HSC Exam Practice

Superposition: deeper layers were deposited first and are older [1]. When fossils in different layers are compared, older strata contain simpler or earlier forms while younger strata contain more recently evolved groups — a time-ordered pattern consistent with descent with modification and change over time [1].

Relative dating determines whether a fossil is older or younger than another by its rock layer position, without assigning an absolute age in years [1]. Radiometric dating calculates an absolute age using the ratio of parent to daughter isotope and the known half-life [1]. For example: carbon-14 is used for recent once-living material because of its shorter half-life (~5,730 years); uranium-lead is used for ancient rocks because of its much longer half-life, appropriate for geological timescales of millions of years [1].

A transitional fossil shows a mix of features from two major groups — it does not need to be a direct ancestor, only to show the predicted combination of traits near a transition [1]. Archaeopteryx has reptile-like features (teeth and a long bony tail) alongside bird-like features (feathers and wings) [1]. This implies that birds and theropod dinosaurs share a common ancestor and that the traits associated with each group evolved gradually, with some overlap during the transition [1].

Reason 1: Soft tissue decays rapidly and is rarely preserved, so the record is biased toward hard parts (shells, bones, teeth). Implication: many lineages and soft-bodied organisms are absent from the record [1+1]. Reason 2: Geological processes such as erosion, metamorphism and tectonic activity can destroy or distort strata. Implication: some fossils that once existed may no longer be available for study, creating apparent gaps that do not reflect non-existence of the organism [1+1].

The site’s oldest exposed rocks were deposited in a marine environment, and more recent rocks were deposited in an aquatic environment that supported fish. Either later rock layers were not exposed or no terrestrial vertebrates lived in this area during the time represented by the accessible strata.

(i) 2 marks. Body size increases from small (dog-sized Hyracotherium at ~55 MYA) to large (modern Equus at ~4 MYA–present) [1]. Toe structure reduces from 4 functional toes in Hyracotherium to a single hoof in Equus, with intermediate stages visible in Mesohippus (3 toes) and Merychippus (3 toes, middle enlarged) [1].

(ii) 3 marks. The data shows directional, time-ordered change in body size and limb structure — this is what would be expected from descent with modification under changing selection pressures (e.g. shifting from browsing in forests to grazing on open grasslands) [1]. Each successive genus shows intermediate forms between the earlier and later extremes, consistent with gradual change over time [1]. The changes are in one consistent direction (increasing size, reducing toe number) rather than random, which is consistent with natural selection acting on inherited variation [1].

(iii) 2 marks. One limitation: fossil data alone cannot confirm that these genera are in a direct ancestor–descendant sequence — they could be closely related side branches [1]. Molecular evidence (DNA or protein comparison from closely related modern species) or additional fossil specimens from transitional time points could help confirm the lineage and the sequence of trait changes [1].

Marking criteria (1 mark each):

• Explains stratigraphy and superposition as a source of evidence for change over time.
• Uses a named transitional fossil with specific features to illustrate evolutionary transition.
• Explains how radiometric dating confirms the time-ordering of the fossil record independently of stratigraphy.
• Acknowledges at least two specific limitations of the fossil record (e.g. soft tissue bias, geological destruction, rare preservation).
• Explains why gaps reflect preservation bias rather than absence of evolution.
• Reaches an explicit evaluative conclusion that the limitations do not make the record unreliable when the preserved evidence shows a consistent, convergent pattern.
• Quality mark: coherent, structured response using precise lesson terminology throughout.