Final answer:
Ancestral echinoderms were likely sessile due to fossil evidence found in sedimentary rock layers, showing early echinoderms without adaptations for active movement and often attached to the sea floor. Structures like Aristotle's lantern suggest sedentary feeding habits, and the Pelmatozoa subclass includes immobilized echinoderms similar to ancestral forms.
Step-by-step explanation:
The evidence that suggests ancestral echinoderms were sessile includes fossil records, which show that early echinoderms did not possess the same structures as modern echinoderms that facilitate active movement. While today's echinoderms, like sea urchins and sand dollars, have structures such as tube feet and in some cases, the ability to attach to objects or other organisms for transportation, ancestral echinoderms lacked these adaptations. Fossilized echinoderms found in sedimentary rock layers, particularly from the Upper Ordovician period approximately 460 million years ago, typically show these organisms in positions that suggest a sessile lifestyle, often attached to the sea floor.
The structure known as Aristotle's lantern which is a complex multipart feeding structure found in echinoderms like urchins also indicates a sedentary feeding habit, consistent with a sessile lifestyle. Additionally, the pentaradial symmetry seen in adult echinoderms is simpler than bilateral symmetry in terms of locomotion, and early echinoderms are thought to have been even less mobile or entirely immobile. The subclass Pelmatozoa, which includes the crinoids, or feather stars, represents echinoderms that are sessile or have limited mobility, reinforcing the idea that their ancestors were also sessile.