PDF Summary:The Rise and Reign of the Mammals, by Steve Brusatte

Chronicling the extraordinary evolutionary journey of mammals, Steve Brusatte's vivid account in The Rise and Reign of the Mammals takes readers from the humble beginnings of mammalian ancestors to the astonishing diversification that followed the dinosaurs' demise. With rich detail, Brusatte unveils the emergence of key mammalian traits—milk production, specialized teeth, keen senses—and explores how environmental upheavals enabled placental mammals to thrive.

From the diminutive shrew-like creatures of the Mesozoic era to the behemoths that roamed the earth in later epochs, this narrative unveils the wonders of mammalian evolution across eons. Brusatte illuminates how mammals conquered land, air, and sea, detailing the rise of iconic species like bats, elephants, and horses.

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The evolution of therians was significantly advanced by the emergence of tribosphenic molars, which led to their specialization as insectivores and subsequently ignited the diversification of the progenitors of both placental and marsupial mammals.

In the era known as the Cretaceous, therians started their rise to prominence, sharing the world with the thriving multituberculates. The group that includes contemporary placental and marsupial mammals is characterized by the unique evolutionary development of the tribosphenic molar.

Steve Brusatte emphasizes the significance of the therians' evolutionary beneficial tooth structure in their rise to dominance. He explains how the complex shapes of these molar teeth improved their effectiveness in crushing the hard exteriors of insects, which was vital for the pollination of flowering plants during the Cretaceous period. Brusatte suggests that the complex configuration of these particular molars was a key factor in spurring evolutionary progress, allowing therians to modify their dietary habits and become resilient to changes in their surroundings by making slight changes to the cusps and crests of their teeth. He comments on the remarkable versatility of therians in adjusting to a variety of environments. Mammals have evolved from their original tribosphenic blueprint into an impressive variety of species we see today, each exhibiting unique dietary behaviors, ranging from shrews that consume insects, to dogs and cats that prey on other animals, dolphins that hunt marine life, and primates such as humans who are omnivorous, consuming a mix of flora and fauna. The emergence of the tribosphenic molar greatly enhanced the ability of therians to hunt insects and also endowed them with the evolutionary flexibility to take on new ecological niches, which in turn paved the way for their supremacy among mammalian groups in later periods.

Other Perspectives

• While mammals and dinosaurs did evolve in parallel, it's important to note that dinosaurs were the dominant terrestrial vertebrates during the Mesozoic era, and mammals were mostly small and nocturnal. The idea of "parallel" evolution might overstate the prominence of mammals during this time.
• The diversity of docodonts is well-documented, but it's also true that their ecological impact was likely limited compared to the dinosaurs, which occupied the top tiers of the food chain.
• The gliding haramiyidans represent a fascinating evolutionary development, but it's worth considering that gliding does not equate to powered flight, which birds (descendants of dinosaurs) achieved. This represents a significant difference in the evolutionary trajectories of mammals and dinosaurs.
• While angiosperms did provide new ecological opportunities for mammals, it's also true that not all mammalian groups benefited equally, and some did not exploit these resources until after the dinosaurs' extinction.
• The success of multituberculates during the Cretaceous is clear, but their eventual decline and extinction suggest that their adaptations were not sufficient to ensure long-term survival beyond the Cretaceous-Paleogene extinction event.
• The tribosphenic molar was indeed a significant adaptation for therians, but it's also true that not all therians or their descendants utilized this tooth structure, as dietary specialization led to a variety of dental adaptations in different mammalian lineages.

The catastrophic events led to the survival and subsequent thriving of some species, while numerous others ceased to exist.

The cataclysmic end of the Permian period almost led to the extinction of all life forms.

The phenomenon known as the Lilliput effect, which highlights the survival benefits of reduced body size due to advantages like burrowing abilities, potential for entering dormancy, and swift attainment of maturity and reproductive readiness in cynodonts, is thoroughly substantiated.

Brusatte recounts how cynodonts, the forerunners of mammals, endured the most devastating extinction event in Earth's history, which brought the Permian period to a close approximately 252 million years ago. A global environmental catastrophe characterized the end-Permian, resulting in the annihilation of up to 96 percent of marine life and over 70 percent of land-based species, a disaster triggered by massive volcanic eruptions that flooded the atmosphere with carbon dioxide and other greenhouse gases.

