Christine Janis

Brown University · Department of Ecology and Evolutionary Biology

Kangaroos (Macropodoidea) display a diversity of locomotor modes, from bounding quadrupedally to hopping bipedally; but hopping has a body mass limit, which was exceeded by a number of extinct taxa. In the Pleistocene, a variety of “giant” kangaroos existed, both within the extinct subfamily Sthenurinae and the extant subfamily Macropodinae (both w...

Using finite element analysis on the astragali of five macropodine kangaroos (extant and extinct hoppers) and three sthenurine kangaroos (extinct proposed bipedal striders) we investigate how the stresses experienced by the ankle in similarly sized kangaroos of different hypothesized/known locomotor strategy compare under different simulation scena...

The larger species in many mammalian clades have relatively longer faces than their smaller relatives. This has been shown to be true for extant kangaroos (Macropodinae), who follow the CREA rule of positive facial allometry; but the extinct short-faced kangaroos (Sthenurinae) have not so far been examined. Using linear measurements, rather than CR...

Sabertoothed mammalian predators, all now extinct, were almost exclusively feloid carnivorans (Eutheria, Placentalia): here a couple of extinct metatherian predators are considered in comparison with the placental sabertooths. Thylacosmilus (the “marsupial sabertooth”) and Thylacoleo (the “marsupial lion”) were both relatively large (puma‐sized) ca...

Kangaroos (Macropodoidea) display a diversity of locomotor modes, from bounding quadrupedally to hopping bipedally, but hopping has a body mass limit, which was exceeded by a number of extinct taxa. In the Pleistocene a variety of "giant” kangaroos existed: members of the extinct subfamily Sthenurinae have been previously considered to have a type...

Bipedal hopping is a mode of locomotion seen today in four rodent lineages and one clade of marsupials. The Argyrolagidae, marsupials from the Oligocene to Pliocene of South America, have also been considered to be hoppers. These lineages all convergently evolved similar general morphologies, with elongated hindlimbs, reduced forelimbs, and elongat...

This chapter focuses on lissamphibians. It details the life histories of lissamphibians, which include egg hatching, parental care, vocalization, and fertilization. The distinct lineages of extant animals generally referred to as amphibians range between Gymnophiona (caecilians), Caudata (salamanders), and Anura (frogs). Since lissamphibians are th...

This chapter discusses early vertebrates. These were more complex and more active animals than the nonvertebrate chordates. Novel vertebrate anatomical features included a tripartite brain enclosed by a cranium, and a muscular pharynx for using the gills for respiration rather than for filter feeding. The chapter mentions gnathostomes, jawed verteb...

This chapter covers the characteristics and systematics of theropods while also considering the origin of birds. It mentions how early theropods were small, agile, lightly built, bipedal carnivores with long arms and legs. The evolutionary history of theropods varies since some gain forms with long and greatly enlarged forelimbs, while other forms...

This chapter discusses the lineages of sarcopterygians in relation to the origin of tetrapods. The vast majority of extant sarcopterygians are tetrapods, most of which are terrestrial. The chapter looks into fossil sarcopterygians which document transitions in organ systems associated with the origin of tetrapods and the move to land. This is where...

This chapter examines the origins of lissamphibia and amniota. It provides an overview of Paleozoic tetrapods, which includes the taxa on the divergence and diversification of lissamphibians and amniotes. Evolutionary changes in skulls, vertebrae, limb girdles, limbs, and ankles have enabled the tracking of the diversification of Paleozoic tetrapod...

This chapter notes the unique structure of turtles. The unique body form of turtles makes them immediately recognizable, but their anatomical rearrangements have obscured morphological characteristics which are used to determine evolutionary affinities among other vertebrates. The shells of different turtle species reveal their habitat and lifestyl...

This chapter looks at chondrichthyans, which are not primitive fishes, but are highly developed. The first definitive chondrichthyan fossils came from the Early Devonian period, although chondrichthyan-like scales are known to have existed from the early Silurian, and possibly the Ordovician periods. The chapter highlights the two main branches of...

This chapter explores the geography and ecology of the Cenozoic era. It cites how changes in continental positions have affected Earth's climates and the ability of vertebrates to disperse between its regions. By the late Mesozoic era, continental separation and epicontinental seas had isolated populations of terrestrial tetrapods and freshwater ve...

