Episode 354 is all about Pisanosaurus, a small Triassic herbivore that is either an ornithischian or a non-dinosaur dinosauromorph.

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In this episode, we discuss:

News:

• A great twitter thread by Brian Engh continues the discussion about how likely Allosaurus was to be a scavenger source
• The Spinosaurus swimming debate continues with a new paper about its possible aquatic adaptations source
• isotopes from titanosaur eggshells help show their nesting behaviors source
• The world’s largest known Triceratops skeleton, “Big John” is going up for auction in France in October source
• The Pacifico Yokohama convention center in Japan has a dinosaur display Called DinoScience: The Dinosaurs of Laramidia source
• Whirinaki Whare Taonga in New Zealand has the exhibit Dinosaur rEvolution: Secrets of Survival source
• A Flintstones spinoff featuring Pebbles and Bamm-Bamm called Yabba Dabba Dinosaurs is coming to HBO Max on September 30 source
• Colin Trevorrow says that Jurassic World: Dominion will explore the question, “If dinosaurs lived amongst us, would you be safe?” source
• The new game Jurassic World Evolution 2 will be released on November 9 source

The dinosaur of the day: Pisanosaurus

• Primitive dinosauriform that lived in the Triassic in what is now La Rioja, Argentina (Ischigualasto Formation)
• Ongoing debate about whether or not it’s a dinosaur
• Small, lightly built, probably bipedal
• Had a small head, long legs (probably a fast runner)
• Herbivorous
• Estimated to be 3.3 ft (1 m) long
• Estimated to weigh between 5 to 20 lb (2.3 to 9.1 kg)
• Found a partial skeleton in 1962
• Fossils found by José Bonaparte, Rafael Herbst, and the preparators Martin Vince and Galileo Scaglia
• Includes a partial skull with parts of the right jaw and teeth, cervical vertebrae, dorsal vertebrae, rib and rib fragments, parts of the pelvis, femora, tibia, fibula, parts of the toes, and more
• Holotype is pretty incomplete, and lots of fragments
• Described in 1967 by Rodolfo Casamiquela
• Type species is Pisanosaurus mertii
• Genus name means “Pisano’s lizard” and is in honor of the paleontologist Juan Arnaldo Pisano
• Species name is in honor of the naturalist Carlos Merti
• Originally thought to be a “very primitive ornithopod, with all probably represents an ad hoc family”
• Casamiquela said it was a surprise of a dinosaur, especially “in light of the paucity of remains of representatives of the Ornithischia throughout the entire geologic column in South America”
• Casamiquela named a new family, Pisanosauridae, but that name is no longer used
• Has been reclassified many times, as a heterdontosaurid, fabrosaurid, hypsilophodont, the earliest known ornithischian, and a silesaurid (a clade of dinosauriforms and the sister group to dinosaurs)
• Mostly because of incomplete fossils
• Orientation of the pubis is unclear. Some reconstructions have it forward pointing (like in saurischians)
• In 1976 José Bonaparte redescribed Pisanosaurus and said it had some distinct characters, such as the hip-joint being open, and the metacarpals in the hand being long
• In 1991 and 2012 Sereno suggested Pisanosaurus was a chimera, and that the skull fragments, pelvis, and distal hind limb could be from one individual, but the rest belonged to another taxon, based on the proportions not resembling the same shape or size of similar bones in heterodontosaurids, and he said there wasn’t much justification for the association of the bones and impressions
• In 2004 Norman and others suggested the skull bones were ornithischian, and the postcrania was similar to bones of a non-dinosaur
• In 2007, Irmis and others said the fossils “cannot be regarded as a chimaera based simply on character incongruence between different regions of the body”
• In 2009, Novas accepted it being one individual. Bonaparte, in 1976, included a sketch of the fossils the way he found them in the field, which shows it belongs to one individual
• In 2017, Federico Agnolin and Sebastián Rozadilla wrote a paper about the phylogenetic reassessment of Pisanosaurus, and agreed the fossils were all from one individual
• Authors redescribed some of the features of Pisanosaurus
• Wrote about the fragmentary skeleton, including partial jaws, dorsal vertebrae, four fragmentary vertebrae in an uncertain position in the column (thought to be caudal vertebrae, but later redescription found it to be cervical vertebrae, and then later thought to be something else (uncertain what), impression of part of the pelvis and sacrum, partial hind limb, a left scapular blade (that was lost), a probably metacarpal III and impressions of some metacarpals (also lost)
• Talked about Pisanosaurus being described from an incomplete, poorly preserved skeleton (makes it hard to describe, and they said parts of the description was tentative because of this), and a recent description of the clade Silesauridae has having teeth like ornithischians
• Suggested Pisanosaurus was part of Silesauridae, the non-dinosaurian dinosauriform, based on the teeth being similar (reduced denticles, as well as teeth fused to the maxilla and dentary bone), and other similarities, such as sacral ribs between two sacral vertebrae
• Thought to be an ornithischian mainly because of its teeth and teeth-bearing bones
• “However, it is worth noting that archosaurs, other than ornithischians, showing such a specialized dentition were unknown before the early 2000s”
• Silesaurus opolensis was described in 2003, and researchers in 2005 and 2007 mentioned how similar dentition in Revueltosaurus, a pseudosuchian, and basal silesaurids show how the diversity of Triassic archosaurs with highly derived “masticatory morphology” (how they chewed) is still not well understood
