Enamel hypoplasia in the Middle Pleistocene hominids from Atapuerca (Spain)

The prevalence and chronology of enamel hypoplasias were studied in a hominid dental sample from the Sima de los Huesos (SH) Middle Pleistocene site at the Sierra de Atapuerca (Burgos, northern Spain). A total of 89 permanent maxillary teeth, 143 permanent mandibular teeth, and one deciduous lower canine, belonging to a minimum of 29 individuals, were examined. Excluding the antimeres (16 maxillary and 37 mandibular cases) from the sample, the prevalence of hypoplasias in the permanent dentition is 12.8% (23/179), whereas the deciduous tooth also showed an enamel defect. No statistically significant differences were found between both arcades and between the anterior and postcanine teeth for the prevalence of hypoplasias. In both the maxilla and the mandible the highest frequency of enamel hypoplasias was recorded in the canines. Only one tooth (a permanent upper canine) showed two different enamel defects, and most of the hypoplasias were expressed as faint linear horizontal defects. Taking into account the limitations that the incompleteness of virtually all permanent dentitions imposes, we have estimated that the frequency by individual in the SH hominid sample was not greater than 40%. Most of the hypoplasias occurred between birth and 7 years (N = 18, X = 3.5, SD = 1.3). Both the prevalence and severity of the hypoplasias of the SH hominid sample are significantly less than those of a large Neandertal sample. Furthermore, prehistoric hunter-gatherers and historic agricultural and industrial populations exhibit a prevalence of hypoplasias generally higher than that of the SH hominids. Implications for the survival strategies and life quality of the SH hominids are also discussed. © 1995 Wiley-Liss, Inc.     

Enamel thickness and development in a third permanent molar of Gigantopithecus blacki

A ground section was prepared from a lower right M3 attributed to Gigantopithecus blacki as close as possible to axial plane of the mesial cusps. Daily cross striations were imaged, measured and counted in each cusp using polarised light microscopy. Long-period striae of Retzius were counted in the lateral enamel and their periodicity determined from counts and measurements of daily cross striations between adjacent striae. Cross striation spacings in the cusps were between 3.8 microm at the enamel dentine junction and 6 microm close to the enamel surface. Cuspal enamel formation times were long (800 days in the protoconid and 620 days in the metaconid). Linear enamel thickness was as much as 3.75 mm in the protoconid. There were 63 and 61 long-period striae of Retzius in the mesial aspects of the lateral enamel and the periodicity was 11 days. Lateral enamel formation took 1493 and 1291 days and when summed with cuspal enamel formation times totalled 4 years in the protoconid and 3.5 in the metaconid. Relative enamel thickness was 23, calculated through the mesial cusps. This falls short of that in the so-called 'thick hyper-thick' enamel described in 'robust' australopithecines to which Gigantopithecus blacki has previously been compared in both its dental and mandibular morphology. With respect to enamel thickness, therefore, Gigantopithecus blacki falls squarely among an increasingly large number of Miocene hominoids that can all be described as having 'thick enamel'.     

Enamel hypoplasia in sympatric chimpanzee and gorilla

The prevalence of enamel hypoplasia in 229Pan andGorilla, from the Republic of Cameroon, is described. A substantially higher proportion of gorillas (76%) than chimpanzees (58%) was affected. The incidence of enamel hypoplasia was not a function of sex, body size, or pathology. A study of tooth formation, from radiographs of a further series of immature apes, indicated that the mandibular canine bore 99% of all information about hypoplasia events. In both species a marked regularity of hypoplastic grooving with an interval of about 11.4% of canine crown height was observed. This appears to reflect a semi-annual cycle of stress which is tentatively linked to the twice-yearly rainy season. Uniform spacing of hypoplastic grooves has been observed in a variety of fossil hominids. Readily observable hypoplastic time markers in the teeth have potential for disclosing growth rates in early Hominidae. This is considered important because of the profound significance which prolonged maturation and longevity characteristic of recent human beings have for the transmission of learned behavior and social bonding.     

