Tiny, long-armed dinosaur leads to rethink of dinosaur miniaturization
The real surprise, however, came when the researchers realized that Alnashetri was not a highly specialized, advanced stage Alvarezsauroid. Instead, although it lived in the Late Cretaceous, it occupied an early branching position among the earlier basal members of the clade.
This combination of small size and early branching status fundamentally breaks our previous model about the evolution of these animals. If the miniaturization of Alvarezsauroids was strictly linked to their insect-killing lifestyle with stubby arms, an early-diverging species like Alnashetri should have some transitional features during a steady clade-wide march toward this extreme endpoint. But that didn’t seem to be the case.
“It’s a very long-limbed animal, so it was probably quite fast. My best analogy would be something like a roadrunner from the American West,” Makovicky said.
Arms and teeth
The later Alvarezsaurids had tiny, robust forelimbs that measured less than half the length of their femur. Alnashetrihowever, sported relatively long forelimbs that accounted for 61 percent of the length of its entire hindlimb. Although it possessed three-fingered hands with a sturdy first digit, a characteristic of its group, it still retained thin second and third digits, unlike its later cousins.
Other features that challenge the established evolutionary pattern of miniature dinosaurs are Alnashetrijaws and teeth. Its dentition has non-serrated teeth inserted in sockets, but importantly, these teeth are not extremely small, as they were in later Alvarezsaurids like Shuvuuia Or Jaculinykus. “This decoupled the evolution of small body size from anatomical specializations,” Makovicky explained.
The team concluded that extreme miniaturization in Alvarezsaurids did not necessarily co-evolve with the evolution of smaller arms more suited to digging or small teeth built to crush ants and/or termites. Instead of a clade-wide trend where the entire lineage gradually declined over time, a new evolutionary model including Alnashetri suggests that the body mass of Alvarezsaurids fluctuated several times. AlnashetriIt turns out that it achieved its 700-gram frame independently of other highly specialized alvarezsaurid species.