Brusatte attributes the endurance of cynodonts through the catastrophic events to their small size and consequent evolutionary changes. He explains that diminutive creatures, especially skilled burrowers, could evade extreme weather and endure unfavorable climatic periods by entering a dormant state known as estivation. The small size of cynodonts contributed to their rapid growth and breeding, enabling a swift recovery in their population following a significant catastrophic event. Brusatte explains that after significant extinction events, the species that survive and thrive often show a tendency towards smaller size, a phenomenon he calls the Lilliput effect. The survival of cynodonts and their kin is linked to a specific occurrence that, although not widespread, definitely took place in their case.

Thrinaxodon, due to its evolutionary characteristics, endured the mass extinction and played a pivotal role in propelling the lineage that would lead to mammals into the subsequent Triassic period.

Brusatte recounts the story of Thrinaxodon, a cynodont comparable in size to a weasel, emphasizing its tenacity and survival through the cataclysmic event that marked the end of the epoch preceding the Triassic. He explains that Thrinaxodon, along with numerous cynodonts, possessed a proficiency for burrowing. Fossils of Thrinaxodon have been unearthed within underground dens, accompanied by the remnants of various species, such as amphibians. Brusatte deduces from these fascinating clusters that Thrinaxodon likely entered a state of dormancy coinciding with the commencement of the wet season.

Thrinaxodon, according to Brusatte, survived the challenging post-extinction environment through several evolutionary changes such as digging habits, estivation, and its quick maturation and swift reproductive cycle. He portrays Thrinaxodon as a species that flourished in the chaotic ecosystems following extinction events, playing a pivotal role in the endurance of the lineage that would eventually give rise to mammals. Brusatte emphasizes the extraordinary endurance of Thrinaxodon, which was among the few species to withstand the most devastating extinction event in history, thereby ensuring the continuous lineage of mammals prior to the emergence of their diverse forms.

The cataclysmic end of the Mesozoic era signified a critical juncture in history.

The asteroid impact on Earth caused extensive devastation, which severely impacted dinosaurs, whereas certain mammals experienced comparatively minor effects.

Brusatte recounts the dramatic events that unfolded 66 million years ago, focusing on the chain of events that began when an asteroid, roughly six miles in diameter, struck Earth. The devastating incident occurring on the Yucatan Peninsula in Mexico is recognized as Earth's deadliest period, resulting in the demise of almost 75% of all species, with only birds remaining as the descendants of dinosaurs. The catastrophic events following the impact, such as firestorms, tsunamis, earthquakes, and a prolonged period of intense cold and darkness, had a devastating effect on global ecosystems and significantly changed the course of Earth's biological evolution.

The impact of the asteroid had a significant impact on mammals, which Brusatte notes narrowly avoided the same fate of extinction that the dinosaurs suffered. He maintains that mammals narrowly escaped total annihilation. Mammals, distinguished by their fur, milk production, the evolution of jaw bones into ear bones, and varied teeth structures, teetered on the edge of complete annihilation, a fate that could have mirrored the demise of the prehistoric reptiles. Brusatte emphasizes that the discovery of fossils in Montana's Hell Creek and Fort Union formations offers undeniable evidence that mammals endured the catastrophic event, signifying the transition from the Cretaceous period's conclusion to the dawn of a novel geological epoch. In North America, only a scant seven percent of mammal species survived the extinction event.

The emergence and spread of small, adaptable mammals following the extinction event can be attributed to their intrinsic adaptability.

Brusatte illustrates how a select few mammalian species flourished as ecosystems recovered from the devastating event marking the end of the Cretaceous period. Following the extinction event, species well-adapted to challenging conditions, akin to the way cockroaches and rats flourish in modern harsh urban settings, discovered that the new environment was especially conducive to their survival.