Enamel extension rate patterns in modern human teeth: Two approaches designed to establish an integrated comparative context for fossil primates

Enamel extension rates (EERs), the rates at which ameloblasts differentiate, determine how fast tooth crowns grow in height. Studies of fossil primate (including hominin) enamel microstructure usually focus on species differences in enamel formation time, but they have also begun to address species-level variation in enamel extension rates. To improve our ability to compare EERs among primate species, a better understanding how EERs vary within species is necessary. Using a large and diverse modern human histological sample, we find that initial EERs and patterns of EER change along the enamel-dentine junction (EDJ) vary in relation to EDJ length. We also find that enamel formation time varies in relation to EDJ length, but that it does so independently of initial EERs. These results suggest that EDJ length variation within a species sample can affect interspecific comparisons not only of EERs but also of enamel formation times. Additionally, these results lend within-species support to the hypothesis, based on comparisons among hominin species, that EERs and crown formation times can vary independently (Dean, 2009). In a second approach, we analyzed EER changes specifically in the lateral enamel of two modern human population samples as these changes relate to the distribution of perikymata. As surface manifestations of internal enamel growth increments, perikymata provide a valuable source of information about enamel growth in fossils. We find that EER declines in the lateral enamel are associated with an increase in perikymata density from first to last-formed lateral enamel. Moreover, variation in the extent of EER decline among individuals is associated with variation in the distribution of perikymata along their enamel surfaces. These latter findings suggest that the distribution of perikymata on the enamel surface provides information about rates of EER decline in lateral enamel, at least in modern humans.     

Dental indicators of stress and reduced age at death in prehistoric Native Americans

Considerable evidence supports the hypothesis that developmental enamel defects represent stress-induced growth disruptions. In this investigation, the relationship between different kinds of enamel defects and age at death is examined in the prehistoric Libben population from Ottawa County, Ohio. The sample consisted of the permanent dentitions of 143 individuals. Defects were classified based on the criteria of the Developmental Defects of Enamel (DDE) Index. The multifactorial age at death determinations of Lovejoy and coworkers (1977) were used in this analysis. Results reveal a significantly lower mean age at death for individuals with enamel defects vs. individuals with normal teeth. This pattern was clearly present for all defect types examined. No significant differences by sex were detected. The age-at-death distribution for individuals with normal teeth approximated the normal curve. The modal value was reached in the 35–40 year age class. The age-at-death distribution for individuals with enamel defects showed two peaks. The mode occurred in the 15–20 year age class, and the second, lower peak occurred in the 30–35 year age class. The early mortality of individuals with enamel defects may be related to biological damage to the immune system during prenatal or postnatal development. © 1996 Wiley-Liss, Inc.     

Natural and abrupt involution of the mammary gland affects differently the metabolic and health consequences of weaning

In most mammals under natural conditions weaning is gradual. Weaning occurs after the mammary gland naturally produces much less milk than it did at peak and established lactation. Involution occurs following the cessation of milk evacuation from the mammary glands. The abrupt termination of the evacuation of milk from the mammary gland at peak and established lactation induces abrupt involution. Evidence on mice has shown that during abrupt involution, mammary gland utilizes some of the same tissue remodeling programs that are activated during wound healing. These results led to the proposition of the “involution hypothesis”. According to the involution hypothesis, involution is associated with increased risk for developing breast cancer. However, the involution hypothesis is challenged by the metabolic and immunological events that characterize the involution process that follows gradual weaning. It has been shown that gradual weaning is associated with pre-adaption to the forthcoming break between dam and offspring and is followed by an orderly reprogramming of the mammary gland tissue. As discussed herein, such response may actually protect the mammary glands against the development of breast cancer and thus, may explain the protective effect of extended breastfeeding. On the other hand, the termination of breastfeeding during the first 6 months of lactation is likely associated with an abrupt involution and thus with an increased risk for developing breast cancer. Review of the literature on the epidemiology of breast cancer principally supports those conclusions.     

