Bakker blogs: Why Dimetrodons had interclavicles — and Uma Thurman doesn’t

HUZZAH! We have the missing bone: the largest unit in the Dimetrodon skeleton, the one bony element we never hoped to find! We thought weʼd have to sculpt a fake one, but now we have the real thing — the INTERCLAVICLE.

Whatʼs that you say? Never heard of an interclavicle? Its common name is the breast-plate. Most of our ancient Permian critters, both reptile and amphibian, have breast-plates.

breastplateNo,not like that.

Our Dimetrodon wasnʼt armored on his chest like Bugs Bunny playing the Valkyrie in “Whatʼs Opera Doc.” The true breast-plate was a long, narrow, strong beam of bone that was fixed to the chest along the midline. Itʼs got a long “stem” in back, and an expanded “bowl” up front. The interclavicle is so named because it was attached to the big collar bones, also known as clavicles. Hereʼs a view of our new find, plus a diagram of how the clavicles and interclavicle attach to each other and to the shoulder blades:

diagramThe diagram is drawn as if the entire shoulder were flattened by a bulldozer so all the
bones are in one plane. Weʼve color-coded the bones so
the osteologically challenged
donʼt get frightened.

volunteer diagram

Our new specimen is held by the volunteer who found it.

To understand the architectural implications of the interclavicle, we must decide on our favorite role played by Uma Thurman. (We just love her — we even have a bone bed named “Uma”). We are impressed with Uma as “Ulla”, the Swedish femme fatale/housekeeper/tidying-upper in The Producers, but our choice would be Mia, the gangster girlfriend in Pulp Fiction.

Mia explains the interclavicle in the famous scene where she is stabbed in the heart with a huge syringe wielded by John Travoltaʼs character, who is trying to resuscitate her after an unfortunate mix-up in recreational prescriptions.

Travolta illustrates an emergency procedure: the needle must be thrust hard through the human breast-bone and into the cardiac cavity to jump-start the heart.

(Note: Do not try this at home. Ever. Not even with the pet gerbil.)

Mia does regain consciousness, with the syringe still sticking out of her breast-bone. (The technical name for her breast-bone is sternum.)

But let’s get to the osteological point. The breast-bone, aka sternum, is NOT the same as the breast-plate, aka interclavicle. Our human breast bone is part of our rib-cage. Itʼs in the middle of our chest and ties the right and left side of our ribs together. Itʼs made from rather soft bone material (so you can, in fact, get a needle through). The turkey breast-bone is the same unit, a sternum, but is much bigger and harder.

Next Thanksgiving, poke around with your fork to see how the birdʼs ribs attach to the sternum.

But back to Ms. Thurmanʼs sternum. Scrutinize this diagram; it shows Umaʼs skeleton:

uma skeleton

Examine the rib cage and breast-bone. Note the collar bones (clavicles). They are slender, graceful bones with swivel joints where they attach to the sternum.

Iʼll give you a minute.

Got it? Now repeat after me: “Sternum ties ribs together; clavicles are slender and have swivel joints.”

Okay. Now look at this chest, from a close kin of Dimetrodon. The clavicles are wide bones, much, much broader than ours. See the interclavicle? Itʼs not attached to the ribs. It lies under the sternum and has a stiff overlapping joint with the clavicles. The entire clavicle/interclavicle apparatus makes an extraordinarily strong, T-shaped apparatus, a rigid support for the shoulders.

shoulder blade

In the Dʼdon clan, the interclavicle is immensely long, far longer than the thigh. If you probe around your own chest you will, I guarantee, not find an interclavicle.

Hereʼs what our chest would look like if we did have one and a set of broad clavicles to match. (If you do find an interclavicle on yourself, call the 800 number at the bottom of the blog).

human interclavicle

We primates donʼt have an interclavicle or wide clavicles attached stiffly to the interclavicle. Neither do cats, dogs, horses, goats, guinea pigs, elephants, ʻpossums, raccoons, dolphins, aye-ayes or numbats (Google those last two). No normal mammal has an interclavicle today. We lost them in the Late Jurassic, about 150 million years ago.

So, what did the interclavicle/clavicle unit do in Dimetrodon, et al? Two things.

One, it helped armor the chest. Since it was dense, hard bone, the interclavicle plus clavicles protected heart and lungs from blows delivered by an opponent. In other words, the bones acted as a chest-protector. And that means all members of a Permian baseball team would be outfitted to play catcher (think about it).

