portentious sign that spring, at long last, is finally here to stay.

The tetrad of creamy white or pink-petaled flowers are not actually flowers at all, but specialized leaves called bracts.  Bracts are usually found as associated supports to flowers and other reproductive structures.  The true flowers are at the center of this arrangement and unfurl as a cluster of tiny yellow blossoms.  In the early autumn, these clusters transform into bright red drupe fruit.

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Image credit: Curious Sengi.

Within the United States, you are most likely to see a cultivar of Cornus florida.  This popular ornamental tree has a native distribution throughout the eastern part of the country from Maine to Florida.  Dogwoods are shade-tolerant denizens of the understory that grow interspersed with other trees forming the canopy above them.  Despite this modest standing, dogwoods are an important component of the local ecosystem.  Their leaves decompose rapidly, releasing mineral nutrients and improving the neighboring soil.  Dogwood fruits, flowers, leaves, and bark are all particularly rich in calcium and fat for the animals that feed on them.  The fruit alone are known to feed over 30 bird species and many mammals.  The wood is hard and smooth-grained, making it desireable as a fine craft wood and for making tool parts subject to heavy usage.

Cedar Waxwing (Bombycilla cedrorum) enjoying some fruit of the Flowering Dogwood (Cornus florida). Image credit: Gypsy Flores via Birds & Blooms.

Cedar Waxwing (Bombycilla cedrorum) enjoying some fruit of the Flowering Dogwood (Cornus florida). Image credit: Gypsy Flores via Birds & Blooms.

The evolutionary history of the whole dogwood clade was a biogeographical mystery, an unknown saga of worldwide meanderings now reconstructed from molecular, morphological, and fossil data.  There are about 58 species in the genus Cornus today and they are distributed in temperate and subtropical regions of North America, Europe, Africa, East Asia, and a sole species outposted in South America.  Earlier studies had suggested an Asian origin for dogwoods, but a recent study incorporating fossil material point towards a European birthplace.  Dogwoods arose in Europe during the Early Paleocene or Late Cretaceous, around 65 million years ago, and within a span of about 10 to 20 million years, the trees made multiple trans-Atlantic dispersals to North America.  The lone South American species was a result of migration from original colonizations in the north.  Though Europe and North America were much closer together during the Paleocene, when the young Atlantic ocean had yet to push the two continents away as distant as they are now, the exact mechanism of how dogwoods could travel over the salt waters has yet to be explained.

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Image credit: Curious Sengi.

 

References

McLemore, B.F.  1990.  “Cornus florida L.  Flowering Dogwood.”  In Silvics of North America:  Volume 2, Hardwoods.  United States Department of Agriculture.  Pp. 278 – 283.

Xiang, Q.-Y. et al.  2006.  “Species level phylogeny of the genus Cornus (Cornaceae) based on molecular and morphological evidence — implications for taxonomy and Tertiary intercontinental migration.”  Taxon 55 (1):  9 – 30.

Origins of an Idea: Stupid Stegosaurus Needs A Second Brain

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Image Credit: Gary Larson via Lapidary Apothegms.

In 1930, English geologist John Parkinson described a species of stegosaur in less than savory terms:

The reptile. . . .belonged to the most uncouth group of all the stegosauria or armoured dinosaurs. . . . Stupidity and slowness seemed to be stamped on every bone of the beast.

One detail, common to the stegosaur type of dinosaur. . .  is the expansion of the canal, which carries the great nerve (the neural cord) from the brain through the arch of the vertebrae, to the sacral or hip region, an expansion so enormous that the enclosed nerve matter exceeded in size that of the brain itself. . . .

In fact, the reptile had two brains. . . . [and the hinder one] looked after the functions of nutrition, digestion, and propagation, practically all that life’s daily routine required in a stegosaur.

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Stegosaurus is a sizable beast, with the largest individuals of certain species growing to nearly 30 ft (9 meters) in length. In comparison to this enormous bulk, the head does appear ludicrously tiny in proportion. Image credit: Yale Peabody Museum / Curious Sengi.