Frequency and chronological distribution of dental enamel hypoplasia in enslaved African Americans: A test of the weaning hypothesis

The dentition of 27 enslaved African Americans from archaeological sites in Maryland and Virginia were examined. All 17 males and 7 of the 10 females in this study exhibited enamel hypoplastic defects indicative of systemic nutritional and disease stresses interfering with amelogenesis. Estimates of the ages of occurrence of these defects show that most occur between 1.5 and 4.5 years of age, 0.5–3.75 years later than historically documented weaning age (9–12 months of age) in similar plantation populations. Comparisons are made with studies of dental enamel hypoplasia in contemporaneous enslaved and free African American populations, including our data on 75 individuals from the First African Baptist Church cemetery in Philadelphia. These populations were highly stressed. While there appears to be a modest effect of early weaning stress, no direct relationship of peak frequencies to weaning age can be shown. These data raise questions about the attribution of peak hypoplasia frequencies to age at weaning or “post-weaning” stresses in previous paleopathological studies. High hypoplasia frequencies during the middle years of enamel development are more likely the result of a combination of 1) multiple environmental stresses, 2) differences in hypoplastic susceptibility in enamel, and 3) random factors. © 1994 Wiley-Liss, Inc.     

On the nature of the opaque and translucent enamel regions of some macropodinae (Macropus giganteus,Wallabia bicolor and Peradorcas concinna)

Summary Teeth of three macropod species, M. giganteus, W. bicolor and P. concinna, have been studied using the techniques of light microscopy, scanning- and transmission-electron microscopy and hardness measurement. Light microscope observations showed that the teeth of these species had a translucent enamel region close to the dentine and an outer opaque enamel region at the tooth's surface. These regions were not related to the presence or absence of tubules which are a characteristic feature of marsupial enamel. Hardness tests showed that the opaque enamel was softer than the translucent enamel. Scanning electron microscope observations revealed that there was no correlation between any particular prism packing or orientation and the opaque and translucent enamel regions. Transmission electron microscope observations showed that the translucent enamel region consisted of well defined prisms and well packed, lath-like crystals, whereas the opaque enamel was disrupted by voids (which ranged in size from enlarged micropores to about 2 m in diameter in extreme cases) between crystals and some randomly oriented, loosely packed crystals. This disruption within the opaque enamel region was more common at prism boundaries but pockets of disrupted enamel were also found within prisms and interprismatic regions. The opacity of the enamel was caused by scattering of light from the voids. The ultrastructure of the opaque enamel region indicated that this region was hypomineralized; hardness tests and polarized light microscope observations were consistent with these results.     

Enamel microstructure in lemuridae (mammalia,primates): Assessment of variability

This study describes the molar enamel microstructure of seven lemurid primates: Hapalemur griseus, Varecia variegata, Lemur catta, Lemur macaco, Lemur fulvus rufus, Lemur fulvus fulvus, and Lemur fulvus albifrons. Contrary to earlier accounts, which reported little or no prism decussation in lemurid enamel, both Lemur and Varecia molars contain a prominent inner layer of decussating prisms (Hunter-Schreger bands), in addition to an outer radial prism layer, and a thin, nonprismatic enamel surface layer. In contrast, Hapalemur enamel consists entirely of radial and, near the surface, nonprismatic enamel. In addition, for all species, prism packing patterns differ according to depth from the tooth surface, and for all species but Varecia (which also has the thinnest enamel of any lemurid), average prism area increases from the enamel-dentine junction to the surface; this may be a developmental solution to the problem of accommodating a larger outer surface area with enamel deposited from a fixed number of cells. Finally, contradicting some previous reports, Pattern 1 prisms predominate only in the most superficial prismatic enamel. In the deeper enamel, prism cross-sections include both closed (Pattern 1) and arc-shaped (Pattern 2 or, most commonly, Pattern 3). This sequence of depth-related pattern change is repeated in all taxa. It should also be emphasized that all taxa can exhibit all three prism patterns in their mature enamel. The high degree of quantitative and qualitative variation in prism size, shape, and packing suggests that these features should be used cautiously in phylogenetic studies. Hapalemur is distinguished from the other lemurids by unique, medially constricted or rectangular prism cross-sections at an intermediate depth and the absence of prism decussation, but, without further assessment of character polarity, these differences do not clarify lemurid phylogenetic relations. Some characters of enamel microstructure may represent synapomorphies of Lemuridae, or of clades within Lemuridae, but homoplasy is likely to be common. Homoplasy of enamel characters may reflect functional constraints. © 1994 Wiley-Liss, Inc.     

