Life and Animals in the Cenozoic Era: Evolutionary Triumphs and the Age of Mammals

 

Abstract

The Cenozoic Era (≈66 Ma to present) marks a transformative interval in Earth’s biological and ecological history, often termed the “Age of Mammals,” during which mammals, birds, and angiosperm-dominated terrestrial ecosystems reshaped global biodiversity. This paper presents a synthetic, theory-driven review of the major evolutionary and ecological changes across the Paleogene, Neogene and Quaternary periods, with particular emphasis on mammalian radiations, climatic and tectonic drivers (such as grassland expansion, mountain uplift and glaciation) and ecosystem restructuring. We introduce a conceptual model in which three interacting drivers—Environmental Forcing (E), Biotic Innovation (B) and Ecological Opportunity (O)—govern radiation and turnover rates: . Applying this framework to the Cenozoic reveals distinct pulses of diversification and turnover: an early-Paleogene mammalian radiation post-K–Pg, a Neogene phase of grassland-driven mammal evolution, and a Quaternary phase dominated by glacial cycles and hominin-driven ecological change. Analysis of fossil- and functional-diversity datasets shows that while mammalian morphological rate peaks occurred early, sustained turnover continued into the Neogene (Goswami et al., 2022; Figueirido et al., 2012). Our model underscores that mammalian diversification during the Cenozoic was neither uniform nor purely gradual but was modulated by pulses of environmental reorganisation and niche creation. These findings contribute to macroevolutionary understanding of how large vertebrate clades respond to climate, habitat transformation and biotic innovation, and they carry implications for understanding modern biodiversity shifts under anthropogenic stress.
Keywords: Cenozoic Era, mammalian diversification, grassland expansion, Cenozoic climate change, macroevolutionary model, ecological opportunity

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Introduction
The Cenozoic Era, spanning roughly 66 million years to the present, constitutes the most recent and biologically dynamic chapter of Earth’s history. This interval witnessed the aftermath of the Cretaceous–Paleogene extinction event (K–Pg), which eliminated non-avian dinosaurs and opened myriad ecological niches, thereby facilitating the diversification of mammals, birds and angiosperm-dominated ecosystems. (Figueirido, Janis, Pérez-Claros, De Renzi, & Palmqvist, 2012; USGS, n.d.). The subsequent Cenozoic record chronicles major climatic, tectonic and ecological shifts—such as global cooling, grassland expansion, mountain uplift and repeated glacial–interglacial cycles—that profoundly shaped terrestrial and marine life. It is therefore timely to examine the Cenozoic not merely as a chronological sequence of epochs (Paleogene, Neogene, Quaternary), but as a framework for understanding how mammals and other taxa radiated, adapted and in some cases declined in response to shifting landscapes and climates.

The purpose of this paper is three-fold: (1) to review the major evolutionary and ecological transformations of life during the Cenozoic Era, particularly focusing on mammals; (2) to present a conceptual theoretical model linking environmental forcing, biotic innovation and ecological opportunity to radiation and turnover dynamics; (3) to apply this model to Cenozoic phases and draw analytic insights into mammal evolution, faunal turnover, and ecosystem restructuring. In doing so, we integrate evidence from paleontology, functional diversity studies and macroevolutionary modelling to provide a coherent narrative of Cenozoic life and its drivers.

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Literature Review
The diversification of mammals during the Cenozoic has generated substantial research. Early works suggested a classic “mammalian radiation” in the Paleogene, but more recent studies nuance this narrative by revealing variation in temporal tempo and morphological disparity (Yu, Xu, Wu, & Yang, 2012). For instance, Goswami et al. (2022) demonstrated that placental mammal morphological evolution peaked soon after the K–Pg boundary and subsequently attenuated. Similarly, Shupinski, Wagner, Smith & Lyons (2024) documented in North America that functional diversity of mammal communities rose rapidly in the first 10 Ma after the extinction, then decoupled across spatial scales.

Climatic and ecological drivers are central to Cenozoic evolution. Figueirido et al. (2012) linked shifts in mammalian faunal associations in North-America with benthic δ¹⁸O records, reflecting global temperature changes. The expansion of C₄ grasslands during the Miocene has been widely documented (Osborne, 2008; National Research Council, 1995) and is closely tied to mammalian herbivore adaptations (Janis, Damuth & Theodor, 2000). In Asia, bursts of mammalian diversification and turnover coincide with tectonic events and reorganisation of monsoon systems (Ye et al., 2022).

Ecological opportunity—especially following mass-extinction events or climatic reshuffling—has been posited as a key driver of radiations (Yu et al., 2012). The post-K–Pg mammalian “opening” of niches is one such instance (Goswami et al., 2022). Yet the pattern is complicated by heterogeneity in diversification rates across lineages, morphological groups and ecological guilds. For example, herbivores appear to have responded more strongly to habitat transformation than carnivores (Figueirido, Janis, Pérez-Claros, De Renzi, & Palmqvist, 2012). Together, these strands of literature suggest that Cenozoic mammalian evolution was shaped by a dynamic interplay of environmental, biotic and ecological factors.

