Why do scientists debate whether the first ancestral chordate resembled a cephalochordate or a tunicate?

The debate regarding whether the first ancestral chordate resembled a cephalochordate or a tunicate centers on how to interpret the evolutionary loss or gain of specific traits.12

The classical theory posits that the ancestor was cephalochordate-like; in this view, one lineage became sessile and evolved into tunicates, while another remained free-swimming and led to vertebrates. This model is often preferred because it aligns well with the observed similarities between chordates and hemichordates.1

Conversely, an alternative theory suggests the ancestor was tunicate-like, with the other subphyla arising through modifications of the tadpole larva. The difficulty in resolving this debate stems from the ambiguity of the fossil record and the uncertainty of whether certain features absent in tunicates were never present or were simply lost over time. Furthermore, while some argue that the simplicity of cephalochordates reflects evolutionary degeneration, there is no definitive evidence to support this claim.12