What does the fossil record show about evolution and mass extinctions?
The evidence for organic evolution from the fossil record (morphological change through time, transitional forms), the major patterns of the history of life, and the causes and consequences of mass extinctions, including the end-Permian and end-Cretaceous events.
A focused WJEC and Eduqas A-Level Geology G3 answer on the fossil evidence for organic evolution (morphological change through time and transitional forms), the broad history of life from the first cells to mammals, and the causes and effects of mass extinctions, focusing on the end-Permian and the end-Cretaceous events.
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What this dot point is asking
G3 is the biological half of interpreting the record. WJEC wants the fossil evidence for evolution, an outline of the history of life, and an account of mass extinctions and what they did to the course of life. The end-Permian and end-Cretaceous events are the set-piece examples. This connects to biostratigraphy (F3) and to palaeoclimate later in the topic.
The answer
Fossil evidence for evolution
The fossil record is the direct historical record of life, and it supports organic evolution in several independent ways:
The broad history of life
Life's history is read from the column: single-celled life early in the Precambrian, the rapid diversification of marine invertebrates in the Cambrian explosion, then fish, the move onto land by plants and amphibians, the rise of reptiles and the dinosaurs in the Mesozoic, and the radiation of mammals and the appearance of humans in the Cenozoic.
Mass extinctions
A mass extinction is a geologically rapid loss of a large fraction of species worldwide. Two dominate the syllabus:
The evidence for the end-Cretaceous impact is a global iridium-rich clay layer, shocked quartz and tektites at the boundary. Extinctions matter because they reset ecosystems: the loss of the dinosaurs opened niches into which mammals radiated.
Examples in context
The horse lineage is a textbook example of traceable evolution, from small, multi-toed ancestors to the large, single-toed modern horse. Archaeopteryx from the Solnhofen Limestone is the classic transitional fossil between reptiles and birds. The K-Pg boundary clay is exposed worldwide with its iridium anomaly, and the ammonites that vanish at it were among the most useful zone fossils of the Mesozoic, ending their long success.
Try this
Q1. Name a transitional fossil and the two groups it links. [2 marks]
- Cue. Archaeopteryx links reptiles and birds (feathers and wings, but teeth, claws and a bony tail).
Q2. State the likely cause of the end-Permian extinction and one of its effects. [2 marks]
- Cue. Siberian Traps volcanism caused warming, anoxia and acidification, removing about 90 percent of marine species.
Q3. State two pieces of evidence that the end-Cretaceous extinction involved a meteorite impact. [2 marks]
- Cue. A global iridium-rich clay layer and shocked quartz (and tektites) at the boundary.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksDescribe the evidence from the fossil record that supports the theory of organic evolution.Show worked answer →
Set out the kinds of evidence the rocks provide, because the marks reward distinct lines of support.
Change through time: fossils in successively younger strata show gradual changes in morphology, so a lineage can be traced changing form over millions of years, for example the increase in size and the reduction of toes in the horse lineage.
Transitional forms: fossils with mixed features link major groups, for example Archaeopteryx with feathers and wings (bird-like) but teeth, claws and a bony tail (reptile-like), bridging reptiles and birds.
Order of appearance: simpler organisms appear first and more complex ones later, with major groups appearing in a consistent order up the column (invertebrates before fish, fish before amphibians, and so on).
Comparative and developmental evidence in living forms, together with the geographical distribution of fossils, reinforces the pattern.
So the fossil record shows directional morphological change, transitional forms and a consistent order of appearance, all consistent with descent with modification.
Markers reward morphological change through time, named transitional forms such as Archaeopteryx, and the consistent order of appearance of groups.
WJEC Eduqas 20215 marksExplain the likely causes and the consequences of the end-Cretaceous mass extinction.Show worked answer →
Give the leading cause and then the biological outcome, because both are asked.
The leading cause is a large meteorite impact at Chicxulub (Mexico), evidenced by a global iridium-rich clay layer, shocked quartz and tektites at the Cretaceous-Palaeogene boundary. The impact threw dust and aerosols into the atmosphere, blocking sunlight, suppressing photosynthesis and collapsing food chains; massive Deccan volcanism in India may have added climatic stress.
The consequence was the extinction of about three-quarters of species, most famously the non-avian dinosaurs, along with ammonites and many marine plankton groups. The removal of the dinosaurs opened ecological niches that mammals then radiated into, leading to the rise of mammals (and eventually primates) in the Cenozoic.
Markers reward the impact evidence (iridium, shocked quartz), the sunlight-blocking and food-chain collapse mechanism, the loss of the dinosaurs and ammonites, and the subsequent radiation of mammals.
Related dot points
- The use of functional morphology to interpret the mode of life of fossil organisms (feeding, locomotion, environment), the concept of trace fossils and their value, and the use of fossil assemblages and adaptations to reconstruct past environments.
A focused WJEC and Eduqas A-Level Geology G3 answer on functional morphology, how the shape and structure of a fossil reveal its feeding, locomotion and environment, the value of trace fossils, and how fossil assemblages and adaptations are used to reconstruct ancient environments and ecological relationships.
- The use of lithological and palaeontological proxies (evaporites, coals, tillites, coral reefs, fossil assemblages) and isotopic and geochemical methods to reconstruct past climates, and the role of palaeoclimate evidence in confirming continental movement.
A focused WJEC and Eduqas A-Level Geology G3 answer on how past climates are reconstructed from lithological proxies (evaporites, coals, tillites, reef limestones), fossil assemblages and oxygen-isotope and geochemical methods, and how palaeoclimate indicators found in unexpected latitudes provide evidence for continental drift.
- The conditions and modes of fossil preservation, the principle of faunal succession, and the use of zone (index) fossils to correlate and relatively date strata.
A focused answer to WJEC and Eduqas A-Level Geology F3 on fossils, covering the conditions and modes of fossil preservation, the principle of faunal succession, and how zone (index) fossils are used to correlate strata between areas and to relatively date rocks.
- The structure of the geological timescale (eons, eras, periods and epochs), how it is built from relative and absolute dating, and the major events that define its main boundaries.
A focused answer to WJEC and Eduqas A-Level Geology F3 on the geological timescale, covering its hierarchy of eons, eras, periods and epochs, how it is constructed from combined relative and absolute dating, and the major events (mass extinctions and the appearance of major groups) that define its principal boundaries.
- The principles of relative dating (superposition, original horizontality, cross-cutting relationships, included fragments and unconformities) and how they are combined to establish the sequence of geological events.
A focused answer to WJEC and Eduqas A-Level Geology F3 on relative dating, covering the principles of superposition, original horizontality, cross-cutting relationships, included fragments and unconformities, and how they are combined to reconstruct the order of geological events in a sequence.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)