Two, it was the attachment for the biggest muscle in the front limb, the pectoralis. Dimetrodon and its friends and relations were all really buff. The pecs were huge. We know that from the bump of bone on the upper arm, the “delto-pectoral crest”, where the pecs attached. Feel the inside of your armpit. The muscle here is the pectoralis. If you were a Dʼdon, the pecs would be four times thicker.

Our new, perfect Dimetrodon interclavicle shows clearly where the pecs attached all along the “stem.” By the way, the buff pecs explain why this bone gets chewed to bits by scavengers nearly every time. The interclavicle is so meaty that it is the first place to bite if you are hungry. Check out this sketch of a Dʼdon relativeʼs chest.

ddon chest

A couple of final points and inquiries. There are very few mammal species alive now who have interclavicle/clavicle apparatus like a Dimetrodonʼs. Who are they? Why is their motherhood so weird? They give us a clue about why most of us Mammalia have lost the bone.

Homework: Move your left shoulder blade up and down and around, as you feel your left collar bone with your right hand. Could Dimetrodon do that? Massage a cat or dog while they are relaxed. See how the shoulder blade moves? Could a Dʼdon or its relatives do that? Ride a horse. Feel the shoulder blade move. Is such mobility possible in an early Permian reptile? Now do you have a notion about why we advanced mammals lost the stiff interclavicle/clavicle arrangement?

Lastly and more importantly, what if Uma Thurmanʼs characterʼs Mia were equipped with a proper Dʼdon interclavicle composed of hard, dense bone? She would not have been resuscitated. The needle would have broken off.

Bakker blogs: Murder by hickey — or a dinner date with a meat head

We’ve been pondering  the problem of Dimetrodon-on-Dimetrodon violence. It’s a Red Beds tragedy; fin-back reptiles were nibbling each other’s brain bones and gouging each others’ cheeks.

But now, maybe, we have some evidence for the softer side of fin-backs: hickeys and love-bites.

Here’s a scientifically precise reconstruction of one fin-back smooching another on the back of the neck, sort of like the cover for a Permian romance novel: Fifty Shades of Red (Beds).

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

Neck-nibbling is quite the thing among living species of predators, both large and small. Cats do it. Go to Animal Planet and see male lions grabbing the lioness by the nape.  Or come visit our Seymour digs in north Texas and meet “Elton,” the male Mountain-Boomer Lizard. Male Mountain Boomers, aka “collared lizards,” are the brightest lizards in all of the Lone Star State.  Not “bright” as in “smart,” but as in wearing “fabulous iridescent blues and pinks and yellows in the mating season.” Elton stakes out a wide, flat area in our quarry where he struts his stuff, doing Marine-style push-ups to attract females and frighten away younger males. Every spring he succeeds in enticing a healthy female, almost as muscular and buff as he is.

Here’s a portrait of Elton, snapped by David Temple, Curator and Herpeto-photographer extraordinaire.

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

(Warning: If you keep Boomers in captivity, never have two males together in a small cage. They’ll beat the coprolites out of each other. The same warning often applies to keeping two curators together.)

Actual Boomer mating includes neck-grabbing. Elton has an extraordinarily wide forehead housing mighty jaw muscles, so the love-nibble has force behind it. If she’s willing, the female displays a hunkered-down posture and shows off her red dots. Therefore, when the female Boomer signals “Bite me!” it’s in fact a “Come hither!” message.

Here’s a fine snap of a female Boomer, from Mike Cong Wild Photography.

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

Elton does NOT view us humans as a higher species. He’ll race to where we’re digging under the shade of a tarp and give us the hairy eyeball, lizard-style, cocking his head right and left. Then out he goes to ascend his viewing stand, a foot-tall sandstone block 20 feet away.  I think he’s checking us out to make sure we are not competition for his favorite lizard-love.

Given such behavior by Elton, we expect that our 400-pound Dimetrodons engaged in some sort of gnathic-cervical love-grabbing. Do we have petrified evidence? You bet. Here’s a cervical vertebra number two, the big bone right behind the head. It belongs to a full grown  D. loomisi, a species nicknamed the “Keira Knightly Finback” because of the excessively long, slender neck. The arrow points to a bite — a  powerful nibble that actually removed a piece of bone.

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

But that’s a bit too big of an ouch. There would be thick muscles running from the vertebra to the back of the skull that flex the head up and down, side to side, and twist the head around. This bite would have gone right through the thick part of the muscles, leading to massive trauma, blood loss and death.

Murder by hickey!

Check out this diagram: On the right you’ll see some of the massive and meaty muscles that are located around the head and neck.