Of all the dinosaurs known to us, Stegosaurus seems to be singled out for being particularly stupid.  So stupid, in fact, that this iconic beast required a second brain in its hip to muddle through life.  It is a strange snippet of information that continues to persist. . . . . so where did this idea come from?

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The cranial cavity that once contained the brain is colored red in this drawing of a cross-sectioned stegosaur skull.  Marsh noted the general elongate shape of the brain, enlarged optic lobes suggesting the relative importance of vision in these animals, and small cerebral hemispheres.  Image credit: public domain via Wikimedia Commons.

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Original collection labels for some of the Stegosaurus fossil material collected for O.C. Marsh.  Note the label on the right describing a natural brain cast, or endocast.  Image credit: Yale Peabody Museum / Curious Sengi.

The first stegosaurs were discovered from Late Jurassic (~150 ma) rocks in Colorado by paleontologist O.C. Marsh (1831 – 1899).  If not otherwise preoccupied with his role as co-villain in the infamous Bone Wars, Marsh ventured into new realms of inquiry including the study of endocasts, which are molds of internal, hollow spaces — in this case, spaces in the skull or vertebrae once occupied by the brain and other nervous system structures.  Looking at these ancient brains not only gave a general sense of their size, but could also reveal finer details indicating heightened development of certain senses such as vision or olfaction.  Marsh found at least one natural fossil endocast of a stegosaur brain.  Based on some quick calculations, he remarked that if a stegosaur and a modern alligator were scaled to the same body size, the stegosaur brain would be 1/100th the size of the alligator’s.  This led to the grim conclusion that:  “Stegosaurus had thus one of the smallest brains of any known land vertebrate (Marsh 1896).”

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Sowing the seeds of suspicion:  comparatively massive white plaster cast of the stegosaur sacral cavity in the background with a natural stone endocast of the brain cavity in the foreground.  The plaster cast captures the enlargement of the spinal cord as it passed through the sacral (i.e., pelvic) vertebrae.  Marsh believed the sacral cavity housed a secondary neural center that was an estimated 20 times larger than the brain.  U.S. quarter for scale.  Image credit: Yale Peabody Museum / Curious Sengi.

To make matters worse, Marsh made an artificial endocast in plaster of the sacral cavity, i.e., the hollow passage for the spinal cord in the vertebrae that are fused to the pelvis.  This cavity was unusually large, more so than would be expected.  While it was observed from a wide variety of vertebrates that the spinal cord bulks up where it sends off nerve branches to innervate the fore- and hindlimbs, the stegosaur’s enlarged neural cavity with its intervertebral bulges was exceptional.  The sacral cavity was estimated to be at least 20 times the size of the brain.

Marsh Stegosaurus Sacrum Plate

One of the plates used in Marsh’s description of Stegosaurus from the American West.  The top figures (2 & 3) show views of the plaster cast made of the sacral cavity, complete with the sideways bulging into the intervertebral space (labeled f, f’, and f”).  Figure 4 shows a schematic cross section of the relative size of the brain and sacral cavity.  Image credit: Marsh 1896.

Given this vaguely brain-shaped hollow and, more interestingly, a particular pattern of growth in size between juveniles and adults, Marsh described the sacral cavity as a “posterior brain case” and “a posterior nervous center” — thus, the notion of a Stegosaurus butt brain was born.

Marsh seemed content with equating the presence of a secondary nerve center with a “. . . . posterior part that was dominant,” which one could imagine referred to the great weight borne upon the hindlegs and the coordination necessary to swing the stegosaur’s spiked tail against foolhardy predators or rivals.  German scientists Branca and Waldeyer went so far as to ascribe a certain independence in function, affirming that this was indeed a proper “sacral brain” delegated to back-half duties:  digestion and sex.

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A set of stegosaur sacral vertebrate.  The hollow through the middle shows the opening into the sacral cavity.  This specimen was also collected for Marsh.  Image credit: Yale Peabody Museum / Curious Sengi.

The role of intelligence — as interpreted through brain size alone — in the evolutionary succession of life on Earth was a particular obsession of the late 19th and early 20th century Western world.  The discord between the gargantuan body size of the dinosaurs and their pathetically small brains captured the public imagination.  It seemed like an obvious rationale for why dinosaurs went extinct.

