Episode 352 is all about Aerosteon, a Late Cretaceous megaraptoran named for its pneumatic bones.
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In this episode, we discuss:
News:
- Two Daspletosaurus braincases have many differences, possibly indicating a new species source
- Two new sauropods, Hamititan & Silutitan, were named among pterosaur fossils in China source
- A key difference in brain shapes may explain why birds survived the K-Pg boundary, but non-avian dinosaurs went extinct source
- Students from the University of Alberta are looking for someone with a boat to help with excavating a hadrosaur source
- If you live in Victoria, Australia, you can vote for a state fossil emblem source
- Tokyo Dome in Japan has a Dinosaur Exhibition until September 5th source
The dinosaur of the day: Aerosteon
- Megaraptoran that lived in the Late Cretaceous in what is now Mendoza Province, Argentina (Plottier Formation)
- Bipedal, with an elongated skull, strong legs and tail, and large claws
- Initial estimates of about 33 ft (10 m) long
- Estimated to be about 20 ft (6 m) long and weigh 1,100 lb (500 kg), according to Gregory Paul
- Molina-Pérez and Larramendi estimated Aerosteon to be 25 ft (7.5 m) long and weigh 2,200 lb (1 metric ton)
- Type species is Aerosteon riocoloradense
- Genus name means “air bone” (Greek)
- Species name refers to it being found 0.6 mi (1 km) north of the Río Colorado
- Species name was originally riocoloradensis, but then changed to riocoloradense to match (neuter gender)
- Fossils found in 1996
- Took years to clean and CT scan the fossils
- First described in 2008 by Sereno and others, but at the time the ICZN didn’t recognize new species as valid if it was only published online. In 2009, PLoS ONE worked with ICZN to make it a valid name
- Holotype includes skull bones, neck and back vertebrae, some ribs, gastralia, furcula (wish bone), shoulder bone, and more
- Bones weren’t fully fused, so probably not an adult specimen
- Part of Megaraptora, lightly built, advanced allosauroids that had large hand claws
- When it was named, didn’t fall into any of the three known groups: abelisaurids, spinosaurids, carcharodontosaurids, found in the Cretaceous in South America
- Aerosteon “represents a distinctive basal tetanuran lineage that has survived into the Late Cretaceous on South America and is possibly linked to the allosauroid radiation of the Jurassic”
- Fossils show a bird-like respiratory system
- Had some pneumatic bones, including in the furcula (wishbone) and ilium and some gastralia
- Sereno and others suggested Aerosteon’s respiratory system may have helped regulate body temperature and then later helped with more efficient breathing
- The respiratory system may have helped with hunting (running long distances) and helped it cool down
- Based on pneumaticity of some of the gastralia, may mean it had air tubes under the skin to help with cooling
- Also may have helped it balance (upper body weighed less, so less likely to fall over when chasing prey)
- May have had a similar air-sac system to modern birds, where air sacs moved air in and out
- Talked about in episode 346, about a new Heterodontosaurus specimen helped show how ornithischians breathe
- Birds are efficient breathers, with unidirectional air flow
- Breathing in fills air sacs, and breathing out empties air sacs
- Lungs always fill from the back (unidirectional)
- Lungs stay full the whole time, which is more efficient (and with birds, helps with flying)
- Aerosteon may help show how bird’s respiratory systems evolved
- From the 2008 paper:
- Fossil evidence of the origin and evolution of air sacs is limited, since lungs don’t fossilize and because the air sacs in modern birds rarely pneumatize their bones and leave evidence that they existed
- In Aerosteon, bones were pneumatized by diverticulae of air sacs (so the air sac pouches led to hollowness in the bones)
- Suggested a four-phase model of the evolution of air sacs and respiratory system in birds
- Phase I: More air sacs along the neck in basal theropods, no later than the earliest Late Triassic
- Phase II: Air sacs differentiated in the Jurassic, to include clavicle and abdominal air sacs. Air sacs could be used as bellows, which may mean there were rigid lungs with flow-through ventilation, like modern birds