Steve Brusatte sheds light on the triumph of certain species following the disaster by examining fossils discovered in Montana. The Z-Line Quarry is renowned for its mammalian fossil deposits, which date back to around 25,000 years following the asteroid impact that resulted in extensive species extinction. The vast array of discovered fossils mainly includes remains from three distinct species: Mesodma, a multituberculate; Thylacodon, a marsupial; and Procerberus, which belongs to the lineage of placental mammals. The diminutive size of these three mammalian lineages or their ancestors, which allowed them to hide by burrowing, coupled with their diverse dietary habits that allowed them to eat a variety of available food, contributed to their endurance during the devastating environmental consequences that originated from the asteroid. Mammals flourished following catastrophic events, as depicted by Brusatte. Our story previously acknowledged their existence after the monumental catastrophe that marked the end of the Permian period. These creatures prosper in chaos, similar to how mammals succeed in conditions characterized by turmoil and filth, comparable to cockroaches.

The evolutionary success of placental mammals may be linked to the emergence of an advanced placental structure in the epoch following the Cretaceous period.

Brusatte emphasizes the swift evolutionary adjustments and subsequent proliferation of species, especially the placental mammals, during the period of initial resurgence after the cataclysmic event at the dawn of the Paleocene. Marsupials, previously flourishing in both abundance and diversity, encountered a phase of near extinction with the end of the Cretaceous period. Eutherians, who are the forebears of current placental mammals, eventually rose to dominance. After the asteroid impact, Brusatte suggests that eutherians took advantage of the newly vacant ecological niches, and he theorizes that the conditions following the extinction event probably promoted the success of mammals possessing an advanced and highly efficient placental system.

Brusatte characterizes the placenta as a crucial evolutionary advancement, functioning as a temporary organ that facilitates the nourishment of the fetus and the disposal of its waste in those species that have this characteristic. Monotremes, including the platypus and echidna, belong to a category of mammals that lay eggs, a characteristic probably inherited from their distant mammalian forebears. Marsupials give birth to offspring that are notably tiny and underdeveloped, which then proceed to mature in a distinct protective pouch. Placental mammals give birth to young that are more developed and equipped to face the initial challenges of their environment, thanks to the sophisticated evolution of their placentas that facilitates prolonged in-womb nourishment. Brusatte suggests that the sophisticated reproductive strategies employed by these animals could have contributed to their success during the unstable and unpredictable Paleocene epoch. Steve Brusatte posits that the reproductive systems of the initial eutherians, which likely evolved to support placental development, emerged in the Paleocene epoch. The offspring of Ectoconus potentially reached a more mature developmental stage upon birth, owing to the sustenance and safeguarding afforded by the connection to the placenta throughout their prenatal growth. Ectoconus displays a more distinct divergence when compared to Carissa. It was considerably larger in size.

Context

• The Lilliput effect is a phenomenon where surviving species after a major extinction event tend to have smaller body sizes. This trend has been observed in various organisms in the fossil record, with some hypotheses suggesting reasons for this size decrease. The term originates from an island in "Gulliver's Travels" and highlights the evolutionary response of species to catastrophic events. The Lilliput effect is particularly notable after the Permian-Triassic mass extinction, where fauna in the Early Triassic period exhibited significantly smaller sizes compared to preceding and subsequent periods.
• Estivation is a survival strategy used by some animals during extreme environmental conditions like drought or high temperatures. It involves entering a dormant state to conserve energy and endure unfavorable circumstances until conditions improve. Animals in estivation reduce their metabolic rate and activity levels to cope with the challenges posed by their environment. This adaptation allows them to survive periods of scarcity or harsh conditions until more favorable circumstances return.
• Eutherians are a group of mammals that include placental mammals and those more closely related to placentals than to marsupials. They are distinguished by specific anatomical traits like the absence of epipubic bones and unique features in their feet, ankles, jaws, and teeth. Eutherians have a reproductive system that allows for expansion of the abdomen during pregnancy, a characteristic not found in marsupials or monotremes. The earliest eutherians date back to the Early Cretaceous period, with fossils found in various regions like China and England.
• Monotremes are a unique group of mammals that lay eggs instead of giving birth to live young. The only existing monotremes are the platypus and echidnas. Monotremes have distinct anatomical features compared to other mammals, such as differences in brain structure and reproductive organs. They are native to Australia and New Guinea, with a history dating back to the Late Cretaceous period.
• Marsupials are a group of mammals characterized by their unique reproductive strategy where young are born in an undeveloped state and nurtured in a pouch. They are found in Australasia, Wallacea, and the Americas, with kangaroos, koalas, and opossums being well-known examples. The evolutionary split between marsupials and placentals occurred over 125 million years ago, leading to the diverse marsupial species we see today. The word "marsupial" is derived from "marsupium," which is the term for the abdominal pouch where marsupial young develop.
• Placental mammals are a group of mammals that nourish their developing offspring through a placenta in the uterus, allowing for more advanced development before birth. They are anatomically distinct from other mammals in several ways, such as having a wide pelvis opening for birthing larger babies and lacking epipubic bones found in other mammals. Placental mammals form the majority of extant mammalian species and are part of the infraclass Placentalia within the class Mammalia.
• The Paleocene epoch lasted from about 66 to 56 million years ago and is the first epoch of the Paleogene Period in the Cenozoic Era. It followed the K–Pg extinction event, which marked the demise of the dinosaurs, and was characterized by significant climatic events like the Paleocene–Eocene Thermal Maximum. During this time, there were geological and biological changes, including the uplift of the Rocky Mountains and shifts in ocean circulation patterns. The Paleocene saw a turnover of species, with new forms replacing those that had become extinct.
• Ectoconus is an extinct genus of terrestrial herbivorous mammal from North America during the Early Paleocene period, approximately 66 to 63.3 million years ago. It was stoutly built, about the size of a sheep, with primitive ungulate-like feet and a heavy tail. Ectoconus is well-known due to the discovery of nearly complete skeletons in New Mexico, making it one of the most thoroughly understood Paleocene mammals.