• Said this means herbivorous-like dentition was more widespread than previously thought, and herbivory happened more than once within archosaurs
• Pisanosaurus maxilla and dentary had about 15 tooth positions, same as Silesaurus and heterodontosaurids
• Teeth are very fragmented, weathered, and cracked, so hard to know exact number of teeth or what they looked like
• Dentary had characters considered to be ornithischian (originally described as having “barricade-like dentition”), but hard to compare them with silesaurids because the published specimens are incompletely preserved
• Later descriptions found the teeth did not form a barricade
• Has some dental features in common with ornithischians, like non-recurved, low-crowned teeth (low-crowned teeth are like what people have), but can see these characters in “several Triassic herbivorous archosaur lineages, including the pseudosuchian Revueltosaurus and silesaurids such as Sacisaurus and Silesaurus”
• Tentatively reconstructed the pelvis to be cup-shaped (“acetabular morphology”), which is different from ornithischians
• Did not have features that made it obvious it was an ornithischian or even a dinosaur, but did have some features similar to other silesaurids
• Agnolin and Rozadilla mentioned how Pisanosaurus was thought to be a heterodontosaurid for a while, but heterodontosaurid teeth are different from Pisanosaurus
• They found that Pisanosaurus is a “stem dinosaur”, probably part of Silesauridae, but may also belong to a “still poorly known clade of stem dinosauriforms”
• But concluded, based on phylogenetic analysis that Pisanosaurus should not be considered the oldest known ornithischian, which the authors said was consistent with a 2011 study by Olsen and others that found ornithischian radiation happened after the Triassic-Jurassic boundary
• In 2017, Matthew Baron wrote “Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?”
• Wrote: “Here I propose that phylogeny could hold the solution to this problem. I examine how an alternative position for Ornithischia – nested either within Theropoda or Sauropodomorpha – could be the reason behind their later appearance and relative rarity in the Early Jurassic. An Early Jurassic origin of Ornithischia would force us to consider that the anatomical similarities between ornithischians and Early Jurassic taxa might not be convergences, and to broaden the current datasets of early dinosaurs to test these ideas.”
• Julia Desojo and others in 2020 looked at fossils from the Late Triassic Ischigualasto Formation
• Said though three papers found Pisanosaurus to not be an ornithischian, the “ornithischian affinity of Pisanosaurus mertii was only questioned on numerical phylogenetic grounds”
• Looked at all the previous studies and the specimen and found it to be more likely to be an ornithischian than a silesaurid
• Reviewed Pisanosaurus anatomical traits and found that the traits that arguably made it a silesaurid were traits that were plesiomorphic for dinosauriformes (an ancestral character) and was in other dinosauromorphs and early dinosaurs
• Also found more than 10 characters shared by Pisanosaurus and heterdontosaurids or other early ornithischians among early dinosauriforms
• Found Pisanosaurus to be from about 229 million years ago, and is the oldest known ornithischian in the latest Carnian (in the Late Triassic)
• Said Pisanosaurus “fills the long-speculated ghost lineage between younger members of that clade and the oldest known saurischian dinosaurs at ca. 233 Ma.”
• Also said in the future there will be a quantitative analysis of the phylogenetic relationships of Pisanosaurus (out of scope for their paper)
• Müller and Garcia, in 2020 had another hypothesis, that Pisanosaurus was a transitional taxon between silesaurids and ornithischians
• Fernando Novas and others came out with a review of the fossil record of early dinosaurs from South American and its phylogenetic implications this year (print version in October of this year, 2021), published in Journal of South American Earth Sciences
• Wrote “The early evolution of dinosaurs was a complex process that occurred in the context of a “crowded ecospace”” and also wrote “there is no consensus about early dinosaur phylogeny, and our paper is not the exception”
• Did support the Ornithischia and Saurischia groupings, not Ornithoscelida
• Studied a bunch of specimens, including Pisanosaurus
• Mentioned some debate that silesaurids may be included within Ornithischia (easy to reinterpret how to classify their characters, because they have a high degree of homoplasy (shared characters, not from a common ancestor)
• Concluded “there are currently major uncertainties concerning the phylogenetic relationships of Pisanosaurus”
• Another case of need more fossils to determine
• If Pisanosaurus is considered to be an ornithischian, because it’s the only one from the Triassic, it fills helps fill a 30 million year gap (next known ornithischians were from the Early Jurassic, like Eocursor and Lesothosaurus)
• Fossils of Pisanosaurus are in the Colección de Paleontología de Vertebrados, Instituto Miguel Lillo in Argentina
• Lived in a warm, humid climate, with ferns, horsetails, and conifers
• Other animals that lived around the same time and place included Herrerasaurus, Eoraptor, Saurosuchus, Hyperodapedon (a beaked reptile), aetosaurs (spiny armored reptiles), and lots of other herbivorous animals

Fun Fact: Vultures have unique adaptations to deal with the increased pathogens of rotting meat, but even they won’t eat meat that’s too old.

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