In chordate evolution, vertebrates and tunicates share which key feature of body plan during at least some stage of their life cycle?

Introduction / Context:
Chordates are a major animal phylum that includes vertebrates, tunicates and cephalochordates. They are defined by a set of key features that appear at some stage in the life cycle. Vertebrates, such as fishes and mammals, and tunicates, also called urochordates, might look different as adults, but they share certain chordate characteristics. This question asks you to identify which such feature they have in common.

Given Data / Assumptions:

• The groups being compared are vertebrates and tunicates.
• We assume standard chordate features such as notochord and dorsal nerve cord are known.
• The options mention neural crest, jaws, skull and exoskeleton in addition to the core chordate traits.

Concept / Approach:
All chordates, at least at some stage, share four main features: a notochord, a dorsal hollow nerve cord, pharyngeal slits or pouches and a post anal tail. Vertebrates retain or modify these features and also possess a vertebral column and usually a skull. Tunicates have free swimming larvae that show a notochord and a dorsal nerve cord, but most adult tunicates become sessile and lose some of these traits. Neural crest, jaws and a well developed skull are more specific to vertebrates and are not shared with tunicates. The option that describes the basic chordate features common to both groups is therefore the correct one.

Step-by-Step Solution:
Step 1: Recall that a notochord is a flexible rod that supports the body in chordate embryos and some adults. Step 2: Recall that the dorsal hollow nerve cord is a tube of nervous tissue located above the notochord, which in vertebrates develops into the brain and spinal cord. Step 3: Recognise that both vertebrate embryos and tunicate larvae display these two structures, so they share them at least during early development. Step 4: Understand that neural crest cells, hinged jaws and a complex skull are special developments within vertebrates and are not present in tunicates. Step 5: Check that the option mentioning a notochord and dorsal hollow nerve cord captures the shared chordate identity of both groups.

Verification / Alternative check:
Textbooks on zoology typically introduce chordates by listing the notochord and dorsal hollow nerve cord as defining characteristics. They then show diagrams of vertebrate embryos and tunicate larvae, both with these structures labelled. Later sections highlight innovations such as vertebral columns, jaws and neural crest cells as features that evolved within the vertebrate lineage. This structure of explanation supports the view that the common feature between vertebrates and tunicates is the basic chordate body plan with notochord and dorsal nerve cord.

Why Other Options Are Wrong:
Option B, neural crest, refers to a group of embryonic cells unique to vertebrates that give rise to structures such as parts of the skull and peripheral nerves, and is not shared with tunicates. Option C, jaws adapted for feeding, describes a feature found only in jawed vertebrates and not in tunicates. Option D, an endoskeleton with a fully developed skull, again applies to vertebrates and not to tunicate adults or larvae. Option E describes a segmented exoskeleton made of chitin, which is typical of arthropods, not chordates.

Common Pitfalls:
A common mistake is to focus on the dramatic differences between adult tunicates and vertebrates and forget that classification is often based on embryonic features. Tunicate adults look more like bags attached to rocks, so students may incorrectly think they have little in common with vertebrates. Remembering that tunicate larvae show a clear notochord and dorsal nerve cord, and that these are hallmark chordate features, will help you identify the correct shared characteristic.

Final Answer:
Vertebrates and tunicates both share A notochord and a dorsal, hollow nerve cord at some stage of their life cycle.