The emergence of modern mammalian groups.

During the Paleocene-Eocene Thermal Maximum, the biogeographical regions experienced a significant shift in their distribution.

The PETM Trinity: the sudden appearance of primates, artiodactyls, and perissodactyls at the start of the Eocene, likely due to global warming-spurred migration

Brusatte recounts that, about ten years following the devastating impact from space which resulted in the extinction of the dinosaurs, our planet experienced a substantial period of increased temperatures referred to as the PETM. Global temperatures during the PETM increased substantially, with a surge ranging from five to eight degrees Celsius over several millennia, due to massive volcanic activity in the North Atlantic.

During the PETM, although it did not trigger a mass extinction, Brusatte's account demonstrates how this profound period of increased temperatures impacted the evolutionary progress and branching out of mammals, which in turn led to their extensive spread and the swift rise of numerous modern lineages. The author suggests that the PETM seemingly initiated the extensive dispersal of various species. The emergence of Trinity fossils, suggestive of a thick swarm of insects, complicates the task of tracing their exact routes of spread. Did the initial evolutionary steps of their journey begin on the Asian continent before dispersing to other landmasses including Europe and North America? Could we contemplate taking a divergent route or opting for a completely distinct direction? During this era, the group comprising primates, artiodactyls with an even number of toes, and their relatives characterized by an uneven number of toes, underwent a rapid diversification. The first perissodactyls were small, nimble mammals with multiple toes, bearing a resemblance to miniature horses.

Mammalian body size experienced considerable changes in conjunction with shifts in the environment at the time of the Paleocene-Eocene Thermal Maximum.

Steve Brusatte illustrates how the increasing warmth and changing environments during the PETM were pivotal in altering the bodily proportions of mammals. As temperatures rose, many mammals became smaller, likely as an adaptation for better regulating body temperature and avoiding overheating, and then, as climates cooled, these mammals rebounded and increased in size once more.

Steve Brusatte's narrative provides insight into how the remarkably intact mammalian fossils discovered in Wyoming's Bighorn Basin shed light on the relationship between climatic shifts and the physical dimensions of ancient mammals. For instance, Sifrhippus, identified as an early equine species, experienced a substantial reduction in size, shrinking by about one-third during a phase of increasing temperatures, yet swiftly regained its initial dimensions after the period of heat ended. The horses that Brusatte discusses underwent a decrease in size as global temperatures rose, and conversely, they grew larger when the temperatures fell. The traits of Ross are especially captivating. Secord's research suggests that the ongoing rise in temperatures could lead to a decrease in the physical dimensions of many mammal species. This may also encompass humans. Humans also belong to the class of mammals. The PETM's body size variations highlight the extraordinary capacity of mammals to adapt swiftly to changes in climate, demonstrating the quick pace at which evolution can react to environmental pressures.

In the Eocene epoch, a wide variety of mammals experienced significant diversification and prosperity.

