Spec-Dinovember Day 20: Reverse Rex, a species that is robust while young, but gracile when older
During the Late Triassic, as other sauropodomorphs transitioned to a quadrupedal stance and began to resemble the Eusauropods they would become, one group took the opposite approach. Agilisauropus velox appears less like its Plateosaurid kin and more like the later ornithomimids. Eschewing the large size of its relatives it relies on speed to escape predation, at least as an adult. The young are, like other sauropodomorphs, quadrupedal. Since they are incapable of the quick retreats the adults employ, young A. velox seek out burrows made by cynodonts and dicynodonts in which to take shelter. Their forelimbs grow rapidly and the young use them to scratch about the walls of whatever shelter they have found in search of edible roots and tubers by day. Under the cover of night they venture out to forage for leafy browse. After a prolonged childhood, once they've grown too fat for any burrow, the dinosaurs go through a rapid and awkward adolescence to reach their adult physiology. This life-stage sees the second highest mortality rate (second only to hatchlings) as the animals are no longer able to shelter underground, but have not yet mastered their speed.
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Sauropods attained their quadrupedal stance through a neoteny event. There exists fossils of hatchling Massospondylus that show they were quadrupedal as infants and matured into bipedal 'prosauropods.' So that sounded like a perfect jumping point for this prompt. Making the adult even more gracile was easy, but thinking up how to make the juvenile more robust was a bit of a challenge. I ended up going with the burrow thing based on aardvark burrows and how a lot of species rely on them. Gives the babies of this speculative species a way to avoid getting eaten without needing to drastically change growth trajectories, just a little allometric growth to kick things off.