Diplocaulus: The Boomerang-Head Amphibian

 Super-sized “Boomerang-head”
amphibian from
290 million years ago

The Houston Museum of Natural Science has just excavated the complete skull of one of the most bizarre animals that ever lived – the amphibian Diplocaulus. With a head shaped like an armor-plated banana, or an Australian boomerang, this distant kin of today’s salamander is so famous that it stars in most kids’ books on dinosaurs – and in college textbooks as well.

The Boomerang-Head (my favorite nickname for the Diplocaulus) was only of modest size – twenty pounds live weight would be an average adult. But since the first discovery in 1878, the extraordinary cranial design has flummoxed the best paleontological minds. Baby Boomer-Heads had a normal salamander-oid shape, with a rounded snout lined with needle-sharp teeth ideal for snapping up worms on the bottom of ponds.

Weirdness entered the growth cycle as Diplocaulus approached adolescence. The rear corners of the skull grew much faster than the rest of the head, so when adulthood was achieved the head was three times wider than long. And the skull corners became pointed horn-like devices composed of thick, dense, armor-like bone material.

No species alive today comes even close.

What did Boomerang-Heads do with their strange skulls?  Theories abound. Perhaps they plowed up crustaceans hiding in freshwater ponds. Perhaps they used the heads as hydrofoils for flying in river currents. Or for staying put on the bottom during floods. The notion I favor is that the adults whacked each other during courtship battles.

More mysteries surrounded the biggest adults. Heads a foot across are common – but a few incomplete specimens showed creatures 30% bigger. Did the giants represent old males who hid in specialized habitats? Or Boomerang-Head matriarchs?

When HMNS began its long-term field survey of Red Beds near Seymour, Texas, getting  Boomerang Heads for the new Fossil Hall was a top priority. Museum crews did find many parts of mid-sized specimens. Many had evidence of being chewed up and dismembered by predators. Who ate Boomerang Heads? Teeth shed during feeding identified the culprit. It was the Dimetrodon,  a reptile close to the direct ancestry of warm-blooded mammals, including us.

No one in the museum field party hoped for a complete Boomerang Head skull of record size, until……

……Kathleen Zoehfeld, long-time museum volunteer and award-winning author of kids’ science books, scouted a shallow arroyo cut into brick-red pond deposits. Boomerang Head bones were everywhere – including neck vertebrae of gigantic size. Then Zoehfeld spotted the front edge of a skull poking out of the rock. Not just a partial specimen of the sort found elsewhere but the entire head, complete from eye-sockets to horn tips.

Zoehfeld christened the specimen “Geoff” in honor of her son, a sophomore at Columbia University.

Geoff’s head was 16 inches or more wide, as big or bigger than any other noted in paleontological journals. And beautifully preserved.

As soon as I saw it, my mind jumped…I could see how Geoff’s skull would star in the Red Beds tableau of our new exhibit. It would make everyone, kids and adults, stop dead in their tracks and stare.

I took charge of cleaning the specimen personally. It’s 90% done. Our friends at the Black Hills Institute will make casts to share with other museums.

And about those mysteries regarding giant Boomerang heads: HMNS is gathering more clues. Parts of several other giant skeletons were secured near Geoff’s site, suggesting that a sort of “old Boomerang men’s club” might have existed in Red Beds time. Or, alternatively, an amphibian-matriarch society.  Skulls were accompanied by evidence from the other end of the animals – beds full of coprolites (fossilized feces) that may well have been produced by big Boomerang-Heads.

We don’t have the final answers. But the new finds will help. Maybe we’re getting closer to understanding these wonderful critters. And the exhibit of bones and coprolites will delight the scientific imagination of museum visitors.

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Dimetrodon, Diplocaulus, and the Permian Treasure Trove

For the past month or so, it has felt very much like Christmas in the cave down in the basement (my nickname for our small but homely Paleontology lab). Recently, we opened a freshly recovered plaster jacket containing the arms of a great beast, the Permian finback Dimetrodon, and hope to exhume his ancient limbs soon. However, there have been several positive delays that have hindered our progress. This jacket has become the gift that keeps on giving, and has yielded much more than was expected.

After retrieving the jacket (click here for an example of this process) and carefully re-depositing it in our lab for meticulous preparation, we began digging from the top of the jacket – however, the Dimetrodon’s arms that we discovered on the surface are at the bottom of our jacket due to the fact that we always flip the jacket over in the field in order to extract it. Thus, we have a certain amount of sediment we must carefully burrow through, much like a blind mole, before we get to our objective – the arms. As a result of this process, anything in between is an unexpected, though happily accepted, added bonus.

CB-BoomBody
The oddly-shaped Diplocaulus.

Our first bonus from the plaster casket was an immediate discovery and occurred during the “jacket flipping” process. This was a very fortuitous event, as we would later discover. After the 300 lb jacket had been flipped, one of our veteran diggers noticed the hint of a small, square bone just barely revealing itself from the moist red clay lining the surface of our upside-down jacket. The identity of the fingerprint pattern on the bone was undeniable: the boomerang-headed amphibian Diplocaulus. 290 million years in the dark red beds of what is now North Texas only to be tossed from its deep sleep in a matter of seconds. Our hearts raced at the thought of a complete Diplocaulus skeleton, as we had but bits and pieces of the odd creature to date. And what an odd creature it was. Imagine a three foot salamander with a boomerang for a head.