The Messel Fossil Site served as a pivotal location for the initial development and eventual spread of placental mammals, culminating in species that possess characteristics we recognize in the present day.

Brusatte describes the period between 56 and 34 million years ago, known as the Eocene Epoch, as an era marked by increased temperatures, widespread forests, and a notable proliferation of mammalian species. The area in Germany, once known for oil shale extraction and now designated as a UNESCO World Heritage Site, offers a remarkable window into a community of mammals from the Eocene epoch, bearing a strong resemblance to modern groups of animals.

Brusatte depicts the slow replacement of early Paleocene mammals with placental ones, a transformation that unfolded over millions of years after the PETM and reshaped animal populations worldwide, with the Messel Fossil Site providing a prime illustration. He argues that the variety within this group of mammals was unmatched in relation to the faunas of New Mexico's Paleocene, surpassing all other mammalian ecosystems of the time in terms of species variety, ecological roles, dietary habits, physical configurations, and behavioral tactics. In the Eocene epoch, the variety of life exceeded that of the Paleocene, which itself had observed a broader array of species compared to the Cretaceous period. Fossilized specimens from Messel that can be identified as ancestors of contemporary species, including horses (Eurohippus), rodents (Ailuravus), primates (Darwinius), pangolins (Eomanis), bats, and carnivorous mammals (Lesmesodon), are integral to Brusatte's engaging narrative, which includes a captivating account of the life of an Eocene-era horse, all of which belong to the assortment of fossils discovered at Messel. Mammals have diversified, expanding into fresh ecological roles and enhancing their sensory systems, while also evolving unique physical traits that have set the stage for the modern variety of mammalian life we observe in the present.

The distinct species of mammals found in South America, such as the hoofed animals first recognized by Darwin, ultimately revealed an evolutionary link to their relatives in the north.

Throughout much of the Eocene epoch, Brusatte characterizes South America as an isolated continental landmass, separated from North America by a considerable stretch of ocean. During his voyage on the Beagle, Charles Darwin came across a unique assembly of plant-eating animals known as the "South American Ungulates," which were part of the extraordinary mammalian fauna of the region and sparked considerable curiosity in him. The proliferation of these ungulates, resulting in the appearance of creatures resembling camels and horses, rhinoceroses, and species similar to rodents, demonstrates the wide variety of species that can develop when they evolve in isolation.

Brusatte outlines the difficulties Darwin faced when classifying animals with hooves. Some species of mammals, however, left Darwin bewildered. Owen recognized Macrauchenia as an animal notable for its extended neck and limbs, with a body spanning roughly ten feet, or three meters, and a weight exceeding one ton. This animal bore a resemblance to a camel but had feet that were significantly larger and sturdier, similar in build to a rhinoceros's. Discussions to identify the nearest relatives and unique traits of these Ungulates have been ongoing among paleontologists for over 150 years. The answer was determined through the use of a genetic lineage verification test. Analysis of the genetic material from the long-extinct hoofed mammals Macrauchenia and Toxodon has revealed that their closest living kin are the perissodactyls, which encompass horses, rhinos, and tapirs. The ancestors of Ungulates likely originated in North America and journeyed to South America during the Paleocene epoch, before the full separation of the two landmasses occurred.

Other Perspectives

• The PETM's role in the distribution of biogeographical regions might be overstated, as other factors like continental drift and ecological competition could also have played significant roles.
• The term "PETM Trinity" oversimplifies the emergence of these mammalian groups, as it implies a sudden appearance rather than a gradual evolution that might have been occurring before the PETM but only became noticeable afterward.
• While the PETM likely contributed to the spread and diversification of mammals, it is possible that some of the diversification events attributed to this period may have been underway prior to the PETM or were influenced by other climatic and environmental factors.
• The relationship between body size and environmental shifts during the PETM is complex, and while there is evidence supporting dwarfism during warming events, other factors like resource availability and predation pressures could also influence body size changes.
• The importance of the Messel Fossil Site might overshadow other significant fossil sites that have contributed to our understanding of Eocene mammals and their evolution.
• The assertion that the Eocene epoch saw unparalleled diversification and prosperity of mammals could be challenged by fossil records from other periods that show similar or even greater levels of diversification.
• The evolutionary link between South American ungulates and their North American relatives is based on genetic evidence, but the exact nature of their relationship and the timing of their migration could be subject to further scrutiny and research.

