Mammal Natural History: From First Mammals to Modern Diversity

Mammals are synapsid amniotes that share a set of derived features including mammary glands, hair, and a lower jaw composed of a single dentary bone. The evolutionary history of this group spans more than 300 million years, beginning with early synapsids in the late Carboniferous and continuing through the diversification of monotremes, marsupials, and placental mammals after the extinction of non-avian dinosaurs. This article traces that history through the fossil record, reproductive biology, physiological innovations, and ecological transitions that shaped modern mammalian diversity. Readers will gain a chronological framework for understanding mammal evolution, with attention to the evidence base, the limitations of current knowledge, and the practical value of this information for life-science professionals and informed general readers.

At a Glance

The table below summarizes the major evolutionary phases in mammal history, the key fossil or living evidence for each phase, and the primary biological innovations that define each transition.

The Synapsid Origin of Mammals

Mammals belong to Synapsida, a clade of amniotes distinguished by a single temporal fenestra behind each eye opening. The earliest synapsids appeared in the late Carboniferous and are often referred to as pelycosaur-grade synapsids. These animals were the dominant terrestrial vertebrates of the Permian and laid the foundation for all later mammal evolution.

The fossil record of early synapsids includes important evidence from the Carboniferous-Permian transition. A new edaphosaurid synapsid, Melanedaphodon hovaneci, recovered from Moscovian-age cannel coal in Linton, Ohio, represents one of the oldest known synapsids and provides the earliest record of the Edaphosauridae family. High-resolution X-ray micro-computed tomography revealed large bulbous, cusped marginal teeth alongside a moderately developed palatal battery. These features distinguish Melanedaphodon from all other known edaphosaurids and suggest adaptations for processing tough plant material already appeared among the earliest synapsids. The study proposes that durophagy, or the ability to crush hard food items, may have provided an early pathway to exploit plant resources in terrestrial ecosystems 15.

Trackway evidence from the earliest Permian of the Boskovice Basin in the Czech Republic adds further detail to our understanding of early synapsid biology. Well-preserved isolated tracks, manus-pes couples, and a slab with trackways composed of approximately 20 tracks in at least four different directions belong to early-diverging pelycosaur-grade synapsids. The material is assignable to the ichnotaxon Dimetropus. The best-preserved specimen shows rare skin impressions from the hands or feet of early-diverging mammal-line amniotes, a preservation type not observed before. This material highlights the significance of the ichnological record for revealing hidden terrestrial tetrapod diversity in equatorial Pangea during the Carboniferous-Permian transitional interval 14.

Herbivory evolved independently in several tetrapod lineages during the late Carboniferous and became more widespread throughout the Permian, eventually leading to the basic structure of modern terrestrial ecosystems. The edaphosaurid evidence from Linton demonstrates that adaptations for processing plant material were present among the earliest synapsids, beyond in later, more derived forms. This early establishment of herbivorous feeding strategies had lasting consequences for ecosystem structure and the evolutionary trajectory of the synapsid lineage.

The Permian and Triassic Radiation of Non-Mammalian Synapsids

Following their origin in the Carboniferous, synapsids diversified into a wide range of body plans and ecological roles during the Permian and Triassic. This radiation included the herbivorous edaphosaurids and caseids, the carnivorous sphenacodontids, and later the therapsids, which include the direct ancestors of mammals. The term non-mammalian synapsid encompasses all members of the synapsid lineage that fall outside the mammalian crown group.

Reproductive biology in these early forms is now better understood through exceptional fossil discoveries. Oviparity was likely the ancestral reproductive condition for non-mammalian Synapsida, yet no definitive fossil eggs of late Paleozoic or early Mesozoic synapsids had been discovered until recently. Three perinate specimens of the dicynodont genus Lystrosaurus from the Early Triassic of the South African Karoo Basin were examined using high-resolution CT and synchrotron scanning. One specimen displays a tightly curled posture suggestive of an in ovo position and completely lacks tusks. The lower jaw symphysis remains unfused, a developmental trait found only in pre-hatching embryos of modern birds and turtles. No calcified eggshell is preserved, suggesting the egg might have been soft and leathery. The large size of the reconstructed egg suggests a precocial, non-milk-feeding developmental strategy 13.