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

It was a sad day when we realized that our love nibble was instead hard evidence of cannibalism. But the head-neck bites also prove something elegant and marvelous about Dimetrodons. We mammals are, supposedly, the Highest Class. We have the most advanced, most efficient anatomical tools for cutting up our food and digesting it quickly. We are far better than the cold-blooded class Reptilia, or so the textbooks say.

Cold-blooded reptiles today do seem sloppy and inefficient. Nile crocodiles and komodo dragon lizards kill zebra, wildebeest and goats — but once their prey is dead, their table manners are primitive. The big reptiles bite their prey anywhere and everywhere, chomping down on bony snouts and chins where there’s not much meat.

Mammal top predators display far greater precision. The tiger examines his prey carefully before removing bite-sized pieces off the meaty zones. The rear teeth slice meat as efficiently as your neighborhood butcher making prosciutto.

You can do this experiment  at home: buy some delicious Texas beef jerky and present a big piece to your hungry dog (or your friend’s). The pup will position the jerky between its rear teeth and slice, slice, slice, GULP. The quick slicing action comes from special features of those rear teeth.

Scrutinize these photos of a wolverine. See the big rear teeth?  When the wolverine bites meat, the upper rear tooth slides against the lower tooth, and the teeth hone each other like metal shears. That’s why mammal meat-eaters can cut even tough meat and tendons swiftly.

Murder by hickey: Dr. Bob explores another side of Dimetrodon-on-Dimetrodon violence in the latest Bakker blog

Fossil predator lairs from the Age of Mammals show that these precision-slicers are old adaptations. When we excavate prey carcasses left by saber-toothed predators like Dinictis and Hoplophoneus (both on display in our new Morian Hall of Paleontology), we see bite marks on the skull bones where there was lots of meat — the rear of the skull, the brain case and the tops of neck vertebrae. The extinct mammals ate like the highly efficient carnivores in today’s world. Saber-toothed cats did not waste much time and energy gnawing bony, meat-poor zones of chin and snout. Neither did the extinct dog-like Hyenodon.

Our Dimetrodon was a very, very primitive reptile. In fact, in most ways, D’don was even more primitive than a crocodile or komodo dragon. One big deficiency was the set of meat-slicing teeth. Dimetrodons didn’t have the enlarged self-sharpening chompers. The upper rear teeth could not slide past the lowers in a honing action. Therefore, so the theory goes, a Dimetrodon would have been sloppy and slow and inefficient when dismembering big carcasses.

If D’dons were really as sloppy as crocs and komodo dragons, then we’d find bite marks all over skulls and necks. But if D’dons were careful and efficient, they would have left tooth marks concentrated on the meaty zones of heads and necks.

When we analyzed bite marks on all the necks and heads from our digs, I was flabbergasted. (Talk to anyone in the lab — Dr. Bob hardly ever gets gabberflasted.) Our supposedly primitive Dimetrodon did not bite a la lizard. Or a la crocodile. Or a la gator.  Bite marks were targeted with consummate precision. Little energy had been wasted gnawing at non-meaty parts. Bony snouts and chins were not chewed upon. Instead, the tooth marks had been concentrated on all the most meaty zones of the head and cervical region. Bites on the braincase are exactly where big, thick muscles attached. Bites on the cheek are where the jaw muscles attached. Bites on the neck are where the thickest cervical flesh was located.

I have new respect for the Texas Red Beds Dimetrodon. Whenever we unearth another D’don victim, I doff my hat in honor of its masticatory prowess.  Our modern mammal efficiency began a hundred million years earlier than we had thought. And now, when we do lunch at Smokey Bros Barbecue and we chew succulent brisket and bring a doggy-bag back to Skippy, we thank our fin-back ancestors.

Dumb as a rock? A lumpy bit of earth reveals a geological timeline in Seymour

Editor’s note: Today’s blog comes to us from paleontologist and field volunteer Neal Immega.

You all know by now that the museum has a dig in Seymour where we are finding fabulous Permian fossils, including the toothy Dimetrodon and the weird boomerang-headed salamander Diplocaulus. We don’t dig in just one place; lots of people go prospecting for new sites (or maybe they are just looking for a private spot to do their business).

A previous blog topic was on a weird lump that turned out to be a caliche ball. Today we have another lumpy rock to look at. This specimen was collected by geologist Gretchen Sparks, who brought it in just to plague me.  Let’s see just how much information we can squeeze out of it:


At first glance, it’s just a rock showing cross-bedded sandstone with low-angled bedding, doubtless caused by water deposition in the Permian creek that crossed our digging area. This is pretty normal stuff. We see cross-bedding everywhere at the dig site, because the sandstone is durable and stands in relief.