- Phase III: Primitive air pump with the sternum and ribs evolved in maniraptoriforms before the end of the Jurassic
- Phase IV: Advanced air pump evolved in maniraptorans before the end of the Jurassic
- Not clear from Aerosteon when unidirectional lung ventilation occurred (haven’t identified via bones uni- or bidirectional lung ventilation)
- Some controversy between the authors and Matt Wedel, who posted on SV POW about findings
- Wedel wrote, “There is no question that the fossil material is pretty stunning” but that there are parts of the paper he disagreed with (had a three part blog post)
- Wedel talked about how only living tetrapods with postcranial pneumaticity are birds
- And said in 2005, O’Connor and Claessens injected and dissected over 200 birds and found cervical diverticula never went further down than the middle of the thorax, so in dinosaurs pneumatic vertbrae further down the body probably were from abdominal air sacs
- With dinosaurs, having pneumaticity all over the body shows they had cervical and abdominal air sacs, which means they had air sacs around the lungs, which means they were at least somewhat equipped to have flow-through breathing like birds
- Wedel disagreed with Sereno et. al saying that cervical air sacs have been found past the thorax in birds (said that was based on papers where the authors didn’t actually say cervical air sacs went past the thorax—pneumatization happened from abdominal air sacs, and that the Aerosteon paper was dismissive of O’Connor and Claessens)
- Wedel also didn’t like the use of the word “pleurocoel” because it’s too broad of a term (word for “big pneumatic cavities that are often present in the vertebral centra of sauropods and theropods” that has been used to mean multiple things) which can lead to confusion
- Said instead of using the word pleurocoel in the paper they should have replaced it with pneumatic foramina (means the pneumatic cavity leads to internal chambers)
- Wedel also defined pneumatic hiatus (a coin he termed in 2003). From a 2009 paper of Wedel’s: “Pneumatic hiatuses are gaps in the pneumatization of the vertebral column and indicate pneumatization from multiple sources” (gaps where there aren’t hollow areas, which means the hollow areas were a result of multiple sources
- In the paper about Aerosteon, the authors said saurischian adults and juveniles do not show a pneumatic hiatus
- Wedel said neither do adult birds, because birds have up to three sets of diverticula in the cervical air sacs, lungs, and abdominal air sacs, which meet up and give a continuous pneumatic foramina down the vertebral column (juveniles have gaps, or pneumatic hiatus, because these sets haven’t met up yet)
- So, Wedel didn’t agree with the part about it being difficult to defend the idea of having three different pneumatic sources that resulted in one pleurocoel (or pneumatic foramina) in Aerosteon
- Or when the authors said it was problematic to directly compare birds to non-avian dinosaurs when it comes to pneumaticity in the posterior axial column
- Wedel said the absence of the pneumatic hiatus is not evidence that there was only one source/route to the bone being pneumatized
- Also said that not enough dinosaurs have been analyzed for pneumatic hiatus (not everyone works on pneumaticity, also Wedel only coined the term in 2003, so not much time had passed by the time Aerosteon was named to look for them)
- Sereno responded and said it was a “misleading, longwinded, ad hominem critique of this paper”, which led to Wedel responding
- A few months later, the third chapter of Wedel’s dissertation was published, about evidence for bird-like air sacs in saurischian dinosaurs (and pneumatic hiatuses)
- An isolated tooth was referred to Aerosteon but later found to be the tooth of an abelisaurid
- Close relative is Murusraptor, described in 2016, that had less pneumatic bones (the authors who described Murusraptor found that the isolated tooth referred to Aerosteon was similar to an abelisaurid tooth that may have been scavenging on the Aerosteen carcass)
- Very similar skull bones and vertebrae
Fun Fact: Albertosaurus & Gorgosaurus also went through a dietary shift as they grew up just like T. rex.
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