More recent scientific studies have revisited the question of stegosaur brains and the results are much more nuanced.

Extrapolation from data collected from living reptiles showed that the vast majority of dinosaurs seemed to fall within the expected range of brain size to body mass proportions.  While the stegosaur brain remained somewhat below the expected prediction, its size was consistent with large animals enjoying an undemanding “slow, herbivorous lifestyle (Buchholtz 2012).”  Even if Stegosaurus was not catastrophically stupid, what about that “second brain”?

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There was nothing bright going on the sacral cavity.  Even though paleontologists no longer accept the idea of a secondary brain, what occupied this space along with the spinal cord remains inconclusive.  Lateral view of sacral vertebrae with the internal cavity illuminated through the intervertebral spaces.  Image credit: Yale Peabody Museum / Curious Sengi.

Paleontologists today emphatically reject the notion of a “sacral brain.”  There are a number of hypotheses proposed for why stegosaurs possessed such enlarged sacral cavities, the most popular of which is that this space housed an agglomeration of tissue called a glycogen body.  Glycogen bodies are only definitively found in the sacral cavities of modern birds and consist of carbohydrate-rich cells.  While this structure has not been extensively studied, it is believed to provide metabolic support for the central nervous system, especially during the formation of myelin, a fatty layer of cells that insulate nerve fibers.  As living descendants of the dinosaur lineage, it seemed plausible that the avian glycogen body is homologous to mysterious item occupying the stegosaur’s pelvis.

Chicken Glycogen Body

The avian glycogen body is an oval-shaped mass on the spinal cord.  The function of this structure is not fully understood, but is probably involved in the metabolism of nervous system tissues.  Image credit: De Gennaro 1982.

It should be noted that sacral enlargements for a putative glycogen body are found only in stegosaurs and another heavy herbivore, those long-necked titans:  the sauropods.  However, neither are stegosaurs and sauropods closely related to each other, nor are they closely related to birds.  Dinosaurs that are in the direct line to modern birds, such as coelurosaurs, lack any kind of enlargement of the sacral cavity.  In the end, the question of why Stegosaurus had such a large hollow in its hip remains a dissatisfying enigma.  But, if anything, it definitely was not a brain with an independent agency as Branca and Waldeyer believed.

Stegosaurs were certainly not very bright, but they did quite well for several million years.  And that is considerably longer than the approximately 200,000 year history of anatomically modern humans and the current future we have set upon.

Preview of the new "Body Worlds" exhibit that will open at the Denver Museum of Nature and Science. Visit to museum on Tuesday, March 9, 2010. Central and peripheral nervous system. Cyrus McCrimmon, The Denver Post

Plastinated specimen of the human nervous system.  Note the thickening of the spinal cord where nerves for the arms and legs branch off.  This is the usual pattern in vertebrates.  From the “Body Worlds” 2010 exhibit at the Denver Museum of Nature and Science.  Image Credit:  Cyrus McCrimmon / The Denver Post.

 

References

Benzo, C.A. & L.D. De Gennaro.  1983.  “An Hypothesis of Function for the Avian Glycogen Body:  A Novel Role for Glycogen in the Central Nervous System.”  Medical Hypotheses 10:  69 – 76.

Buchholtz, Emily.  2012.  “Dinosaur Paleoneurology.”  In The Complete Dinosaur. 2nd edition.  M.K. Brett-Surman, T.R. Holtz, Jr., & J.O. Farlow, editors.  Indiana University Press.  Pp. 191 – 208.

De Gennaro, Louis D.  1982.  “Chapter 6:  The Glycogen Body.”  In Avian Biology, Vol. VI.  D.S. Farner, J.R. King, & K.C. Parkes, editors.  Academic Press.  Pp. 341 – 372.

Galton, Peter M. & P. Upchurch.  “Chapter Sixteen:  Stegosauria.”  In The Dinosauria, 2nd edition.  2004.  D.B. Weishampel, P. Dodson, & H. Osmólska, eds.  University of California Press.  Pp. 343 – 362.