This Lystrosaurus embryo evidence is significant for several reasons. As a non-cynodont synapsid, Lystrosaurus offers a glimpse into reproductive biology far removed from the mammalian crown group. Unlike the more derived, mammal-like cynodont Kayentatherium, whose egg size aligns with lactation, Lystrosaurus anchors the ancestral condition deep within Synapsida. Its reproductive strategy may have played a crucial role in its resilience and ecological dominance following the end-Permian mass extinction 13.

The digit morphology of non-mammalian synapsids has also been a subject of study. Research presented at the International Conference on Creationism examined whether the fossil record of non-mammalian synapsid digits shows an increasing mammal-ness 12. This line of inquiry addresses the gradual acquisition of mammalian features in the synapsid lineage, though the abstract for this work was not available for detailed review.

The Transition to Mammalian Features

The transition from non-mammalian synapsids to mammals involved the gradual acquisition of a suite of features that define the mammalian crown group. These include the reorganization of the jaw joint, the development of three middle ear bones, the evolution of hair and mammary glands, and changes in reproductive physiology.

The lower jaw transition is one of the most well-documented transformations in vertebrate evolution. Early synapsids possessed a lower jaw composed of multiple bones, including the dentary, articular, and prearticular. Over evolutionary time, the dentary expanded while the postdentary bones were reduced. In mammals, the dentary is the sole bone of the lower jaw, and it articulates with the squamosal bone of the skull. The articular and quadrate bones, which formed the jaw joint in early synapsids, were incorporated into the middle ear as the malleus and incus.

The embryological basis for these transitions has been clarified through comparative studies of organogenesis. A comprehensive embryological dataset reconstructed the ancestral chronology of organogenesis and life-history modes in placental mammals. The study found that the ancestor of marsupial and placental mammals was placental-like at birth but had a long, marsupial-like infancy. The authors hypothesize that mammalian viviparity might have evolved in association with the extension of growth after birth, enabled through lactation, and that mammalian altriciality is inherited from the earliest amniotes. The precocial lifestyle of extant sauropsids and that of many placental mammals were acquired secondarily 8.

This research provides a framework for understanding how reproductive strategies evolved in concert with other mammalian features. The combination of viviparity, lactation, and extended postnatal care distinguishes mammals from other vertebrates and has profound implications for life history evolution.

The Evolution of the Placenta and Viviparity

The placenta is a defining feature of therian mammals, though placental-like structures have evolved in other vertebrate groups as well. A widely cited definition describes the placenta as the apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange. By this definition, placentas have evolved within every vertebrate class other than birds, and they have evolved on multiple occasions, often within quite narrow taxonomic groups 4.

The evolution of the placenta in mammals represents a story of convergent evolution at both the macromolecular and molecular levels. As the placenta and the maternal system associate more intimately, such that the conceptus relies extensively on maternal support, the relationship leads to increased conflict that drives adaptive changes on both sides. This evolutionary arms race has produced remarkable diversity in the morphology and physiology of eutherian placentas 4.

The transition to viviparity in mammals is closely tied to the evolution of lactation. The ancestral mammal likely laid eggs, as monotremes do today, and nourished young through milk secreted from mammary glands. The evolution of viviparity allowed for longer gestation and more developed young at birth, but it also imposed new physiological demands on the mother. The placenta mediates nutrient and gas exchange between mother and fetus, and its evolution enabled the extended internal development characteristic of marsupials and placentals.

The embryological evidence suggests that the ancestor of marsupials and placentals was placental-like at birth but had a long, marsupial-like infancy. This combination of features indicates that the evolution of viviparity and placentation preceded the diversification of therian mammals and that the altricial developmental pattern of marsupials reflects the ancestral condition 8.

Physiological Innovations in Mammal Evolution

Mammals are distinguished by several physiological features that evolved in the synapsid lineage. These include endothermy, the ability to maintain a constant body temperature through internal heat production, and the associated cardiovascular and respiratory adaptations that support high metabolic rates.