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Methodology / Theoretical Model
To structure and interpret the patterns of mammalian evolution during the Cenozoic, this paper proposes a conceptual model defined by three interacting drivers:

1. Environmental Forcing (E): palaeoclimatic change (e.g., global cooling, glaciation, monsoon intensification), tectonic/landscape change (mountain uplift, continental rearrangement) and vegetation shifts (forest to grassland).
2. Biotic Innovation (B): lineage-specific morphological or ecological novelties (e.g., endothermy, high-crowned dentition, volant mammals, large body size) that enable exploitation of new niches.
3. Ecological Opportunity (O): the availability of under-utilised niches, whether created by extinction, habitat restructuring or major ecosystem change.

We express net radiation/turnover rate for a clade across time as:

R(t) = f\bigl(E(t),\,B(t),\,O(t)\bigr)

Here, large values of E(t) may increase O(t) by disrupting incumbents and creating new habitats, while B(t) reflects the intrinsic ability of taxa to exploit opportunity. The model is qualitative: for each major interval (e.g., Paleogene, Neogene, Quaternary) we assign relative high/medium/low status to E, B and O based on palaeontological and palaeoenvironmental evidence, then infer relative radiation (R) values, and compare with empirical data on mammal diversity, morphological rate or turnover.

Operational steps:

1. Segment the Cenozoic Era into three major periods: Paleogene (66–23 Ma), Neogene (23–2.58 Ma) and Quaternary (2.58 Ma–present).
2. For each period, evaluate E, B and O qualitatively based on current literature.
3. Infer expected relative radiation/turnover R.
4. Compare with empirical proxies: mammalian functional diversity, morphological rate attenuation, evidence of grassland expansion and mammalian adaptation, and faunal turnover records (Goswami et al., 2022; Figueirido et al., 2012; Osborne, 2008).

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Results and Analysis
Applying the model yields the following assessments:

Paleogene (66–23 Ma):

• E(t): High, due to aftermath of K–Pg extinction, warm climates transitioning to cooling, landscape restructuring.
• B(t): Medium to High, as mammals radiated into many niches, along with diversification of birds and angiosperm ecosystems.
• O(t): High, because ecological niches vacated by dinosaurs and other Mesozoic lineages created opportunities.
  → Predicted R: High radiation/turnover.

Empirical data support a rapid burst in mammalian morphological evolution early in the Paleogene, with studies showing declining morphological rates thereafter. (Goswami et al., 2022; Shupinski et al., 2024). The dominance of forested ecosystems gave way to more seasonal habitats, enabling new mammal groups (Rose, 2006; Figueirido et al., 2012). Thus the Paleogene fits a high-radiation phase consistent with the model.

Neogene (23–2.58 Ma):

• E(t): High to Medium, with major climatic cooling, grassland expansion (especially Miocene), mountain building, tectonic shifts.
• B(t): High, due to innovations such as high-crowned teeth adapting ungulates to grasslands, apes and early hominins, aquatic mammals, etc.
• O(t): Medium to High, as new open-habitat niches emerged but many terrestrial niches were already filled.
  → Predicted R: Moderate to High radiation for certain clades, but not uniformly across all mammals.

Evidence: The spread of C₄ grasslands and adaptation of herbivores deepened (Osborne, 2008; National Research Council, 1995), and analyses indicate mammalian diversification bursts are linked to tectonic/climatic events in Asia during this interval (Ye et al., 2022). Predator functional morphology in North America shifted at the Eocene–Oligocene and Miocene (Figueirido et al., 2024). The model thus aligns with selective radiations and turnover during the Neogene—but not a uniformly high rate for all mammals.

Quaternary (2.58 Ma–present):

• E(t): High, driven by repeated glacial cycles, human impact, major ecological restructuring.
• B(t): Medium, as major mammal body plans are established but human technology and culture become the dominant innovation.
• O(t): Low to Medium, because many niches are saturated and human-driven extinctions reduce opportunity for radiation.
  → Predicted R: Lower radiation, with higher turnover (extinctions) rather than radiations.

Analyses of recent mammal functional diversity show decoupling and regional variability (Shupinski et al., 2024). Human-driven extinctions and climate change dominate ecological dynamics (Koch & Barnosky, 2006). Thus the Quaternary era corresponds to attenuated net radiation, consistent with the model.

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Discussion
The conceptual model offers insight into mammalian evolution during the Cenozoic by emphasising that diversification is not simply a function of time but a dynamic interplay of environment, innovation and opportunity. Several key implications emerge:

First, while the Paleogene represents a classical mammalian adaptive radiation following the K–Pg event, the model reveals how subsequent diversification was modulated by environmental and ecological saturation effects. Goswami et al. (2022) show morphological rates peak early and decline, consistent with the model’s expectation of declining O(t) over time as niches fill.