Griffin, Emily.  1990.  “Gross Spinal Anatomy and Limb Use in Living and Fossil Reptiles.”  Paleobiology 16 (4):  448 – 458.

Lull, Richard Swann.  1917.  “On the Functions of the ‘Sacral Brain’ in Dinosaurs.”  American Journal of Science 44:  471 – 477.

Marsh, O.C.  1896.  “The Dinosaurs of North America.”  Sixteenth Annual Report of the U.S. Geological Survey, Pt. I:  133 – 244.

Parkinson, John.  1930.  The Dinosaur in East Africa:  An Account of the Giant Reptile Beds of Tendaguru, Tanganyika Territory.  H.F. & G. Witherby.

Notes from the Field No. 4: Darwin’s Remedy

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Painting of Botafogo Bay, Rio de Janeiro, Brazil by Augustus Earle (c. 1783 – 1838).  Earle was an artist specializing in exotic scenes captured during his travels.  He was briefly a part of the HMS Beagle crew alongside Darwin.  Image credit: newtonsapple.org.uk.

Charles Darwin (1809 – 1882) landed in Rio de Janeiro in the spring of 1832 during the first year of the HMS Beagle voyage.  Traveling by boat made Darwin horribly seasick, but the tropical jungles were full of potential health dangers — yet another source of anxiety.  Throughout his adult life, Darwin’s health was a complicated flux of chronic symptoms with perhaps even a tendency towards hypochondria.   Despite these difficulties, he still kept his eye on the natural world and recorded his observations.

11 April 1832

Passed through several leagues of forest. very impervious trees not large: I here first began to feel feverish shivering & sickness. much exhausted: could eat nothing at one oclock which was the first time I got anything. — travelled till dark: miserably faint & trouble with faintness.

At night we slept 2 miles S of Marica: felt very ill in the course of day I thought I should have dropt off the horse: horrors of illness in foreign country: during the morning C Frio appearing from refraction like inverted tumblers. Gneiss dipping to the South (& then the north).

12 April 1832

Started in the morning & doubted whether I could proceed. — Cinnamon & port wine cured me

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Life-size figure of a young Darwin on display in Sao Paulo, Brazil.  Honestly, he still looks pretty queasy.  He could also be completely bummed out because upon arriving in Rio de Janeiro, he received letters informing him that his sweetheart married another man just days after Darwin left England.  Image credit: Rodrigo Barbassa via The Dispersal of Darwin.

 

Quotation Source

The Complete Work of Charles Darwin Online: Rio notebook.  John van Wyhe, editor.  2002.  http://darwin-online.org.uk/

 

Hands Behind the Masterpiece: Audubon’s “Birds of America”

Lush and beautiful.  Dynamic.  Faintly fragrant with the mystery and romance of the man who created it.  This is the enormous double elephant folio, Birds of America, by artist, naturalist, and outdoorsman John James Audubon (1785 – 1851).

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“Blue Grosbeak,” Image credit: Yale Peabody Museum / Beinecke Library / Curious Sengi.

In order to realize his ambition to publish illustrations of every living bird species in America, Audubon was forced to leave the depths of a frontier wilderness and the nascent cities of a new republic.  In Britain, Audubon had a chance to find wealthy subscribers to fund and talented printers to produce his masterwork.  Birds of America was ultimately entrusted to the London engraver Robert Havell, Jr. (1793 – 1878).  The project encompassed the production of 435 hand-colored plates with text and accompanying figures, printed in four volumes in double elephant folio size:  26 ½ by 39 ½ inches.  This endeavor would take a team of fifty men over fourteen years to produce.

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Color plates were generated in a multi-step process that involved engraving a copy of Audubon’s original watercolor image onto a copper plate, printing, and hand-coloring.   Work on the 400+ plates began in 1827 with the illustration of a Wild Turkey and was finished in 1838.  Image credit: Yale Peabody Museum / Curious Sengi.

Of the 180 original copies of the double elephant folio, about 110 still survive today and are sought out as great treasures.  In 2010, Sotheby’s in London broke the record for any printed work when a copy was auctioned to a bird-loving art dealer for £7.3 million ($11.5 million).  In contrast, a Shakespeare 1623 First Folio sold for a paltry £1.5 million at the same auction.