The evolution of high blood pressure in mammals is linked to the development of a separate pulmonary circulation. When vertebrates conquered land as amphibians, the evolution of the lung required a low systemic blood pressure, which explains why early land vertebrates such as amphibians and reptiles are low performers. Gaining independence from water required the evolution of an impermeable skin and a water-retaining kidney. This was accomplished twice with two different solutions in the two major branches of vertebrate evolution. Mammals excrete nitrogenous waste products as urea, which can be utilized by the kidney as an osmotic agent to produce more concentrated urine. Dinosaurs and birds have a distinct nitrogen metabolism and excrete nitrogen as water-insoluble uric acid, so their kidneys cannot use urea to concentrate as well. Instead, some birds have developed the capability to reabsorb water from their cloacae 7.

The convergent development of a separate small circulation of the lung in mammals and birds allowed for the evolution of high blood-pressure animals with better capillarization of the peripheral tissues, allowing high endurance performance. This physiological innovation underpins the active, endothermic lifestyle that characterizes modern mammals 7.

At the cellular level, mammals share enzymatic systems that manage oxidative stress. Catalase is an enzyme with one of the highest turnover numbers of all enzymes and is essential for neutralizing hydrogen peroxide. It was among the first protein crystals to be isolated, though its three-dimensional structure was discerned some forty years later. Catalase is known for suicide inactivation by its own substrate and is implicated in various medical scenarios, with its levels serving as a marker in that capacity 6. While catalase is not unique to mammals, its role in cellular protection is relevant to understanding the physiological demands of endothermy and high metabolic rates.

The Diversification of Crown Mammals

The crown group Mammalia includes monotremes, marsupials, and placentals. The timing of the divergence between marsupials and placentals is estimated from molecular and fossil evidence to have occurred in the Jurassic or early Cretaceous. Following the extinction of non-avian dinosaurs at the end of the Cretaceous, mammals underwent a major adaptive radiation.

The earliest African placental mammals are documented from the Ouled Abdoun phosphate Basin in Morocco. First carbon isotope chemostratigraphy of this basin has provided constraints on the dating and evolution of these early placentals 16. The phosphate deposits of Morocco have yielded some of the most complete early Cenozoic mammal faunas from Africa, providing critical evidence for understanding the biogeographic history of placental mammals.

Body-size evolution in early Cenozoic mammals has been a major focus of research. Brontotheres, which are among the first radiations of mammals that consistently evolved multitonne sizes, provide a case study in macroevolutionary body-size change. Several scenarios have been proposed to explain rapid net size increases in some early Cenozoic mammalian lineages, including sustained and gradual directional change, successive occupation of adaptive zones associated with progressively larger body sizes, and nondirectional evolution associated with branching events in combination with higher diversification potential of the larger lineages. Testing these hypotheses in brontotheres revealed that body-mass evolution mainly occurred during speciation and had no preferential direction. Long-term directional change stemmed from the higher survival of larger lineages in less-saturated herbivore guilds. This study emphasizes the role of differential species proliferation in explaining the long-term phenotypic trends observed in the fossil record, which are more than an accumulation of steady microevolutionary changes 9.

Life history optimization provides a broader framework for understanding body-size evolution across mammals. An analytical life history model of optimal size was tested using data on mammal life history and phylogeny and was shown to explain several well-defined macroevolutionary patterns. The evolution of body size is explained in large part by life history optimization with respect to adult mortality under metabolic constraints on productivity. The model also explains plausible effects of climate change, diet, feeding mode, cursoriality, aquatic living, powered flight, and island endemicity on the evolution of body size 11.

Locomotion and Ecological Specialization

Mammals have evolved a remarkable diversity of locomotor modes, including cursoriality, arboreality, fossoriality, aquatic swimming, and powered flight. The evolution of cursoriality, or running specialization, is particularly well documented in the fossil record.

Cursoriality evolved in the Artiodactyla, Perissodactyla, and Carnivora coincident with global cooling and the replacement of forests with open landscapes in the Oligocene and Miocene. The majority of mammal species, though, remained non-cursorial, plantigrade, and small, weighing less than 1 kg. A unique case of cursoriality in mammals smaller than 1 kg, termed micro-cursoriality, has been documented in elephant-shrews of the genus Elephantulus from Namaqualand, South Africa. These animals achieve maximum running speeds higher than those of most mammals smaller than 1 kg and possess exceptionally high metatarsal-to-femur ratios of 1.07, which are typically associated with fast unguligrade cursors 5.

