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Difficulty: HardEvidence for Evolution: Paleontology and Fossil Records

The table below shows the distribution of three distinct index fossils (XX, YY, and ZZ) found within four undisturbed sedimentary rock strata (Layer 1 being the lowermost and oldest, and Layer 4 being the uppermost and youngest):

Rock LayerFossils Present
Layer 4 (Top)Fossil ZZ
Layer 3Fossil YY, Fossil ZZ
Layer 2Fossil XX, Fossil YY
Layer 1 (Bottom)Fossil XX

Based on the law of superposition and paleontology principles, which of the following deductions regarding the evolutionary timeline of these organisms is correct?

  1. Fossil XX represents the oldest ancestral organism, while Fossil ZZ evolved most recently in the geological timeline.Answer
  2. B
    Fossil ZZ is the oldest organism because individuals in upper strata accumulated somatic changes through environmental use and disuse over time.
  3. C
    Fossils XX and ZZ represent analogous structures that evolved simultaneously within the same geological period.
  4. D
    Fossil YY represents a discontinuous genetic variation of Fossil XX that appeared spontaneously in all rock layers.

Answer

Fossil XX represents the oldest ancestral organism, while Fossil ZZ evolved most recently in the geological timeline.
The law of superposition dictates that in undisturbed sedimentary rock sequences, the deepest layer (Layer 1) is the oldest and the uppermost layer (Layer 4) is the youngest. Because Fossil XX is found in Layer 1, it represents the earliest organism in the record. Fossil ZZ, occurring in Layer 4, represents the most recently evolved organism.

Step-by-Step Solution

1
Analyze the rock strata sequence using the Law of Superposition.
Layer 1 (bottom) is the oldest sedimentary deposit, followed by Layer 2, Layer 3, and Layer 4 (top, youngest).
In undisturbed sedimentary rock layers, deeper layers are deposited first and are older than overlying layers.
2
Map the occurrence of each fossil to its corresponding geological timeframe.
Fossil XX is present in Layers 1 and 2 (oldest timeframe); Fossil YY is present in Layers 2 and 3 (intermediate timeframe); Fossil ZZ is present in Layers 3 and 4 (youngest timeframe).
Fossils preserved in specific strata indicate the geological period during which those organisms lived.
3
Deduce the relative age and evolutionary succession of the organisms.
The evolutionary chronological order from oldest to newest is Fossil XX →\rightarrow Fossil YY →\rightarrow Fossil ZZ.
Sequential appearance of index fossils across rock strata reflects the chronological order of biological evolution over time.

Key Concept

Evidence for Evolution: Paleontology and Fossil Records
Estimated Time:2m 0s
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Evidence for Evolution: Paleontology and Fossil Records

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