This chapter considers the sequential acquisition of the most important mammalian characters within the synapsid lineage. It explores the evolutionary history of synapsids in relation to their feeding, hearing, locomotion and breathing. The three clades of Mammalia range between Prototheria (monotremes and extinct relatives), Allotheria (multituber...

This chapter covers the origin of Osteichthyes and the radiation of actinoptergians. It details the key differences between actinopterygians and sarcopterygians before turning to actinopterygian evolution, diversity, swimming, reproduction, and ecology. After the industrial world brought increasing destruction and pollution of resources, different...

This chapter explores earth's history and divides it into the Precambrian and the Phanerozoic. No life was present during the first part of the Precambrian, informally known as the Hadean, while the Phanerozoic represents the last 540 million years of Earth's 4-billion-year history. Yet it is from this time that most multicellular life is known. Th...

This chapter discusses the relationship between primate evolution and the emergence of humans. Molecular techniques show that chimpanzees and bonobos are the closest extant relatives to humans. Humans and their fossil relatives belong to a tribe within Hominidae called Hominini. In comparison to other hominids, humans exhibit three major derived ch...

This chapter explains how the crown group Lepidosauria is one of the two largest clades of tetrapods. It looks into the diverse groups of lepidosaurs and their body forms and habitats. Extant lepidosaurs branched into two lineages: a single species of rhynchocephalian (also known as the tuatara), and more than 11,000 species of squamates. Mriad spe...

This chapter looks into early avemetatarsalians and the origin of Dinosauria. From the period of late Triassic and Mesozoic, Avemetatarsalia is a large and diverse clade including many familiar extinct vertebrates, such as pterosaurs and non-avian dinosaurs like birds. Moreover, Avemetatarsalians decoupled the functions of the fore and hindlimbs, w...

This chapter explores the diversity, classification, and evolution of vertebrates. It highlights how evolution is central to biology since its overarching principles allow further understanding of how living organisms operate and organize their diversity. Phylogenetic systematics produces branching evolutionary diagrams or phylogenetic trees which...

This chapter looks at vertebrates which live successfully in water. A vertebrate must adjust its buoyancy to remain at a specific depth and must force its way through a dense medium to pursue prey or escape predators. Thus, aquatic vertebrates have evolved solutions to the physical challenges of life in water. The chapter primarily focuses on fishe...

This chapter covers two ways of living on land with reference to synapsids and sauropsids. The mammalian lineage is characterized by synapsid skulls that have a single fenestra, while members of the reptilian lineages have diapsid skulls with two fenestrae. Skull structure is only one way in which the evolutionary histories of Synapsida and Saurops...

This chapter details the geography and ecology of the Mesozoic period. It explains that the Mesozoic period was a time of major diversification and radiation, leading to large-scale changes in flora and fauna by the end of the era. In Mesozoic oceans, neoselachians diversified, while the total diversity of tetrapods changed relatively slowly. Meanw...

This chapter tackles the evolutionary history of Theria and its two clades, Marsupalia and Eutheria. Therians are viviparous amniotes which give birth to young after a period of internal gestation. Although most therians are terrestrial, there are also therians with burrowing, aquatic, and flying forms. Thus, the evolution of these different ways o...

This chapter focuses on the demands of terrestrial life. It highlights how the viscosity of water compared to forces in the air and on land resulted in change. Since the most important difference between living in water and on land is the effect of gravity on support and locomotion, the skeletons of terrestrial tetrapods needed to be able to suppor...

This chapter looks at the concept of ectothermy and endothermy in relation to regulating body temperature. It highlights the importance of controlling body temperature. Vertebrates employ a variety of behavioral and physiological mechanisms to control body temperature. The evolution of endothermy was a major event in vertebrate evolution, especiall...

This chapter explains that vertebrates belong to the phylum Chordata, which along with the echinoderms and the hemichordates comprise the Deuterostomia. Deuterostomes and protostomes are the subdivisions of the more complex animals with bilateral symmetry and bodies that develop from three layers of tissue. The chapter looks at non-chordate deutero...

This chapter focuses on the taxonomic diversity of extant birds. It shows how extant birds are divided into Paleognathae and Neognathae, in accordance with their palatal anatomy. The main characteristics of birds include flight at a structural level and diurnality and this has played a significant role in their lives. The chapter details the myriad...

This chapter provides an overview of extant crocodylians. The group of crocodylians include the largest extant species of reptiles and the only ones that sometimes regard humans as prey. Moreover, all extant crocodylians are semiaquatic predators and nearly all live in the tropics or subtropics. Contrary to the impression that comes from seeing wel...

