Podokesaurus Holyokensis

''Podokesaurus'' is a genus of coelophysoid dinosaur that lived in what is now the eastern United States during the Early Jurassic Period. The first fossil was discovered by the geologist Mignon Talbot near Mount Holyoke, Massachusetts, in 1910. The specimen was fragmentary, preserving much of the body, limbs, and tail. In 1911, Talbot described and named the new genus and species ''Podokesaurus holyokensis'' based on it. The full name can be translated as "swift-footed lizard of Holyoke". This discovery made Talbot the first woman to find and describe a non-bird dinosaur. The holotype fossil was recognized as significant and was studied by other researchers, but was lost when the building it was kept in burned down in 1917; no unequivocal ''Podokesaurus'' specimens have since been discovered. It was made state dinosaur of Massachusetts in 2022.

Estimated to have been about in length and in weight, ''Podokesaurus'' was lightly constructed with hollow bones, and would have been similar to ''Coelophysis'', being slender, long-necked, and with sharp, recurved teeth. The were very light and hollow, and some were slightly concave at each end. The (neck) vertebrae were relatively large in length and diameter compared to the (back) vertebrae, and the (tail) vertebrae were long and slender. The (upper-arm bone) was small and delicate, less than half the length of the (thigh-bone). The pubis (pubic bone) was very long, expanding both at the front and hind ends. The femur was slender, nearly straight, had thin walls, and was expanded at the back side of its lower end. The three of the lower leg were closely appressed together forming a compact structure.

Since it was one of the few small theropods known at the time it was described, the affinities of ''Podokesaurus'' were long unclear. It was placed in the family Podokesauridae along with other small theropods, and was speculated to have been similar to a proto-bird. It was suggested it was a synonym of ''Coelophysis'' and a natural cast specimen was assigned to it, but these ideas are not currently accepted. The family Podokesauridae is not in use anymore, having been replaced by Coelophysidae, and ''Podokesaurus'' is thought to have been a coelophysoid. As such, ''Podokesaurus'' would have been a fleet-footed predator, with powerful forelimbs and grasping hands. It is estimated it could have run at . ''Podokesaurus'' is thought to have been collected from the Portland Formation to the Hettangian–Sinemurian stages of the Early Jurassic, between 201 and 190 million years ago.

Discovery and history

In 1910, the American geologist Mignon Talbot was walking with her sister Ellen to Holyoke, Massachusetts, in the eastern US, when they passed a farm and noticed a small hill nearby. It had a gravel pit at one side, and was formed by an accumulation of sand, gravel, and boulders left by a receding glacier. Talbot noticed a white streak on a sandstone boulder at the bottom of the gravel pit, and upon discovering these were bones, she told her sister she had found a "real live fossil". This was because many fossil tracks had previously been discovered in the Connecticut Valley (that she had often taken her students to see), but few actual dinosaur skeletons, and none at Mount Holyoke. She was granted permission by the land owner to collect the specimen for Mount Holyoke College (an all-women's college a few miles from there), where she was head of the geology department.

The next day she brought a group of workmen to collect the specimen, and found another piece of sandstone that contained the rest of the fossil as well as impressions of those in the first slab. The specimen appeared to have been exposed for years with no one noticing it, the boulder having been broken open by people or frost. The fossil was brought to the laboratory where it was prepared and photographed. The incomplete specimen preserved 5 (neck), 11 (back) and 24 (tail) vertebrae, a fragment of the left (shoulder blade) and right (part of the ), a partial left (upper arm bone), of 3 fingers, including 2 (claw bones), ribs, the and (bones forming the lower front and lower back of the pelvis, respectively), the (thigh bones), the left (lower leg bone), a fragment of the right (a bone in the ankle), articulated (foot bones) of the left foot, followed by 3 pedal phalanges (toe bones), partial metatarsals and possible phalanges of the right foot, and fragments of uncertain identity. The light and delicate bones were in their natural position or nearly so within the rock, except for the tail and uncertain fragments, which were a few centimeters away from the skeleton, following a long gap in the skeleton behind the pelvis. The front half of the neck was missing.