Second, the Neogene phase illustrates that diversification is not necessarily monotonic: although overall radiation may slow, specific clades (e.g., grassland herbivores, hominins, elephants) exploit new ecological opportunities created by grassland expansion and tectonic events. The model highlights that B(t) and O(t) vary by clade—herbivores adapted to grasslands may see high R even when other mammals are plateauing.

Third, the Quaternary emphasises that high environmental forcing (glaciations, human impact) does not automatically produce high radiation; rather, the availability of unoccupied niches declines, and turnover (i.e., extinction) dominates. This reinforces macroevolutionary arguments about declining net diversification rates in saturated ecosystems.

Fourth, the model underscores the importance of scale and clade-specific nuance. The functional diversity study by Shupinski et al. (2024) found that during the first 10 Ma of the Cenozoic, local and continental diversity rose in lockstep—but thereafter decoupled. This suggests spatial scale matters in assessing O(t) and R(t), and regional environmental dynamics (e.g., monsoon intensification, mountain uplift) may localise radiations (Ye et al., 2022).

Moreover, the model resonates with broader macroevolutionary theory: major environmental perturbations (e.g., K–Pg event, Miocene cooling, Pleistocene glaciations) create resets in ecosystem structure, but the subsequent exploitation of opportunity depends on the intrinsic traits of lineages (B) and the state of available niche space (O). Put another way: environment sets the stage, biology takes the lead, and opportunities determine the pacing.

However, limitations exist. The qualitative nature of the model means it does not produce quantitative predictions of speciation/extinction rates. The fossil record is uneven, and sampling biases cloud diversity curves (Figueirido et al., 2012). Also, differences among continental regions (e.g., Asia vs North America) highlight that E, B and O may have distinct values regionally and taxonomically rather than globally constant.

From a broader perspective, understanding Cenozoic mammalian evolution has relevance for contemporary biodiversity crises. The decoupling of functional diversity across scales observed by Shupinski et al. (2024) may illuminate how modern mammals respond to rapid anthropogenic change: when O(t) contracts and E(t) intensifies, turnover (extinction) may dominate rather than radiation. Thus, the Cenozoic record provides a deep-time analogue for the interplay of climate change, habitat transformation and biological adaptation.

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Conclusion
The Cenozoic Era stands as an exemplary case study of how life on Earth responds to sequential episodes of disturbance, innovation and opportunity. By applying a simple yet powerful conceptual model—where radiation and turnover are functions of Environmental forcing (E), Biotic innovation (B) and Ecological opportunity (O)—we have shown that mammalian evolution during this era was neither uniform nor purely gradual, but characterised by pulses and plateaux, dependent on the complex interaction of drivers.

Key conclusions include:

1. Paleogene Phase: In the wake of the K–Pg extinction, high environmental forcing (E) and abundant ecological opportunity (O) enabled mammals to radiate widely, supported by novel adaptations (B). Morphological and functional diversity rose rapidly.
2. Neogene Phase: Although environment continued to change markedly (grasslands, tectonics), ecological opportunity for many clades reduced as niches became occupied. Nevertheless, key innovations (e.g., high-crowned teeth in ungulates, hominins in savanna ecosystems) allowed selective radiations within particular clades.
3. Quaternary Phase: While environmental forcing was high, ecological opportunity declined and human influence increased, resulting in lower net radiation and increased turnover/extinction rather than diversification.
4. The model emphasises that biotic innovation and ecological opportunity—not simply time or taxonomic potential—govern diversification dynamics. In ecosystems approaching saturation, even strong environmental forcing may drive turnover rather than radiation.
5. From a mac­roe­volutionary perspective, the Cenozoic illustrates how major transitions (e.g., grassland ecosystems, hominins, volant mammals) often follow background disturbance and the opening of niches more than steady-state gradualism.

For future research, several avenues are recommended. Quantitative calibration of the model (e.g., estimating numerical values of E, B and O over time using proxies such as δ¹⁸O, morphological rate data, functional diversity indices) would allow formal testing of the model’s predictive power. Regional comparisons (e.g., Asia vs North America vs Africa) could reveal how geography modulates driver values. Investigations of functional diversity across trophic levels may unpack how herbivores, carnivores and omnivores responded differentially to Cenozoic change.

In sum, life in the Cenozoic did not simply “take off” and proceed smoothly upward; instead, it ebb­ed and flowed, surged and stalled, as mammals, birds and plants adapted to a planet in flux. By viewing the “Age of Mammals” through the lens of environmental forcing, innovation and ecological opportunity, we gain a richer understanding of how major vertebrate clades evolve—and how modern biodiversity may respond to the next wave of change.

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References
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