The real shock, however, came when researchers discovered that Alnashetri wasn’t a highly specialized, late-stage Alvarezsauroid. Instead, although it lived in the Late Cretaceous, it occupied an early-branching spot among the more basal members of the clade.
This combination of diminutive size and basal placement overturns our earlier model of how these animals evolved. If alvarezsauroid miniaturization were tightly linked to a stubby-armed, insect-eating lifestyle, an early-diverging form like Alnashetri should display transitional traits trending gradually toward that extreme. But it didn’t.
“It had very long limbs, so it was probably quite quick. My closest analogy would be something like a roadrunner from the American West,” Makovicky said.
Arms and teeth
Late alvarezsaurids possess tiny, stout forelimbs less than half the length of their femurs. Alnashetri, by contrast, had relatively long forelimbs equal to 61 percent of its hindlimb length. Although it retained the group’s three-fingered hand with a robust first digit, its second and third digits remained slender, unlike those of its later relatives.
Other aspects that challenge the orthodox view of miniature dinosaur evolution are Alnashetri’s jaws and dentition. Its teeth were non-serrated and set in sockets, and notably they were not the extremely reduced teeth seen in later alvarezsaurids such as Shuvuuia or Jaculinykus. “This decoupled the evolution of small body size from anatomical specializations,” Makovicky explained.
The team inferred that extreme miniaturization in alvarezsaurids did not have to co-evolve with either shortened, digging-adapted arms or tiny, ant-crushing teeth. Rather than a lineage-wide, steady shrinkage, a model incorporating Alnashetri suggests alvarezsaurid body mass rose and fell repeatedly. Alnashetri appears to have reached its roughly 700-gram mass independently of the other highly specialized alvarezsaurid species.