The significance of the fossil was confirmed at an intercollegiate meeting of geology departments, and when the American paleontologist Richard Swann Lull subsequently encouraged Talbot to describe the specimen, she replied she did not know anything about dinosaurs, but Lull suggested she should study them and then describe it. In December 1910, Talbot read a preliminary description of the fossil at the Paleontological Society meeting at Pittsburgh, and in June 1911 she published a short scientific description, in which she made the specimen the holotype of the new genus and species ''Podokesaurus holyokensis''. The generic name is derived from the Ancient Greek words ''podōkēs'' (), which means "swift (or fleet)-footed", an epithet commonly used in reference to the Greek hero Achilles, and ''saura'' () meaning "lizard", while the specific name refers to Holyoke. In full, the name can be translated as "swift-footed lizard of Holyoke". The discovery and naming of ''Podokesaurus'' made Talbot the first woman to find and describe a non-bird dinosaur. The American paleontologist Robert T. Bakker stated in 2014 that while old professors grumbled that women were unfit for working with fossils during his time at university, Talbot's discovery of ''Podokesaurus'' was a counterargument to that.

By the time the description was published, Talbot had sent the fossil to the Peabody Museum of Natural History at Yale University for further preparation and study, where cast replicas were also made of the bones as they lay in the rock. There, Lull drew a reconstruction of the skeleton, basing the parts missing from the fossil on the equivalents in ''Compsognathus'', and created a model of the animal in life, which Talbot later described as having a "sardonic smile". Lull expanded on Talbot's article in a 1915 publication, wherein he also proposed other identities for some of the bones than what she had originally suggested, including a partial coracoid instead of a scapula and part of the tibia instead of the (part of the hips). Lull suggested that the boulder containing the fossil must have come from a ledge which lay on the south side of the Holyoke Range, about two or three miles north of where it was found, and specified that it was from the Longmeadow Sandstone.

Lull had sent his manuscript to the Danish ornithologist Gerhard Heilmann for criticism prior to publication. Heilmann published his response in a 1913 article in which he included previously unpublished photos of the fossil received from Talbot, as well as his own restorations. He disagreed with some of Lull's anatomical interpretations, and had corresponded regarding the fossil with the American zoologist Robert Wilson Shufeldt about his contentions. Heilmann's article was one in a series about the origin of birds, wherein he examined the skeletons of prehistoric reptiles to find traits that may have been ancestral to birds. In a 1916 Washington Academy of Sciences meeting, Shufeldt gave an account of his correspondence with Lull, Talbot, and Heilmann, and agreed with the latter in some of his criticisms of Lull's restoration of ''Podokesaurus''.

Talbot wanted the fossil to stay at Yale or Washington on permanent exhibit, where it could "be with its kind", but it was kept at Mount Holyoke in the old science building, Williston Hall, as a local specimen, where it became a "pet curiosity" for the students. During the Christmas break of 1917, Williston Hall burned down, and no remains of the ''Podokesaurus'' fossil were found in the rubble. The American writer Christopher Benfey pointed out in 2002 that ''Podokesaurus'' therefore had the peculiar distinction of being the dinosaur that vanished twice. The German paleontologist Friedrich von Huene was the last to study and describe the holotype specimen, in a 1914 publication before it was destroyed (he had studied it at Yale before Talbot's description was published). While the college's fossil collections were almost entirely destroyed by the fire, its facilities and collections continued to grow and improve afterwards due to Talbot's efforts.

No other unequivocal ''Podokesaurus'' specimens have since been found, but cast replicas of the type specimen remain at the Peabody Museum of Natural History and the American Museum of Natural History in New York. ''Podokesaurus'' received little further attention until new and abundant fossils of ''Coelophysis'' were discovered in the late 1940s, and the anatomy of small theropod dinosaurs became clearer; the American paleontologists Edwin Harris Colbert and Donald Baird compared ''Podokesaurus'' with ''Coelophysis'' in 1958, using the casts of the former. Along with the sauropodomorph ''Anchisaurus'', ''Podokesaurus'' was among the first substantial collections of dinosaur skeletal material known from the East Coast of North America. Apart from a tooth (assigned to ''Coelophysis'' in 1976), all other Early Jurassic theropod records on the East Coast are footprints.

In 2021, ''Podokesaurus'' and ''Anchisaurus'' were proposed as contenders for state dinosaur of Massachusetts by Representative Jack Patrick Lewis, with ''Podokesaurus'' winning the online poll. On May 2, 2022, Governor Charlie Baker signed a law making it the official state dinosaur.

