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