Arm-Jacket Amphib 3
Notice the unique texture on the fossil -
a sure sign you’ve found
a Diplocaulus fossil.

Ignoring the desire to uncover the bone further, we shrouded the exposed surface with layers of aluminum foil and entombed the new specimen once more in plaster for the long haul to Houston. We now had two specimens that would need scrupulous attention upon reaching the lab.

The jacket was finally opened about a month later. Our newest paleo volunteer, Meredith, was assigned to work with me on the jacket. I opened the aluminum foil and plaster lid, re-exposing the red clay that I knew housed the Dimetrodon limb some three hundred millimeters or so below the surface. The small piece of Diplocaulus skull peered up at us. It was beautiful. The fingerprint pattern of the skull was astonishingly clear and well preserved.

Meredith
Volunteer Meredith Fontana
holds the maxilla, the facial
skull bone that contains most
of the teeth of a large
Dimetrodon

The prep process began under Meredith’s careful fingers. Our speed of prep was severely diminished due to the presence of our newest Diplocaulus – and hopes that it would reveal more than a fragment of skull. Soon more skull fragments appeared. And more. Then ribs. Then vertebrae. All so very tiny. The largest vertebra was a mere seventeen millimeters. The entire animal in life was no larger than a cat. The ribs are quite peculiar; very flat and uncurved. Imagine this curious creature with a remarkably flat belly that clings advantageously to the bottom of a mud-filled wallow or other small body of water.

The eruption of bone continued, and is continuing. The possibility of having our very first complete Diplocaulus, or boomer-head as we call them, is a very distinct possibility. I realized this animal did not yet have a name. All our Permian pets receive names, mind you. After discussing the possibilities with our well-seasoned digger Johnny “The Mole Man” Castillo, we agreed that “Meredith” would be the name of our little amphibian. After all, our volunteer Meredith had been the sole prep-tech for this jacket.

johnny
Johnny Castillo prepares Meredith for
removal by adding a mini plaster jacket to
the skeleton to keep each bone in place.

The next bonus appeared a week ago; a beautiful, small, jaw full of teeth. Twenty of them, to be exact. Upon first glance at this handsome little jaw, I assumed I was seeing more of our dear Meredith, though something seemed rather odd. The teeth were not as needle-like as they should have been to be Diplocaulus; the jaw was not as round. I noticed that one of the teeth was loose. This was both good and bad. Re-attaching teeth to jaws is exceedingly complicated, and when you have a jaw that is only six-and-a-half centimeters long with itty-bitty teeth… We like to be perfectionists when it comes to prep and pre-dino dentistry is quite difficult.

This loose tooth was my chance to attempt to i.d. its owner. Under a microscope I stared at this magnificent crown in the palm of my hand. My heart stopped. Tiny serrations lined the outer edge. This wasn’t Diplocaulus. Who had serrations at this time? Dimetrodon did. But was this Dimetrodon?

chis jaw 1
Extremely tightly packed teeth may offer
an idea on how young Dimetrodons
regrow their teeth.

I looked at the tiny jaw. All the teeth were the same size.  Except for a missing tooth at the front of the jaw which would have been obviously larger, but not by much. My heart skipped a beat this time. It was strikingly similar to a specimen possibly new to science discovered right here in our very own labs. It was only just recently we discovered a jaw which may soon prove to be a new genus of Sphenacodont. Sphenacodonts, a family of Pelycosaurs which includes our favorite fin-back Dimetrodon,  were the first animals to evolve a specialized set of teeth which include a large canine tooth as well as smaller cutting teeth. Thus they are less like reptiles and more like mammals. Our distant cousins, these mammal-like-reptiles, would probably have been endothermic, or warm-blooded, as well.

I compared the two jaws closely. Both jaws contained the two sizes of teeth; the larger canine and the smaller canines, but the position of the teeth was a bit different.

I stared in disbelief; my heart had resumed beating as I thought about the other possibilities. Loomisi was another Dimetrodon species we were familiar with who also had serrated, tightly-packed teeth, and as Dr. Bakker suggested to me, a young Loomisi may be similar to the new specimens. Unfortunately, part of the jaw is missing. Consequentially much more analysis will have to be done before a final identification can be given.

chris jaw 2
What could this be?

The arms of the great Dimetrodon are still buried, all thanks to an amphibian named Meredith and some other strange beast who got in the way. The arms will have to wait while Christmas in the cave continues. The dust from the white plaster jackets fall to the floor like a fine powder snow, and our plaster gift keeps on giving. A Permian present wrapped in plaster and burlap, filled with bizarre creatures most of us only dream about; amazing creatures that lived 290 million years ago that tell us a story of what our planet was like before the first dinosaurs.