Description

The preserved body of the only known ''Podokesaurus'' fossil was long. Lull estimated the complete length of ''Podokesaurus'' to have been , the neck accounting for , the back for , the (the vertebrae of the pelvic region) for , and the tail for . Von Huene estimated the animal to have been long, with the tail accounting for about , more than 1.5 times the rest of the skeleton together. Colbert estimated the animal's length at about . In 1995, the writer Jan Peczkis estimated ''Podokesaurus'' to have weighed , through pelvic height determination (based on von Huene's measurements). The paleontologist Gregory S. Paul estimated it to have weighed in 2016 (based on a 1 m length estimate). Talbot stated in 1937 that those scientists who had seen the specimen did not think it was a young individual as there were no indications of cartilage that would turn to bone with age, while Paul thought it was possibly juvenile. In general, the animal was lightly constructed, with hollow bones. As a coelophysoid, it would likely have been similar in build to for example ''Coelophysis'', being slender, long-necked, and with sharp, recurved teeth.

Since few other small theropods were known at the time ''Podokesaurus'' was described, it was mainly compared to the later genera ''Compsognathus'', ''Ornitholestes'', and ''Ornithomimus'', which were thought related. Due to the fragmentary nature of the ''Podokesaurus'' specimen and the fact that it is lost, the identity and shape of some of its bones are today unclear. As only casts of the specimen now remain, it is only possible to get a general impression of the preserved skeleton and its proportions. Talbot identified bone fragments next to the tail as skull bones because two of them were bilaterally symmetric, and one was broadly convex with a sulcus (or furrow) at the midline, and according to Lull, was possibly placed at the upper wall of the skull, not far from the . Lull thought other of these elements may have been part of the . Von Huene instead interpreted these bones as caudals from the middle of the tail.

Postcranial skeleton

The vertebrae were very light and hollow, and some were slightly concave at each end. The cervical vertebrae were relatively large in length and diameter compared to the dorsals, and had styliform (resembling a pen or bristle in shape) cervical ribs which were long, straight, and narrow, as in ''Anchisaurus''. The front dorsals were much shorter than the cervicals, then again increased in size towards the pelvis. They measured in length. The of the dorsals were narrow, high, and curved backwards, but not as expanded from front to back as in ''Compsognathus''. The dorsal ribs were strongly curved and very slender and hollow, reminiscent of those in ''Saltopus'' and ''Hallopus'', the largest being long and wide. The (that connected with the ribs on each side of a vertebra) formed a circular arc downward from each side of a dorsal, extending about as far from the (the "body" of a vertebra) as the was high. The (processes that connected adjacent vertebrae) were well-developed. The centra were relatively longer than those of ''Ornithomimus'', while those of ''Compsognathus'' resembled those of ''Podokesaurus'' more, but were shorter.

While the hind part of the tail was located some distance away from the rest of the skeleton, Lull believed it was in its natural position and that it would have been very long, the intermediate vertebrae having been swept away. The caudal vertebrae were very long and slender, and along with their (bones on the undersides of the caudals), resembled those of ''Ornitholestes''. A typical caudal was long and in diameter. The caudals were very similar in length and did not decrease in length until the last few in the succession. Just in front of the pubis there was a cluster of slender (abdominal ribs), the longest of which was long. The frontmost gastralia had broadened ends, which is often the condition in sternal ribs.

The humerus was small and very delicate, long, less than half the length of the femur. It had a strong processus lateralis (a process directed to the side) above its lower end. Lull found it similar to that of ''Ornitholestes'', apart from the radial crest (that connected with the of the lower arm) not being as high, showing little muscular power. Von Huene estimated the radius to have been long. Lull found the hand more similar to that of ''Compsognathus'' than of ''Ornitholestes''. Only slender phalanx bones of the three fingers were preserved, including sharp, curved claws.

The pubis was very long, expanding both at the front and hind ends. It was wide and long, and its lower expansion was in diameter. Talbot found the pubis similar to that of the then undescribed ''Procompsognathus''. Lull argued that the pubis lay approximately in position in the fossil, pointing forwards. Heilmann instead believed the bone had been moved out of position before burial, and would have been directed backwards. Colbert and Baird suggested the pubis was directed forwards, but that it curved slightly upwards instead of downwards due to natural warping and cracking of the bone. The ischium was long, wide at the end closest to the body, and its slender shaft was in diameter, and somewhat thickened at its lower end. Lull found it similar to those of ''Compsognathus'' and ''Ornitholestes''. Below the (where the femur connects with the pelvis) there was a broad lamella running towards the pubis.

