From ScienceDaily:
Paleontologists at the University of Toronto (U of T) and the Royal Ontario Museum (ROM) in Toronto have entirely revisited a tiny yet exceptionally fierce ancient sea creature called Habelia optata that has confounded scientists since it was first discovered more than a century ago.
In those days, we had it all sewn up except for a few outliers. Then…
The researchers argue that this difference in anatomy allowed Habelia to evolve an especially complex head that makes this fossil species even more peculiar compared to known chelicerates. The head of Habelia contained a series of five appendages made of a large plate with teeth for mastication, a leg-like branch with stiff bristle-like spines for grasping, and an elongate, slender branch modified as a sensory or tactile appendage.
“This complex apparatus of appendages and jaws made Habelia an exceptionally fierce predator for its size,” said Aria. “It was likely both very mobile and efficient in tearing apart its preys.”
That implies a sophisticated nervous system controlling the apparatus.
The surprising outcome of this study, despite the evolutionary relationship of Habelia with chelicerates, is that these unusual characteristics led instead the researchers to compare the head of Habelia with that of mandibulates from a functional perspective. Thus, the peculiar sensory branches may have been used in a similar fashion as mandibulates use antennae. Also, the overlapping plate-like appendages in the middle series of five are shown to open and close parallel to the underside of the head — much as they do in mandibulates, especially those that feed on animals with hardened carapaces. Paper. (public access) – Cédric Aria, Jean-Bernard Caron. Mandibulate convergence in an armoured Cambrian stem chelicerate. BMC Evolutionary Biology, 2017; 17 (1) DOI: 10.1186/s12862-017-1088-7 More.
The early evolution of sophisticated heads should be seen as an argument against the long, slow growth of intelligence by purely Darwinian means.
See also: Cambrian worm find from the Cambrian (541-485 mya) “helps rewrite” our understanding of annelid head evolution. So the complex head has been around longer than we thought.