The NOW database of fossil mammals came to be through a confluence of several initiatives spanning multiple decades. The first public version of NOW database was released in 1996 and the first Advisory Board was established the year after. Originally, NOW stood for Neogene of the Old World but with the gradual expansion of the database the acronym...

NOW ( New and Old Worlds ) is a global database of fossil mammal occurrences, currently containing around 68,000 locality-species entries. The database spans the last 66 million years, with its primary focus on the last 23 million years. Whereas the database contains records from all continents, the main focus and coverage of the database historica...

The NOW database of fossil mammals was released in 1996, slightly more than 25 years ago. Initially the acronym stood for ‘Neogene Old World’, but as the database later expanded to include data for Cenozoic mammals worldwide it was changed to ‘New and Old Worlds’. NOW is a comprehensive research database and its creation approximately coincided wit...

The equid subfamily Equinae radiated 17.5 Ma in the New World and reached the Old World at around 11 Ma. The New World radiation comprised three tribes (Hipparionini, Protohippini, and Equini), with a maximum diversity of 31 equine species in the early Late Miocene, while the Old World radiation comprised only Hipparionini until the arrival of Equu...

This volume presents an array of different case studies which take as primary material data sourced from the NOW (‘New and Old Worlds’) database of fossil mammals. The NOW database was one of the very first large paleobiological databases, and since 1996 it has been expanded from including mainly Neogene European land mammals to cover the entire Ce...

Horse evolution is popularly known and portrayed as an “Eohippus to Equus” sequence, with the modern horse as the pinnacle of the sequence. This portrayal not only ignores the bushy and branching pattern of equid evolution that we now know took place, but in particular ignores the radiation of large browsing horses (the Anchitheriini) that occurred...

The traditional story of the evolution of the horse (family Equidae) has been in large part about the evolution of their feet. How did modern horses come to have a single toe (digit III), with the hoof bearing a characteristic V-shaped keratinous frog on the sole, and what happened to the other digits? While it has long been known that the proximal...

The timing of the placental mammal radiation has been the focus of debate over the efficacy of competing methods for establishing evolutionary timescales. Molecular clock analyses estimate that placental mammals originated before the Cretaceous-Paleogene (K-Pg) mass extinction, anywhere from the Late Cretaceous to the Jurassic. However, the absence...

Masticatory muscle features allow for an understanding of how dietary habits and masticatory functions have evolved across mammalian lineages. Herbivorous mammals were traditionally classified as pertaining to either ‘ungulate-grinding’ or ‘rodent-gnawing’ morphotypes, but those classes might not adequately encompass the diversity of past and prese...

Vertebrate Life integrates information from vertebrate anatomy, physiology, ecology, and behavioral studies and then helps students see important connections across levels of biological organization. The result is students come to understand how organisms function in their environments and how lineages of organisms change through evolutionary time....

Widely praised for its comprehensive coverage and exceptionally clear writing style, this best-selling text explores how the anatomy, physiology, ecology, and behavior of vertebrates interact to produce organisms that function effectively in their environments and how lineages change through evolutionary time.

Previous studies of the morphology of the humerus in kangaroos showed that the shape of the proximal humerus could distinguish between arboreal and terrestrial taxa among living mammals, and that the extinct “giant” kangaroos (members of the extinct subfamily Sthenurinae and the extinct macropodine genus Protemnodon ) had divergent humeral anatomie...

The extinct sthenurine (giant, short-faced) kangaroos have been proposed to have a different type of locomotor behavior to extant (macropodine) kangaroos, based both on physical limitations (the size of many exceeds the proposed limit for hopping) and anatomical features (features of the hind limb anatomy suggestive of weight-bearing on one leg at...

The name “Cetartiodactyla” was proposed in 1997 to reflect the molecular data that suggested that Cetacea is closely related to Artiodactyla. Since then, that taxon has spread in popularity, even outside the scientific literature. However, the implications of the name are confusing, because Cetacea and Artiodactyla are not sister-taxa. Instead, the...

Jaw morphology is closely linked to both diet and biomechanical performance, and jaws are one of the most common Mesozoic mammal fossil elements. Knowledge of the dietary and functional diversity of early mammals informs on the ecological structure of palaeocommunities throughout the longest era of mammalian evolution: the Mesozoic. Here, we analys...