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The Hooping Crane [sic]. Image credit: Yale Peabody Museum / Beinecke Library / Curious Sengi.

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Havell’s engraving were based upon watercolors like this one that Audubon provided.  The lively gestures and life-like poses characteristic of Audubon’s work is attributed to his method of using freshly shot birds supported by wire armatures.  Image credit:  New York Historical Society.

As cherished and celebrated as the folio volumes are, the original copper engraved plates have met with an extraordinary history of their own.  After Birds of America was printed in London, the plates were shipped to New York and stored in a warehouse which burned down in 1845, damaging a good number of them.  After that episode, the plates were stored in a special fire-proof storage vault Audubon had constructed on his property.  By 1869, Audubon’s impoverished widow, Lucy, was forced to sell the plates as mere scrap metal to Ansonia Brass & Copper Company in Connecticut.

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Copper plate engravings, such as this one of American Scoter Ducks, were made by Robert Havell, Jr. of London.  He and his father were both printer/artists who specialized in natural history images.  Havell would later leave London and move to upstate New York, where he is buried in Sleepy Hollow Cemetery.

Not all the copper plates were melted down, however.  Charles Cowles recounts the extraordinary story:

At that time I was about fourteen years old.  I was beginning the study of taxidermy and was naturally deeply interested in birds.  I happened to be at the refinery watching the process of loading one of the furnaces, and noticed on one of the sheets of copper that a man was throwing into the furnace, what appeared to me to be a picture of a bird’s foot.  I took the plate from him, cleaned it with acid, and thereupon discovered the engraving [of the Black Vulture]. . . . I appealed to the superintendent, but without avail.  I next brought the matter to. . . . my father, from whom I received no encouragement. . . . I appealed to my mother and interested her to such an extent that she drove to the factory and looked at one of the plates.  She of course recognized that they were Audubon plates; and instructions were given by my father to keep them intact.  (Quoted from Deane 1908.)

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Could this be the very same Black Vulture plate snatched from the furnace by an astute teenage boy? One shudders at the thought of what could have been lost.  Image credit: Yale Peabody Museum / Curious Sengi.

The surviving plates were then distributed to various museums, universities, and individuals.  The clever young Cowles talks of two plates that “. . . particularly struck my fancy, so much so that when the plates were first discovered I managed to secure them on the quiet, cleaned them myself and hid then; and when the plates were distributed no one knew of the existence of these two and they later became my property (quoted from Deane 1908).”

Approximately 80 plates survive today.  In 1985, to celebrate the 200th anniversary of Birds of America, a handful of plates in the care of the American Museum of Natural History in New York City were taken back to London for restoration and a special limited edition of reprints, which were quickly snapped up by wealthy enthusiasts.  But this masterpiece has not been completely sequestered by private collectors.  A number of copies of the double elephant folio are on public display at various universities and museums.

Read, view, and download high-resolution images from Birds of America via the Audubon Society webpage.

Images from this post were taken at the “Audubon and the Double Elephant Folio” exhibit at the Yale Peabody Museum.

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The double elephant folio is huge!  These dimensions were adopted according to Audubon’s plan to have each bird depicted at life size.  Image credit:  Yale Peabody Museum / Beinecke Library / Curious Sengi.

 

References

Deane, Ruthven.  1908.  “The Copper-Plates of the Folio Edition of Audubon’s ‘Birds of America,’ with a Brief Sketch of the Engravers.”  The Auk 25 (4):  401 – 413.

Hart-Davis, Duff.  2005.  Audubon’s Elephant:  America’s Greatest Naturalist and the Making of the “Birds of America.”  New York:  Henry Holt and Company, LLC.

Reyburn, Scott.  7 December 2010.  “‘Birds of America’ Book Fetches Record $11.5 Million.”  Bloomberg.  Accessed 1 May 2016.

Thomas, Michael.  2006.  “The extraordinary tale of an eight point eight million dollar book.”  In Consuming Books:  The Marketing and Consumption of Literature.  S. Brown, ed.  London:  Routledge.  Pp. 32 – 45.