The kingdom of mammals displays a remarkable range of extremes.

Land Giants: Elephants

The Afrotherian Radiation highlights the diversity of the afrotherian group, their restriction to the African continent, and their concurrent evolution with mammals originating from the upper half of the globe.

In his book, Brusatte explores the development and proliferation of the Afrotheria lineage, an essential branch within the four primary subdivisions of the placental mammal family tree, which primarily occurred in isolation on the African continent. The assortment of creatures including elephants, manatees, elephant shrews, tenrecs, and hyraxes demonstrates how mammals have branched out into various specializations to fill similar ecological niches, illustrating an instance of convergent evolution.

Brusatte depicts the Afrotheria's evolution on the secluded African continent, where they developed a variety of ecological adaptations akin to those seen in distantly related placental mammals across North America and Eurasia. He describes Laurasiatheria undergoing a phase of diversification in the northern hemisphere, recognized as one of the four principal evolutionary branches of placental mammals. In the northern regions, mammals with hooves, specifically perissodactyls and artiodactyls, thrived, whereas Africa was the habitat of hyraxes. In the northern areas, pangolins dined on ants and moles burrowed beneath the surface, while aardvarks along with the elusive golden moles established their habitats on the African continent. In the northern areas, hedgehogs and shrews thrived, while the wildlife of Africa featured elephant shrews and tenrecs, commonly known as 'Madagascar hedgehogs'. The Afrotheria, as Brusatte elucidates, underwent convergent evolution as they adapted to comparable environmental challenges, establishing themselves in habitats similar to those of the Laurasiatherian placentals, yet with distinct variations.

Elephants evolved from smaller ancestors into larger beings, a change characterized by the elongation of their trunks and the enlargement of their tusks, changes that were driven by their feeding habits, social behaviors, and adjustments to environmental shifts.

Elephants evolved from humble ancestors into the massive creatures that currently hold the title of the largest mammals on Earth, a transformation that occurred over a prolonged span of time. He recounts the evolution of prehistoric elephants in Morocco, charting their progression from creatures the size of dogs to immense behemoths, characterized by their extended snouts and sizable ivory.

Brusatte highlights the importance of Eritherium, a ten-pound animal from the period following the dinosaurs, which, despite its smaller size compared to many household cats, is acknowledged as the earliest known progenitor of today's elephants. The Numidotherium, which appeared early in the Eocene epoch, had an estimated weight of around 650 pounds and showed early signs of diminutive tusks as well as a shorter snout, indicative of the early stages of trunk formation. The progenitors of modern elephants underwent considerable size increases during the Oligocene and Miocene epochs. During the Miocene epoch, species such as Palaeomastodon in Egypt evolved to match the size of contemporary elephants, characterized by extended trunks and tusks that curved downward. The incomplete skeletal remains of Palaeoloxodon indicate that its weight might have soared to twenty-two tons, making it potentially among the heaviest land mammals that have ever lived. The enlargement of these elephants, as detailed by Brusatte, is linked to their herbivorous feeding habits and the evolution of their trunks, which aided in the gathering of sustenance. He emphasizes the evolutionary progression in early elephants, noting their distinctive foreheads and repositioned nasal openings, initial adaptations that ultimately resulted in the formation of their distinctive trunks. Elephants, with their distinctive trunks, revolutionized their method of obtaining food and water, enabling them to access these necessities without having to shift their entire mass, thereby facilitating their evolution into larger forms.

Nocturnal Sovereigns: Bats

Bats evolved from an ancestor skilled in gliding and gained ecological advantages through the development of sustained flight and echolocation navigation.

Brusatte recounts the remarkable evolutionary milestone where bats evolved to fly, a trait shared exclusively among three groups of vertebrates: pterosaurs, avians, and bats, which emerged during the Eocene period. Bats developed a distinctive mode of flying, with their wings being made up of skin stretched across elongated finger bones, essentially transforming their hands into wings.