The extraordinary running speed and digitigrady of elephant-shrews was established in the Early Eocene in the earliest macroscelid Prodiacodon, but was probably inherited from Paleocene, Holarctic stem macroscelids. Micro-cursoriality in macroscelids evolved from the ancestral plantigrade foot of the possum-like ancestral mammal earlier than in other mammalian crown groups. It evolved first in forests, presumably in response to selection for rapid running speeds in order to avoid predators. During the Miocene, micro-cursoriality was pre-adaptive to open, arid habitats and became more derived in the newly evolved Elephantulus and Macroscelides elephant-shrews with trail running 5.

Island environments have also driven distinctive patterns of morphological evolution in mammals. Research on island mammals has shown that morphological evolution is accelerated among these populations 17. Island dwarfism and gigantism are well-known phenomena, and the accelerated rates of morphological change on islands reflect the distinct selective pressures of island ecosystems, including reduced predation pressure, limited resources, and competition dynamics that differ from mainland environments.

Coloration and Signaling in Mammals

Mammalian colors and color patterns are some of the most diverse and conspicuous traits found in nature and have been widely studied from genetic, developmental, and evolutionary perspectives. The proximate causes underlying variation in pigment type, or color, and pigment distribution, or color pattern, are distinct developmental processes. Ultimate factors that have driven the evolution of coloration differences in mammals include background matching, intra- and interspecific signaling, and physiological influences 3.

The evolution of coloration in mammals is relevant to understanding both ecological adaptation and social behavior. Background matching provides camouflage from predators or prey, while signaling functions include warning coloration, mate attraction, and social communication. Physiological influences on coloration include thermoregulation and protection from ultraviolet radiation. The diversity of mammalian coloration reflects the interplay of these selective pressures across different lineages and environments 3.

Mammals and the Evolution of Plant Interactions

Mammals have played significant roles in the evolution of plant communities, particularly through seed dispersal and pollination. The mistletoe lineage provides a striking example of mammal-plant coevolution. Mistletoes are the only woody, parasitic plants to infect host canopies, and this growth habit represents a key innovation. How this aerially parasitic habit originated is unknown, as mistletoe macrofossils are relatively recent, from long after they adapted to canopy life and evolved showy, bird-pollinated flowers, sticky, bird-dispersed seeds, and woody haustoria diverting water and nutrients from host branches 10.

Since the transition to aerial parasitism predates the origin of mistletoes contemporary avian seed dispersers by 20 to 40 million years, the question of who the original mistletoe dispersers were remains open. By integrating fully resolved phylogenies of mistletoes and aligning the timing of historic events, two ancient mammals have been identified as likely candidates for planting Viscaceae and Loranthaceae in the canopy. Just as modern mouse lemurs and galagos disperse viscaceous mistletoe externally by grooming the sticky seeds from their fur, Cretaceous primates such as Purgatorius may have transported seeds of root-parasitic understory shrubs up into the canopy of Laurasian forests. In the Eocene, ancestors of today mistletoe-dispersing marsupials, Dromiciops, likely fed on the nutritious fruit of root-parasitic loranthaceous shrubs, depositing the seeds atop western Gondwanan forest crowns. Once mistletoes colonized the canopy, subsequent evolution and diversification coincided with the rise of nectar- and fruit-dependent birds 10.

The Cenozoic Radiation and Modern Mammal Diversity

The extinction of non-avian dinosaurs approximately 66 million years ago opened ecological opportunities for mammals. The Cenozoic Era witnessed the diversification of major mammalian lineages, including primates, cetaceans, proboscideans, perissodactyls, artiodactyls, and carnivorans. This radiation is documented in the fossil record across multiple continents.

The story of mammal evolution in the Cenozoic includes many enigmatic transitions. The evolution of whales from terrestrial ancestors, the diversification of horses from small forest-dwelling forms, and the origins of various other mammalian groups are documented in the fossil record and popularized in works such as Beasts of Eden: Walking Whales, Dawn Horses, and Other Enigmas of Mammal Evolution 18. These transitions illustrate the capacity of mammals to adapt to diverse ecological niches and the importance of the fossil record for understanding evolutionary processes.

The earliest African placental mammals, documented from the Ouled Abdoun phosphate Basin in Morocco, provide critical evidence for understanding the biogeographic history of placental mammals. Carbon isotope chemostratigraphy of this basin has provided constraints on the dating of these faunas and their evolutionary relationships 16. The African fossil record is essential for testing hypotheses about the origins and dispersal of placental mammal lineages.