The femur was slender, nearly straight, had thin walls, and was expanded at the back side of its lower end. It was long and in diameter just below the (a flange placed mid-length at the back of the femur). The fourth trochanter was strong, long and about high, and was situated past the middle of the shaft, towards the lower end. The condyles, as seen in birds. The length of the metatarsals was , while their width was . The most complete pedal phalanx was very slender and long.

Classification

Early interpretations

When reading her preliminary description in 1910, Talbot suggested ''Podokesaurus'' to have been an "herbivorous dinosaur", but further work at Yale University showed that some bones had been incorrectly identified, and the ischium of the pelvis with a well-developed ridge was found to resemble that of ''Compsognathus''. She refrained from making a definite classification of the specimen in her 1911 description due to the lack of jaw and foot-bones which could have aided in this, but concluded it would have belonged to a "carnivorous dinosaur" based on the shape and position of the pubis, as well as the absence of a . She considered the fossil to be Triassic in age (the first of the three Mesozoic periods). In 1914, von Huene named the new family Podokesauridae, wherein he, in addition to ''Podokesaurus'', included ''Saltopus'', ''Procompsognathus'', ''Coelophysis'', and ''Tanystropheus''. He placed this group under Coelurosauria, which at the time was used to include small theropods in general.

Heilmann considered ''Podokesaurus'' very similar to the early bird ''Archaeopteryx'' in 1913, and wondered why Talbot had not made any comparisons to it. He particularly found the legs and the pelvis, when the pubis was interpreted as pointing backwards (as in birds), similar to what it might look like in a bird ancestor, while considering Lull's reconstruction of a forward-directed pubis unlikely. He also found parts of the shoulder blade, forelimbs, and even tail bones similar to those of birds. He furthermore pointed to the presence of abdominal ribs and that the bones were hollow, and concluded that nothing precluded ''Podokesaurus'' from being a very primitive proto-bird, with not yet fused middle-foot bones. The only feature he found inconsistent with this interpretation was the short finger with a claw, if it was not instead part of a toe. He suggested that the issue could be determined if the skull, sternum, and were found in the rock, bones that would be important clues for classification.

Heilmann wished for a clearer explanation of the placement of the middle-foot bones in relation to each other, and found it surprising that Lull's reconstruction of ''Podokesaurus'' showed an even more appressed middle-foot than was present in the later supposed descendant ''Ornithomimus'', which he thought would have developed a completely bird-like middle-foot by that time. Heilmann found ''Podokesaurus'' very unusual for a dinosaur from the Triassic, and thought it was one of the earliest bipedal vertebrates. He suggested that since it was so different from ''Thecodontosaurus'' and ''Plateosaurus'', those may not have been dinosaurs, or ''Podokesaurus'' could instead have been related to ''Saltopus'' or parasuchians, but found its fossils too insufficient to say anything definite about the issue.

Shufeldt elaborated on his and Heilmann's interpretation of the pubis in 1915, and stated the bone had probably been displaced during fossilization. He pointed out that if it had faced forward, it would have been in forcible contact with the abdominal ribs that would have been dangerous for internal organs during movement. Lull found Heilmann and Shufeldt's criticisms of his pelvis reconstruction probable in 1915. He listed Podokesauridae under the superfamily Compsognatha, and suggested ''Podokesaurus'' was perhaps ancestral to the later North American genera ''Ornitholestes'' and ''Ornithimimus''. In 1916, Heilmann pointed out that early dinosaurs, parasuchians, and pterosaurs all had similarities to birds, as well as to each other, and that Triassic reptiles like '' Scleromochlus'', ''Saltopus'', and ''Podokesaurus'', were difficult to separate. He proposed that ''Podokesaurus'' and other bipedal reptiles from the Triassic were descended from pseudosuchians (which many types of archosaurs were classified as at the time) that may have evolved bipedality by the Permian. He concluded that birds were descended from ornithosuchian pseudosuchians rather than from dinosaurs, due to their bipedality and bird-like legs.

The paleontologist Henry Fairfield Osborn used ''Podokesaurus'' as an example of a "carnivorous dinosaur of the bird-like type" in 1917, while stating that similarities between birds and dinosaurs were due to parallel evolution. The paleontologist Franz Nopcsa von Felső-Szilvás created the subfamily Podokesaurinae in 1928, within which he included ''Podokesaurus'', '' Procerosaurus'', ''Saltopus'', and ''Tanystropheus''. Von Huene grouped ''Podokesaurus'', ''Coelophysis'', '' Spinosuchus'', ''Halticosaurus'', ''Saltopus'', '' Avipes'', and ''Velocipes

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