Sthenurine kangaroos, extinct “giant kangaroos” known predominantly from the Plio-Pleistocene, have been proposed to have used bipedal striding as a mode of locomotion, based on the morphology of their hind limbs. However, sthenurine forelimb morphology has not been considered in this context, and has important bearing as to whether these kangaroos...

Many studies have shown a correlation between postcranial anatomy and locomotor behavior in mammals, but the postcrania of small mammals (<5 kg) is often considered to be uninformative of their mode of locomotion due to their more generalized overall anatomy. Such small body size was true of all mammals during the Mesozoic. Anatomical correlates of...

Background: Saber-toothed mammals, now all extinct, were cats or "cat-like" forms with enlarged, blade-like upper canines, proposed as specialists in taking large prey. During the last 66 Ma, the saber-tooth ecomorph has evolved convergently at least in five different mammalian lineages across both marsupials and placentals. Indeed, Thylacosmilus...

Cenozoic mammal evolution and faunal turnover are considered to have been influenced and triggered by global climate change. Teeth of large terrestrial ungulates are reliable proxies to trace long‐term climatic changes due to their morphological and physicochemical properties; however, the role of premolar molarization in ungulate evolution and rel...

Savanna-like ecosystems were present at high latitudes in North America during much of the Neogene. Present-day African savannas, like the Serengeti, have been proposed to be modern analogs of these paleosavannas, particularly those from the middle Miocene of the Great Plains region of the United States. Both these extant and extinct savannas conta...

The involvement of mineralized tissues in acid–base homeostasis was likely important in the evolution of terrestrial vertebrates. Extant reptiles encounter hypercapnia when submerged in water, but early tetrapods may have experienced hypercapnia on land due to their inefficient mode of lung ventilation (likely buccal pumping, as in extant amphibian...

Significance The six “evolutionary faunas” of large mammal taxonomic diversity described for the North American Cenozoic have a nonrandom ecomorphological spectrum and show a long-term trend toward greater ecological specialization over the past 66 My. We show here that each successive fauna was characterized by a change toward more specialized eco...

The traditional story of horse evolution is well-known: over time, horses became larger, they attained higher-crowned teeth, and they changed from having three toes (tridactyly) to a single toe (monodactyly). Evolution is often perceived as a progression toward some optimum outcome, in this case the “Noble Steed.” However, the evolutionary advantag...

Equids have often been discussed regarding tooth morphological change due to the evolution of highly hypsodont teeth over time, the hyper-grazing habits of modern horses, and an older view that the acquisition of hypsodonty and the widespread appearance of grasslands were synchronous. Many more recent studies, however, have reported asynchrony in t...

A new study by Fraser et al (2018) urges the use of phylogenetic comparative methods, whenever possible, in analyses of mammalian tooth wear. We are concerned about this for two reasons. First, this recommendation may mislead the research community into thinking that phylogenetic signal is an artifact of some sort rather than a fundamental outcome...

Locomotor mode is an important component of an animal's ecology, relating to both habitat and substrate choice (e.g., arboreal versus terrestrial) and in the case of carnivores, to mode of predation (e.g., ambush versus pursuit). Here, we examine how the morphology of the calcaneum, the 'heel bone' in the tarsus, correlates with locomotion in extan...

Because body size interacts with many fundamental biological properties of a species, body size evolution can be an essential component of the generation and maintenance of biodiversity. Here we investigate how body size evolution can be linked to the clade-specific diversification dynamics in different geographical regions. We analyse an extensive...

Locomotion is intimately linked to the ecology of an animal, and so understanding the relationship between form and function in the calcanea of extant Carnivora permits comparative analyses with extinct Carnivora and other carnivorous mammals to better understand their paleoecology. The calcaneal heel is the point of attachment of the gastrocnemius...

Significance Our study links diversity dynamics of fossil large mammals through time to primary productivity, i.e. net production of plant biomass. Spatial diversity patterns of extant terrestrial animals are often correlated with present-day primary productivity, but it is unclear whether the relationship holds throughout the geological past. Here...

The ungulate ecomorphological diversity of the mid-Miocene of North America has often been compared with that of present-day savannahs of East Africa. We compared the ecomorphology of Serengeti ungulates with those of the early Clarendonian (~12 Ma) Burge Quarry fauna from Nebraska. Ecometric traits, indicative of dietary preference and habitat cho...