But why is it lumpy? The bulge in this picture is not exactly standard:


It gets better. The rock is too heavy to be only quartz. A heavy, light-colored sandstone is likely to be cemented by barite (barium sulfate). Let’s cut the rock in half and polish the face.

cutLumpLabeledThis is turning out better than I expected. You can see a seam of barite cutting the nodule vertically in half. The sandstone shows horizontal layers which correspond to the cross-bedding.

What about the red-colored area? The area we are working in North Texas is called the Permian Red Beds because everything got oxidized from prolonged exposure to the atmosphere. It was a really dry time, and the critters stayed close to the Permian creek which deposited these sediments. It is good for us because the fossil remains are concentrated in a small area (our dig site is just about the size of a tennis court).

Let’s consider this possible sequence of events.

1.    250 million years ago, sandstone was deposited in a creek. It is all cross-bedded.

2.    The sand grains were likely coated with iron oxide and thus turned red.

3.    The sandstone was buried by maybe 1000-plus feet of additional rock.

4.    Shales deeper in the geological section were heated by the normal geothermal gradient to hundreds of degrees and adsorbed water was squeezed out, taking with it the barium that was also adsorbed (from the ocean) on to the clay surfaces. The water moved vertically along cracks in the rock.

5.    When the barium reached the rocks we are digging in, the barium precipitated because the pore water is very “hard” with dissolved gypsum. The barium reacts with the local sulfate, producing barium sulfate which is essentially insoluble in water. It is the ultimate “hard water” scum.

6.    The barite precipitated as the vertical seam and filled the pore space in the red rock.

7.    A whole lot of rock was eroded in the next 250 million years to bring us to the present and the rocks back to the surface.

8.    10 to 100-thousand years ago or so, North Texas was in an Ice Age, and it was really wet with lots of vegetation. Decomposing vegetation created a reducing environment which dissolved iron right out of the rock. Barite is very chemically resistant, and this lump could have been at the surface through part of the ice age. The iron could have partially leached out of the lump at that time. You can see that the leaching went deeper into the lump where there are horizontal fractures in the rock.

All this from a lump of rock. Thanks, Gretchen!

The Bakker Beat: The Texans loss in Monday Night Football delivers a big, wet kiss on our Permian cheekbone

(Plus, a key to the Fin-back Zombie Mystery!)

Where were you guys during the Texans-Patriots game on Monday Night Football?  Your curator of paleontology was hunched over his best microscope, looking up every 45 seconds to check out the action on his black-and-white Zenith television, a full 16 inches across the picture tube.

Of course I was hurling scholarly imprecations at the screen: “Pass, rush, PASSSS RUSHHHH!!!!! Hit ‘em in the infundibulum!!”

Three TDs in 19 minutes from Tom Terrific??? Groan.

As our humiliation unfolded, I had one consolation. Under the ‘scope was an unexpected delight, a Happy Holidays kiss on the cheek from the Permian Red Beds of Seymour, Texas. It was the latest discovery in our five-year analysis of violence among our ancestors 285 million years ago.

I always have the TV on when doing delicate fossil-cleaning with a high-power ‘scope. That’s because, for best eye health, you must look away from the fossil and focus on something a dozen feet away. The look-away is good exercise for your eye-focusing muscles. I know too many colleagues who became severely near-sighted after 40 years of ‘scope work.

Anyway, late in the second quarter I noticed something weird on the petrified rear cheekbone I was cleaning. Known technically as a “squamosal”, the rear cheek bone is a keystone element in skulls — our skulls and those of our antecedents, the fin-back reptile Dimetrodon. Squamosals make the lower-rear corner of the skull — check out this diagram that shows the bone in a mean guard dog and a paleo department volunteer. If you tap your cheek just behind the eye, you’ll be in touch with your squamosal.

cb mammal-one-holers labelF color


In Dimetrodon, the squamosal ties the jaw-joint bone, the quadrate, into the rest of the skull. Observe this diagram.

cb ddon willi squam D. limb label copy

In our human skulls, too, the squamosal performs a unique role in the jaw action: the quadrate disappears and the squamosal bone forms the upper jaw joint where the lower jaw attaches. Medical doctors call this region the “temporal-mandibular joint,” or “TMJ,” and it’s the locus of chronic pain if your grind your teeth during Texans games.