Records and Measurements in Mammal Evolution Research

Research on mammal evolution relies on a range of observational and measurement techniques. These include morphological measurement of fossil specimens, phylogenetic analysis of morphological and molecular data, and geochemical analysis of fossil-bearing sediments.

Morphological measurements used in mammal evolution research include body-mass estimation from skeletal dimensions, dental measurements for dietary inference, and limb proportions for locomotor reconstruction. The brontothere study, for example, used body-mass estimates to test hypotheses about the mode and tempo of size evolution 9. The elephant-shrew study measured running speeds and limb morphology, including metatarsal-to-femur ratios, to document micro-cursoriality 5.

Geochemical techniques provide chronological control for fossil localities. Carbon isotope chemostratigraphy of the Ouled Abdoun phosphate Basin in Morocco has been used to date the earliest African placental mammals 16. High-resolution X-ray micro-computed tomography has been used to examine internal structures of fossil specimens, as in the study of Melanedaphodon hovaneci 15 and the Lystrosaurus embryos 13.

Embryological datasets provide another source of evidence for understanding mammal evolution. The comprehensive embryological dataset published in the organogenesis study includes data on the chronology of organ development across placental mammals and was used to reconstruct ancestral life-history modes 8.

Common Failure Patterns in Interpreting Mammal Evolution

Several common errors arise when interpreting the evolutionary history of mammals. One failure pattern is the assumption that the fossil record is complete and that the absence of a fossil from a particular time interval indicates the absence of the lineage. The fossil record is inherently incomplete, and new discoveries frequently push back the origins of major groups.

Another failure pattern is the conflation of ancestral and derived traits. The embryological evidence that mammalian altriciality is inherited from the earliest amniotes challenges the assumption that altriciality evolved recently in mammal history 8. Similarly, the evidence that micro-cursoriality in elephant-shrews evolved in forests before being pre-adaptive to open habitats demonstrates that the current ecological associations of a trait may not reflect its original selective context 5.

A third failure pattern is the assumption that evolutionary trends are driven by steady, directional microevolutionary change. The brontothere study demonstrated that body-mass evolution mainly occurred during speciation and had no preferential direction, with long-term directional change stemming from the higher survival of larger lineages 9. This finding emphasizes the role of differential species proliferation in explaining long-term phenotypic trends.

Limitations and Open Questions in Mammal Evolution Research

Several limitations constrain current understanding of mammal evolution. The fossil record of early synapsids is sparse, particularly for the Carboniferous-Permian transition. The trackway evidence from the Boskovice Basin highlights the importance of ichnological records for revealing hidden diversity that is not represented by body fossils 14.

The reproductive biology of extinct synapsids remains poorly known. The Lystrosaurus embryo discovery provides the first definitive evidence of oviparity in non-mammalian synapsids, but the soft-shelled nature of the egg means that fossil eggs may be rarely preserved 13. The transition from oviparity to viviparity in the synapsid lineage is not directly documented in the fossil record and must be inferred from embryological and phylogenetic evidence.

The origins of mammalian physiological features, including endothermy and lactation, are also incompletely understood. These soft-tissue features do not fossilize directly, and their evolution must be inferred from skeletal correlates and comparative biology. The embryological approach used in the organogenesis study provides one avenue for reconstructing ancestral states 8.

Professional Escalation Criteria

Researchers and professionals working with mammal evolution should recognize when questions exceed the available evidence and require specialized expertise. Consult a vertebrate paleontologist when interpreting fossil specimens or when questions involve the taxonomic identification of fossil material. Consult a molecular phylogeneticist when questions involve the timing of lineage divergences or the relationships among living mammal groups. Consult a geochronologist when questions involve the absolute dating of fossil localities or the correlation of stratigraphic sections.

For professionals working with living mammals, consult a wildlife biologist or conservation geneticist when questions involve the evolutionary history of particular populations or species. Consult a reproductive physiologist when questions involve the evolution of reproductive strategies or the interpretation of embryological data. The embryological dataset published in the organogenesis study provides a foundation for such consultations but should not be used as a substitute for domain-specific expertise 8.