 

Figures: Origins of a Stem Hero

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Image credit: Brower & Veinus 1974.

Millions of years before Peter Parker, the stem hero Eurypterid Man came into glorious being, only to meet an immediate tragic end.

 

About This Image

What on earth is this figure doing in a scientific paper?  And what does it mean?!?

This figure, appearing in the journal Mathematical Geology, is a playful visual metaphor for two different statistical approaches to analyzing complex data sets.  The statistical zap utilizes a single, targeted method of analysis that goes “straight to the heart” to reveal the underlying structure within a data set, but has the distinct potential of missing the mark entirely and yielding no useful information.  Instead, the authors advocate a multivariate shotgun approach that “literally overwhelms the target data and blasts it into oblivion.”  Brower and Veinus used the example of ontogenetic changes in a species of eurypterid, demonstrating that as the animal grows, it not only grows in overall size, but the relationship between different proportions and structures (e.g., eyes) changes in biologically relevant ways.  Such subtlety can be gleaned from the data using the shotgun approach.

Eurypterids are a group of extinct aquatic arthropods that are known to have existed from the Middle Ordovician (~460 mya) to the Permian (~260 mya).  These sleek predators are casually called “sea scorpions,” despite only a distant relationship with true modern scorpions.  Eurypterids belong to a larger group called Chelicerata, which includes arthropods such as horseshoe crabs, daddy longlegs (harvestmen), mites, scorpions, and spiders.  Since eurypterids are an extinct group within Chelicerates with no living descendants, they are a stem group.

The zapped and shotgunned eurypterid cartoon is no Marvel Comic, but  “. . . the senior author acknowledges son Jeffrey for the loan of his comic book collection in connection with [this figure].”

Special thanks to James Lamsdell for suggesting and providing this figure.

Image Source

Brower, James C. & J. Veinus.  1974.  “The Statistical Zap versus the Shotgun Approach.”  Mathematical Geology 6 (4):  311 – 332.

References

Lamsdell, James C. et al.  2015.  “The oldest described eurypterid:  a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa.”  BMC Evolutionary Biology 15:  169.

Iron Shrew: Sorex cinereus & Blarina brevicauda

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Small, but fierce:  Sorex cinereus next to the eye of a needle. Image credit: Yale Peabody Museum / Curious Sengi.

The tiny shrew is a fearsome creature, a ravenous beast constantly seeking out prey to fuel its metabolic fire.  It seems appropriate that the teeth of this assiduous hunter are a striking blood red.

The red comes from a pigment deposited in the enamel.  Tooth color has been traditionally used to distinguish between two subfamily groups of shrews, the red-toothed Soricinae and the white-toothed Crocidurinae.  Soricine shrews are a prolific group of insectivores with over 260 species spread throughout the world.  If there is a patch of well-watered greenery near you, it is likely that a shrew is on the hunt tonight.

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A feisty little conspiracy of shrews, Blarina brevicauda, gathered in a museum drawer.  Image credit: Yale Peabody Museum / Curious Sengi.

Pigmented teeth of various colors have been reported in different species of cyprinid fish (carps, minnows), lungfish, salamanders, frog (a fossil from the Eocene, 36 – 40 million years old), snakes, lizards, extinct early mammals, rodents, and shrews.  While the red teeth of soricine shrews have been undoubtedly known for centuries, only in recent years have scientists been able to reveal the details though nano-scale structural and chemical analyses.

The red is a result of an ultrafine-grained iron oxide called magnetite (Fe3O4) concentrated in the first 40 microns of the tooth enamel.  Magnetite was deposited during tooth development by ameloblasts, cells that secrete the protein precursors to enamel.

Shrew Tooth Cross Section Micro

Cross-section of a tooth from B. brevicauda imaged under a microscope.  Pigmentation is localized in the outer aprismatic layer of enamel.  Teeth have a hard cover of enamel composed of a crystalline form of calcium in a mineral called hydroxyapatite; the red-colored magnetite is distributed around those crystals of hydroxyapatite.  Dentine is a softer mineralized substance containing a higher percentage of organic materials.  Image credit:  Dumont et al. 2014.