The Dimetrodon squamosal proves that it is a human ancestor, even though in most ways this fin-back is more primitive than a lizard. In our human skulls, the squamosal makes the rear rim of the temporal fenestra, an opening behind the eye. Powerful jaw muscles used for chewing attach to the inner edges of the fenestra rim.Try this: bite down hard on some West Texas jerky. Touch your temporal fenestra with you index finger. You’ll feel the jaw muscles bulging. All mammals have a temporal fenestra built just like Dimetrodon’s. Dinosaurs, lizards, snakes and all other vertebrates don’t have this osteological badge of the Mammalia. Dimetrodon and its kin evolved the fenestra late in the Coal Age and eventually passed it down to all us furry mammals, from possums to gorillas and bats to blue whales.

But back to the Texans and Patriots. After Brady’s third scoring toss, I was comforted by the fact that this particular D’don squamosal was well-preserved. Almost a miracle, because when I saw it last, the bone and the rock it was in were sodden with rain water. Water is the enemy of delicate bones. Our Permian quarry is made up of mudstone, layers of clay-rich sediment that accumulated on the bottom of a pond in an ox-bow. When dry and fresh, the Permian mudstones are so hard that cold-chisels are required. But when water seeps in, the clay minerals get squishy and messy. The bones split and splinter; some bones actually dissolve.

The squamosal in question had been victimized by a sneak attack of ground water. When the squamosal was first discovered and still embedded in rock, associate curator of paleontology David Temple and I had dug drainage trenches all around the bone and then covered the specimen with a tarp to protect it from the thunderstorm we could see coming. After the rain stopped, we lifted the tarp. The top of the rock was dry, but then we saw water percolating up from below the bone. Rain water had soaked into a hill that rose 10 feet above the level of the quarry. The water then traveled  through the red rock layers. Naturally, the water flowed down from the hill until it hit a hard layer, a dense blue limestone that made the floor of our quarry.

Our skeleton was in the red mudstone a foot above that limestone. After the rain stopped falling, the rain water in the rock kept flowing down and across the hard layer, through the red sediment. The flow then had enough energy to come up through a foot of mudstone and drown our specimen. Here’s a diagram of how the seepage works. (By the way — we’re not kidding that you must avoid red harvester ants — one of our crew was swarmed by these hazardous haploids and got 10 bites and multiple stings. Each ant grabs your skin in its vice-like jaws and then proceeds to rotate its rear end in a semicircle, stabbing with its stinger. Our victim swears that the pain is worse than wasp stings. Worse than childbirth!  She is still all itchy and scratchy two weeks after the attack.)

cb ddon willi drowningFFF copy

We did the emergency glue drill, which involves dumping low-velocity super glue on the wet specimen. Super-glue hardens moist rock and bone.Then we waited. A year later, we removed the block of rock inside a coating of aluminum foil and plaster. I had little hope of the squamosal surviving.

Another year went by and I opened the block, during that now-infamous Monday night. Surprise! The squamosal was 95 percent intact. The glue drill had succeeded!

It got better. In the third quarter, when our offense showed some sign of life, I spotted a strange groove in the squamosal just below the temporal fenestra.  I cranked the ‘scope up to maximum power and teased away flecks of rock with a fine steel needle.

Was the mystery a channel for an artery? No — there is no blood vessel in this part of the cheek in any vertebrate skull.  A nerve?  No. Towards the end of the sad, sad fourth quarter, the the bone was fully cleaned of rock. The groove showed raised ridges on both sides, as if a miniature bulldozer had plowed up the depression. Check out this close-up.

cb ddon willi squam Ddon bite bw

Bulldozers of any size had not evolved in the Permian, but there was one thing that could gouge a groove in bone. A Dimetrodon fang!  The shape of the groove matched the geometry of a big killing tooth from the front of the mouth in a large D’don.

Our fin-back had been bitten, hard, by another fin-back. Intraspecific violence, as the PhDs say. Cannibalism! Which is how we return to our Dimetrodon Zombie question.  It turns out that the squamosal belongs to the same skull that shows bite marks on the braincase. Remember our last post — it makes no sense for a carnivorous Permian reptile to bite at the brain of a Dimetrodon. There is simply not enough brain-meat to be worth the trouble. Now we had evidence that the same victim had been bitten along the temporal fenestra.

Why bite the squamosal here? The correct answer will lead you to getting the answer to why the cannibal Dimetrodon had bitten the bones around the brain. Get both questions answered, and you’ll discover that our ancestor Dimetrodon was a very clever, very efficient predator.