Frequently Asked Questions

What is the first mammal in the fossil record?

The identity of the first mammal depends on how the term mammal is defined. If mammal is defined as the crown group Mammalia, which includes monotremes, marsupials, and placentals, then the first mammals appeared in the Late Triassic or Early Jurassic. If mammal is defined more broadly to include all synapsids more closely related to mammals than to other synapsids, then the first mammaliamorphs appeared earlier. The fossil record of early synapsids extends back to the Late Carboniferous, with forms such as Melanedaphodon hovaneci representing one of the oldest known synapsids 15.

How did mammals survive the extinction that killed the dinosaurs?

Mammals survived the end-Cretaceous extinction that eliminated non-avian dinosaurs, but the reasons for their survival are complex. Small body size, generalist diets, and burrowing or semi-fossorial habits may have provided buffers against the environmental disruptions following the asteroid impact. The fossil record shows that mammals underwent a major adaptive radiation after the extinction, occupying ecological niches that were vacated by dinosaurs. The earliest African placental mammals are documented from the Paleocene, shortly after the extinction event 16.

What is a synapsid and how is it related to mammals?

A synapsid is a member of the clade Synapsida, which includes mammals and all extinct amniotes more closely related to mammals than to reptiles. Synapsids are distinguished by a single temporal fenestra behind each eye opening. The synapsid lineage includes pelycosaur-grade forms, therapsids, cynodonts, and mammals. Non-mammalian synapsids such as Lystrosaurus provide evidence for the ancestral reproductive and developmental conditions of the lineage 13.

When did the placenta evolve in mammals?

Placental-like structures have evolved multiple times in vertebrates, including in every vertebrate class other than birds. In mammals, the placenta evolved in the therian lineage, which includes marsupials and placentals. The embryological evidence suggests that the ancestor of marsupials and placental mammals was placental-like at birth but had a long, marsupial-like infancy 8. The evolution of the placenta involved convergent evolution at both the macromolecular and molecular levels 4.

How do scientists reconstruct the body size of extinct mammals?

Scientists reconstruct the body size of extinct mammals using correlations between skeletal measurements and body mass in living mammals. Commonly used measurements include skull length, tooth dimensions, and limb bone dimensions. Body-mass estimates for brontotheres were used to test hypotheses about the mode and tempo of size evolution in this lineage 9. Life history models that incorporate adult mortality and metabolic constraints also provide predictions about optimal body size

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Coloration in Mammals.. Trends in ecology & evolution, 2020.
• The evolution of the placenta.. Reproduction (Cambridge, England), 2016.
• The evolution of micro-cursoriality in mammals.. The Journal of experimental biology, 2014.
• Catalase and its mysteries.. Progress in biophysics and molecular biology, 2018.
• The evolution of blood pressure and the rise of mankind.. Nephrology, dialysis, transplantation : official publication of the European Dialysis and Transplant Association - European Renal Association, 2015.
• Evolution of organogenesis and the origin of altriciality in mammals.. Evolution & development, 2016.
• A macroevolutionary pathway to megaherbivory.. Science (New York, N.Y.), 2023.
• Did Mammals Bring the First Mistletoes into the Treetops?. The American naturalist, 2020.
• Life history optimization and the macroevolution of mammal body size.. 2026.
• Does the Fossil Record of Non-Mammalian Synapsid Digits Show an Increasing "Mammal-ness?". The Proceedings of the International Conference on Creationism, 2023.
• The first non-mammalian synapsid embryo from the Triassic of South Africa. PLoS ONE, 2026.
• Synapsid tracks with skin impressions illuminate the terrestrial tetrapod diversity in the earliest Permian of equatorial Pangea. Scientific Reports, 2023.
• A new Carboniferous edaphosaurid and the origin of herbivory in mammal forerunners. Scientific Reports, 2023.
• First carbon isotope chemostratigraphy of the Ouled Abdoun phosphate Basin, Morocco, implications for dating and evolution of earliest African placental mammals. Gondwana Research, 2014.
• Morphological evolution is accelerated among island mammals. Plos Biology, 2006.
• Beasts of Eden: Walking whales, dawn horses, and other enigmas of mammal evolution. Beasts of Eden Walking Whales Dawn Horses and Other Enigmas of Mammal Evolution, 2004.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.