The obvious question is why soricine shrews should have red, iron-containing teeth.  The familiar properties of iron would suggest that the presence of this material would make teeth stronger.  Scientists have proposed a number of hypotheses for the functional adaptation of having iron pigment in teeth such as increasing the hardness of enamel; increasing resistance to acids; increasing resistance to abrasive wear; reducing catastrophic cracking of teeth; and generating different wear rates in regions of the tooth to maintain sharp cutting edges.

A functional study by Strait & Smith (2006) on the Short-Tailed Shrew (Blarina brevicauda) found higher densities of iron in teeth — and individual tooth cusps — that were subject to more mechanical stress and consequent wear and fracture.  They found a “functionally significant pattern” in the molars of these shrews, where pigmented enamel showed the highest concentration of iron on the crushing and grinding cusps (the hypoconid and entoconid of the talonid, if you are into that sort of thing).  In contrast, the shearing cusps of those same molars had relatively low concentrations of iron.  In addition, of the three molars, the anteriormost first molar (m1) was the largest in size and the most iron-rich.  Usually, molars tend to increase in size from front to back.  The unexpected inversion of this pattern might be due to opportunistic omnivory in shrews.  These tiny mammals often take down large prey items.  In order to scarf down and begin chewing prey that exceeds the narrow gape of their jaws, the primary chewing surface was displaced forwards.  To deal with the challenge of the initial breakdown of prey, the first molar is hypothesized to have increased in size and strength.

Shrew Teeth Cusps

Illustration of the left lower jaw of the Short-Tailed Shrew, B. brevicauda,with (a) showing the extent of pigmentation in lateral view of the large procumbent incisor, small canine, single premolar, and three molars and (b) showing the same tooth row in occlusal (chewing) view with cusps labeled.  Iron concentrations were highest in the hypoconid and entoconid, least in the metaconid and paraconid.  Image credit: Strait & Smith 2006.

In terms of the material performance, Dumont et al. (2014) performed nano-scale indentation tests showing that pigmented enamel is harder than unpigmented enamel by an average of 30%.  Despite this seemingly large difference in hardness, this is well within the range observed in other mammalian teeth, meaning that the presence of iron pigment does not bestow any special super-shrew powers of tooth hardness.  Ultimately, the question of how the inclusion of iron pigments in teeth translates into strength is still unresolved.  The lack of conclusive evidence comes from the inherent complexity of studying biological material properties.  Teeth are surprisingly sophisticated composite structures and it is not always easy to predict how they will behave under different stresses and strains.

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Skulls of three individuals of B. brevicauda.  Note the patterns of pigment deposition on the occlusal (chewing) surfaces of the molars as well as the variation in color.  There is some evidence that the extent of pigmentation, especially of the incisors, is an indicator of the age of the animal, with lesser pigmentation suggestive of more wear and greater age.  Image credit: Yale Peabody Museum / Curious Sengi.

The essence of shrew life is eating.  Regardless of our human struggle to understand how iron oxide pigments confer strength to teeth, shrews are definitely consuming large amounts of tough and highly abrasive items such as insects, snails, and earthworms full of grit.  They consume anywhere from 80 – 125% of their body weight of this stuff, so the critical importance of controlling tooth wear is real.  Shrews grow and then lose their first set of teeth in utero, thus entering the world with only one set of teeth to last a lifetime.  There are anecdotal reports of adult shrews with worn down dentition being found out in the open, just dropping dead the moment their engines ran out of fuel.

This frenetic need to eat might explain why there are red-toothed soricine shrews and white-toothed crocidurine shrews.  There is plenty of circumstantial evidence that red iron pigments give soricine shrews an edge in the metabolic race.  Soricine shrews have much higher energy demands than crocidurine shrews, which might be related to their respective northern and tropical climate distributions.  It has also been observed that soricine shrews are specialized to support a fast, efficient metabolism:  higher chewing rates, more efficient reduction of food into small particles readily broken down by digestive enzymes, and jaw configurations allowing for greater variety and flexibility of movement compared to crocidurine shrews.  It would not be surprising if wear-resistant red pigmented teeth were included in this suite of adaptations as well.

