Tuesday, August 28, 2018

Species That Don't Get Enough Publicity #14: Achillobator giganteus

Dromeosaurs are one of the last dinosaur groups to really become superstars. In the 80s, Deinonychus was the most popular dinosaur: a unique sprinter, jumper, kicker, gangster. A well-armed social hunter, Deinonychus mobbing and dismembering their prey became a defining image of the new “hot blood” look of dinosaurs as science finally came to terms with dinosaur endothermy.  When Michael Crichton gave them the name of Velociraptor and Steven Spielberg put them on screen in the most terrifying depiction of any dinosaur, they became superstars.

Immediately after the release of the film, a new giant dromaeosaur was discovered: Utahraptor. Utahraptor showed that Deinonychus was no longer the earliest or largest dromaeosaur. Earlier dromaeosaurs have since been found, but not larger ones. At over 1,000 lbs, Utahraptor remains the largest (and most famous) of the dromaeosaurs. In the 2010s, a new giant raptor has made headlines. Extremely rare, Dakotaraptor finally allows for the fantasy “Utahraptor meets Tyrannosaurus” scenarios dinosaur fans have dreamed of for 20 years.  

But there are two other giant raptors, two big dromaeosaurs that have been found and ignored by popular culture. One I will deal with later along with its family, but today I will talk about Velociraptor’s giant predecessor in Mongolia: Achillobator.

In 1989 a Soviet-Mongolian expedition was collecting in Burkhant, Dornogovi, part of the Bayan Shireh formation. This mysterious expedition (nothing on it exists on this side of the paywall) collected turtles, fragments of sauropods, azdarchid pterosaurus, and a mysterious new theropod. This theropod was a disarticulated fragmentary skeleton-left maxilla with teeth, 11 vertebrae, bits of rib, a scapula and coracoid, the right half of the pelvis, a radius,, a left femur and tibia, and an incomplete foot and hand. 

10 years later, increased Mongolian ties with the American Museum of Natural History caused their leading dinosaur paleontologists Mark Norell and Jim Clark to examine the remains together with the great Mongolian paleontologist Altangerel Perle. They concluded it was a new dromaeosaur-a big one, the biggest one since Utahraptor. They named it Achillobator: Achillo- refers to the large heel but also works with the -bator. Achilles was also the Greek hero that the heel was named after, while bataar/baattur is Mongolian for hero.

Achillobator was unusual for Mongolian dromaeosaurs. Other species like Adasaurus and Velociraptor were much smaller, and similar to the North American species like Dromaeosaurus and Deinonychus. The pelvis is shockingly different-the pubis is like that of a Tyrannosaur, jutting straight down with a large “boot” at the end, while all other dromaeosaurs have a birdlike pubis that curves along with the ischium. A team of theropod workers including Burnham, Ostrom, Bakker, Currie, Zhou and Destier considered it a possible chimera in their paper on Bambiraptor.  Norell took their criticisms in hand and further examination proved him right-Some of the bones were still articulated, the bones were of the same color, and every cladistic analysis he and Mackovicky applied still kept Achillobator as a dromaeosaur, no matter how primitive the pelvis was. 

Aside from the pelvis, Achillobator is unique in terms of size. Other dromaeosaurs of the region-Adasaurus, Velociraptor, Luanchuanraptor, Zhenyuanglong, etc, were only about 2 meters in length and less than 20 kg. Judging by the known material of Achillobator and comparing it to those relatives, the animal could have been as long as 6 meters and 350 kg. This would make it larger than Austroraptor, about the same size as Dakotaraptor and second only to the 500 kg Utahraptor.  In build, it’s more robust than Dakotaraptor, but still not as robust as Utahraptor.

As a Dromaeosaur, Achillobator would have had a full coat of plumage, even on the tip of the tail and with nonfunctional wings.  It would have a long snout full of curved, serrated teeth, long talons on the hand, and a single giant talon on each foot. The tail would be long and stiff, and the legs and neck flexible and agile. Dromaeosaurs  were not as cursorial as other coelurosaurs, but maneuverable and agile, built for quick motions, ducking, dodging, leaping. Achillobator itself has a longer femur than tibia, and may not have been agile as even Dakotaraptor.   Paleoartist Mark Witton has an intriguing speculation that Achillobator could have been a badger-like digger hunting mammals and small ornithiscians (art below)

The use of the hand and foot claws has been debated: they could have been used for stabbing, slashing, climbing, or nothing at all. Unfortunately, the foot claws of Achillobator are missing. Using Dakotaraptor as a model, the inner toe claw would be 6 inches long.

Achillobator is a dromeosaurine-in Dromeosaurinae, it is more derived than the earlier Deinonychus and later Atrociraptor, but still less derived than the earlier Utahraptor and later Dromaeosaurus and Dakotaraptor.  Dromaeosaurinae is defined by boxier, wider skulls, teeth serrated on both sides, and shorter and more powerful legs for jumping rather than running.

Dating Achillobator is difficult: the Bayan Shireh formation stretches 98 to 83 million years, 15 million years in length. Why this formation is so difficult to date is unclear-it seems to be a problem with the amount of magnetic material, Potassium, Argon, Lead and Uranium in the rock, as well as the deposition of the strata itself.   This 15 million years encompasses 4 geologic ages: The Cenomanian, Turonian, Coniacian and Santonian. This is a general problem with other Mongolian Strata: The overlying Iren Dabasu formation ranged from 85 to 70 million years ago. So the two species assigned to Alectrosaurus may prove to either be the same, or belong to Tyrannosaurs separated by 30 million years. 

The Bayan Shireh is similar in environment to the contemporary wetlands-The Moreno Hill, Upper Cedar Mountain and Wahwheap Formations in North America,  the Bahariya formation in Africa, and the Candeleros, Lissandro, Portezuelo and Bajo de La Carpa Formations  in South America. The upper Cretaceous for the most part was very wet, only with some parts of Mongolia and the Americas drying out in deserts in the Campanian.

Achillobator was one of the main predators of the ecosystem, sharing the role with the small Tyrannosaur Alectrosaurus. Alectrosaurus’ close anatomical similarities in both the Bayan Shireh and Iren Dabsu indicates possibly a later date for the Bayan Shireh, closer to 80 mya than 100.  Prey would have consisted of the undescribed sauropod’s young, the small ceratopsians Graciloceratops and Microceratus, the Therizinosaurs Erlikosaurus, Enigmosaurus, and Segnosaurus,  the Ornithomimid Garudrimimus, and probably early Hadrosauromorphs and Oviraptors. The environment also contained azdarchid pterosaurs and the ankylosaurs Talarurus, Tsagantegia and Maleevus.  

Hopefully more of the Bayan Shireh and Achillobator in particular will be found. It’s this mystery that prevents it from reaching the fame of the even bigger Utahraptor, and the “contemporary-with-T.rex” Dakotaraptor, let alone the complete Velociraptor and Deinonychus. The late date of discovery doesn’t help either-while it would fit the size of the “Velociraptor” in Jurassic Park, Deinonychus was the species referred to in the novel and Velociraptor in the film and publicity material.  Achillobator is one of the many dromaeosaurs overshadowed by the main 5 genera in pop culture, especially since the Bayan Shireh isn’t particularly diverse or late, with smaller animals than South America,  in a country without the media of North America. 

So anyway, Hollywood, give this guy a break. Museums, put him on display. Toy companies, here’s a new fresh face. Authors, think about the crap you’re writing and how this genus could perk up the place. And remember, shop Achillobator where you work or play.

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