In his account, Brusatte emphasizes the significance of the discovery made by paleontologist Nancy Simmons, which sheds light on the evolutionary progression of flight in bats through the species Onychonycteris. Onychonycteris displayed clear traits typical of bats, such as wings formed by skin and a shorter tail, as well as a more compact shape, but it also had primitive features like claws on its digits suitable for climbing, along with wings that were uniquely shaped and proportioned, suggesting a clumsier and less skillful flight capability. Steve Brusatte suggests that bats evolved from a progenitor skilled in arboreal gliding, akin to contemporary flying squirrels, and that their progression to active flight took place as these ancestors elongated their finger bones, transforming their gliding membranes into wings capable of flapping. Although there is a limited number of fossils that capture this evolutionary change, Brusatte underscores that the story is consistent with our current knowledge.

The evolution of echolocation is vital to bats as it enables them to pursue prey, orient themselves, and interact with one another in darkness.

Brusatte delves into the evolutionary journey of bats, emphasizing their mastery of flight and the sophisticated sensory system that allows them to navigate in darkness. He explains how bats navigate using echolocation: they emit high-pitched noises or clicks with their tongues and interpret the echoes that bounce back to mentally map their surroundings.

Brusatte suggests that the evolution of echolocation, similar to the advent of flight, probably occurred gradually after the initial shift to powered flight. The first bat species unearthed at the Messel fossil site had slightly larger cochleas, crucial for auditory processing within the inner ear, and lacked the intricate features necessary for producing high-frequency sounds; however, later bat species developed sophisticated systems for both emitting and perceiving sounds, enabling them to adeptly move through the dark. He explains that the ability to navigate and locate food by sound was crucial for the widespread success and prevalence of bats, enabling them to seek food at night, and he also highlights the lack of bat-like species among marsupials or Afrotherians, suggesting that their separation by location hindered the evolution of their own airborne types. Brusatte highlights the pivotal evolutionary development that enables bats to employ echolocation for orientation and food gathering in darkness.

Context

• The Afrotherian Radiation is a term used to describe the evolutionary diversification of a group of mammals known as Afrotheria, which includes elephants, manatees, elephant shrews, tenrecs, and hyraxes. These mammals are primarily restricted to the African continent and have evolved unique adaptations to fill various ecological niches. The Afrotherian Radiation showcases how mammals from this group have independently evolved similar traits to adapt to their environments, demonstrating a fascinating example of convergent evolution. This evolutionary process highlights the distinct evolutionary path taken by Afrotherian mammals compared to other placental mammals from different regions of the world.
• Laurasiatheria is one of the four major groups of placental mammals, alongside Afrotheria, Xenarthra, and Euarchontoglires. It includes diverse mammals like bats, carnivores, ungulates, and cetaceans. Laurasiatheria originated in the northern hemisphere and diversified there, with species like hooved mammals thriving in those regions. The group played a significant role in mammalian evolution, showcasing adaptations to various ecological niches across different continents.
• The elephant evolution section discusses the development of elephants from smaller ancestors to larger beings over millions of years. Specific species like Eritherium and Numidotherium are highlighted as early ancestors showing traits that eventually led to the evolution of modern elephants. The text mentions the Oligocene and Miocene epochs as periods when significant size increases and trunk development occurred in elephant ancestors. Notable species like Palaeomastodon and Palaeoloxodon are referenced as examples of ancient elephants that evolved into larger forms with distinctive features like elongated trunks and tusks.
• Bats evolved from ancestors skilled in gliding, transitioning to active flight by elongating their finger bones to form wings capable of flapping. The species Onychonycteris represents a pivotal point in bat evolution, displaying primitive features like claws for climbing and wings suited for less skillful flight. This transition from gliding to powered flight is supported by fossil evidence, indicating a gradual development towards the sophisticated flight capabilities seen in modern bats. The evolution of flight in bats is a remarkable example of adaptation to exploit ecological niches and thrive in diverse environments.
• Bats use echolocation as a sophisticated sensory system to navigate in darkness by emitting high-pitched noises or clicks and interpreting the echoes that bounce back to mentally map their surroundings. This ability allows bats to pursue prey, orient themselves, and interact with each other in the absence of light. The evolution of echolocation in bats probably occurred gradually after the initial shift to powered flight, with later bat species developing sophisticated systems for both emitting and perceiving sounds. Echolocation is crucial for the widespread success and prevalence of bats, enabling them to seek food at night and thrive in various environments.