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Skull of B. brevicauda. Image credit: Yale Peabody Museum / Curious Sengi.

Outside of soricine shrews, the other major group of mammals to exhibit pigmented teeth are the rodents.  The yellow or orange front teeth of rats and beavers are the most prominent examples.  Unlike shrews which have pigment across all their teeth, rodents restrict pigmentation to their perpetually-growing incisors.  Analysis on beaver teeth found the pigment to be a poorly crystallized ferrihydrite, another iron oxide but distinct from the magnetite found in shrews.  Pigmented teeth have also been found in the fossil record, not only in shrew and rodent ancestors, but in two unrelated groups of enigmatic extinct mammals known as Multituberculates and Apternodontidae, indicating this morphology has evolved a number of times independently.

Multituberculate Teeth 2

Multituberculate Teeth 1

Teeth from the Late Cretaceous (~70 Ma) multituberculate Barbatodon transylvanicus flaunting some impressive enamel pigmentation.  The large, serrated blade-like fourth premolar (p4) is characteristic of this extinct group of early mammals.  This is the earliest known occurrence of red teeth in mammals.  The oldest red-toothed shrew is Domnina gradata from the Early Oligocene (29 – 34 Ma) of North America, suggesting that tooth pigmentation is a basal ancestral trait for soricines.  Image credit:  Thierry & Codrea 2015.

It is an easy epithet to call our early mammalian ancestors “shrew-like creatures,” as though shrews are so dismissively small, indistinct, and full of stupidly unrealized potential to the point of cliché.  But shrews have traveled the evolutionary road for a long time and proved themselves to be as fascinating and formidable as any other.  Did I mention that they are also venomous?

Shrews Chew Through Trap

Sorex araneus (A, B, and C) and Neomys fodiens (D) are two soricine shrews native to the Netherlands.  They have both inflicted significant chewing damage on aluminum live traps used in a population study.  The author states that these traps are designed to last up to 30 years, but with the increasing number of shrews being captured and turning their toothy fury upon their prisons, these traps can last no more than 5 to 10 years.  Image credit: van Boekel 2014.

 

References

Dumont, M., T. Tütken, A. Kostka, M.J. Duarte, & S. Borodin.  2014.  “Structural and functional characterization of enamel pigmentation in shrews.”  Journal of Structural Biology 186:  38 – 48.

Furió, Marc, J. Agustí, A. Mouskhelishvili, O. Sanisidro, & A. Santos-Cubedo.  2010.  The Paleobiology of the Extinct Venomous Shrew Beremendia (Soricidae, Insectivora, Mammalia) in Relation to the Geology and Paleoenvironment of Dmanisi (Early Pleistocene, Georgi).”  Journal of Vertebrate Paleontology 30 (3):  928 – 942.

Holman, Alan & D.L. Harrison.  2003.  “A new helmeted frog of the genus Thaumastosaurus from the Eocene of England.”  Acta Palaeontologica Polonica 48 (1):  157 – 160.

Jin, Chang-Zhu & Y. Kawamura.  1996.  “A new genus of shrew from the Pliocene of Yinan, Shandong Province, northern China.”  Trans. Proc. Palaeont. Soc. Japan 182:  478 – 483.

Miles, A.E.W.  1963.  “Pigmented Enamel.”  Proceedings of the Royal Society of Medicine 56 (10):  918 – 920.

Smith, Thierry & V. Codrea.  2015.  “Red Iron-Pigmented Tooth Enamel in a Multituberculate Mammal from the Late Cretaceous Transylvanian ‘Hateg Island.'”  PLoS ONE 10 (7):  e0132550.

Strait, Suzanne G. & S.C. Smith.  2006.  “Elemental analysis of soricine enamel:  pigmentation variation and distribution in molars of Blarina brevicauda.”  Journal of Mammalogy 87 (4):  700 – 705.

van Boekel, Wim H.M.  2014.  “Damage to Longworth live-traps by shrews.”  Lutra 57 (1):  39 – 42.