Analyses of craniodental and calcaneal material of extant macropodoids show that both dietary and locomotor types are statistically distinguishable. Application of the craniodental data to fossil macropodoids from the Oligo-Miocene of South Australia (Lake Eyre Basin) and Queensland (Riversleigh World Heritage Area) shows that these taxa were prima...

It has long been thought that environmental perturbations were the key driving force behind the succession of three distinct mammal faunas in the Pleistocene Ailuropoda-Stegodon faunal complex (sensu lato) of South China: the lower Pleistocene Gigantopithecus-Sinomastodon fauna, the middle Pleistocene Ailuropoda-Stegodon fauna (sensu stricto) and t...

Thylacoleo carnifex , or the “pouched lion” (Mammalia: Marsupialia: Diprotodontia: Thylacoleonidae), was a carnivorous marsupial that inhabited Australia during the Pleistocene. Although all present-day researchers agree that Thylacoleo had a hypercarnivorous diet, the way in which it killed its prey remains uncertain. Here we use geometric morphom...

The spread of open grassy habitats and the evolution of long-legged herbivorous mammals with high-crowned cheek teeth have been viewed as an example of coevolution. Previous studies indicate that specialized predatory techniques in carnivores do not correlate with the spread of open habitats in North America. Here we analyse new data on elbow-joint...

Patterns of late Palaeogene mammalian evolution appear to be very different between Eurasia and North America. Around the Eocene–Oligocene (EO) transition global temperatures in the Northern Hemisphere plummet: following this, European mammal faunas undergo a profound extinction event (the Grande Coupure), while in North America they appear to pass...

The fossil record is widely informative about evolution, but fossils are not systematically used to study the evolution of stem-cell-driven renewal. Here, we examined evolution of the continuous growth (hypselodonty) of rodent molar teeth, which is fuelled by the presence of dental stem cells. We studied occurrences of 3,500 North American rodent f...

Carnivorous mammals use their forelimbs in different ways to capture their prey. Most terrestrial carnivores have some cursorial (running) adaptations, but ambush predators retain considerable flexibility in their forelimb movement, important for grappling with their prey. In contrast, predators that rely on pursuit to run down their prey have sacr...

Sthenurine kangaroos (Marsupialia, Diprotodontia, Macropodoidea) were an extinct subfamily within the family Macropodidae (kangaroos and rat-kangaroos). These "short-faced browsers" first appeared in the middle Miocene, and radiated in the Plio-Pleistocene into a diversity of mostly large-bodied forms, more robust than extant forms in their build....

Here we review published molar wear rates, measured in terms of tooth height loss per year (mm yr-1) published on natural populations of ungulates (25 species), rodents and lagomorphs (Glires; 14 species) and macropodid marsupials (seven species). Although the data are limited, they nevertheless reveal consistent patterns, and raise new questions....

The horse lineage (family Equidae) represents one of the clearest acquisitions of complicated derived dental morphology from a more generalized ancestor. Here we investigate the change in dental complexity (orientation patch count rotated, OPCR) during the evolution of key members of this group. A clear linear increase in dental complexity over evo...

While the identity and validity of the extant families of ruminants are undoubted, there are significant problems with the determination of the interrelationships among the families, notably within the families of the Pecora, or horned ruminants. The morphological features used to construct ruminant phylogeny have been a source of controversy: many...

A major step in mammalian evolution was the shift amongst many herbivorous clades from a browsing diet of leaves to a grazing diet of grasses. This was associated with (1) major cooling and increasing continentality and the enormous spread of grasslands in most continents, replacing closed and open forests, and (2) hypsodonty, the possession of hig...

Neogene cooling and aridification in the Northern Hemisphere have long been recognized, but there are no studies comparing patterns of aridity gradients or differences between North America and Eurasia. Large herbivorous mammals are an excellent source for understanding large-scale environmental and climatic patterns because their molar crown heigh...

The dermal bone sculpture of early, basal tetrapods of the Permo-Carboniferous is unlike the bone surface of any living vertebrate, and its function has long been obscure. Drawing from physiological studies of extant tetrapods, where dermal bone or other calcified tissues aid in regulating acid-base balance relating to hypercapnia (excess blood car...

Asymmetry of the knee (distal femur), where the medial trochlear ridge is larger than the lateral one, has been observed in certain ungulates, but until now there has not been a comprehensive study of this anatomy across a range of extant and extinct mammals. A degree of knee asymmetry is present in most ungulates: this asymmetry increases in exten...