Enamel microstructure and molar wear in the greater galago,Otolemur crassicaudatus (mammalia,primates)

This study describes the molar enamel microstructure of the greater galago, based on SEM study of four individuals. Galago molar enamel consists primarily of radially oriented Pattern 1 prisms. However, the most superficial enamel is characterized by regions of poorly developed prisms or nonprismatic enamel, and Pattern 3 prisms can be found at depths intermediate and deep to the enamel surface. Orientations of prism long axes relative to wear surfaces differ among functionally distinct regions (cuspal facets, Phase I/II facets, and crushing basins). Consequently, orientations of enamel crystallites relative to these surfaces also differ. Because crystallites are the structural unit involved in enamel abrasion, these differences in orientation may have important effects on molar wear patterns. Crystallite orientations differ most between cuspal facets and Phase I/II facet surfaces. Cuspal facets are characterized by near surface-parallel interprismatic and surface-oblique prismatic crystallites. Previous experimental studies suggest that this arrangement is most resistant to wear when surface-normal (compressive) loads predominate. In contrast, prismatic and interprismatic crystallites intercept Phase I/II facet surfaces obliquely, an arrangement expected to resist abrasion when surface-parallel (shearing) loads predominate. Superficial enamel is preserved at most basin surfaces, indicating that these regions are subject to comparatively little abrasive wear. These results support the hypothesis that galago occlusal enamel is organized so as to resist abrasion of different functional regions, a property that may prove important in maintaining functional efficiency. However, this largely reflects constraints of occlusal topography on a microstructure typical of many mammals and thus does not appear to represent a structural innovation. © 1993 Wiley-Liss, Inc.     

Dietary level of fibre and age at weaning affect the proliferation of Clostridium perfringens in the caecum, the incidence of Epizootic Rabbit Enteropathy and the performance of fattening rabbits

An experiment was conducted to investigate the effects of dietary fibre content and weaning age on Clostridium perfringens proliferation in the caecum and fattening mortality in growing rabbits farmed in a facility having Epizootic Rabbit Enteropathy. The experiment consisted in a 2 × 2 factorial arrangement with two weaning ages (28 days vs. 42 days) and two levels of dietary neutral detergent fibre assayed with a heat stable amylase and expressed exclusive of residual ash (aNDFom; 330 g/kg vs. 425 g/kg). Controls were made during two consecutive experimental periods that differed in hygienic environmental conditions by modifying the intensity of cleaning and disinfection in the farm previous to the trial. An interaction (P<0.001) was detected among the independent variables studied on Cl. perfringens enumeration in the caecal contents, as minimal values for this trait were obtained in non-medicated animals reared in a clean environment, and especially when they were weaned at a later age and fed the diet with the lower fibre content. The treatments studied also led to a variation in fattening mortality (from 4.7% to 34.0%), which was highly and positively correlated (P<0.001) to the average Cl. perfringens caecal counts in each combination of treatments. The results of the current study indicate that high counts of Cl. perfringens in the caecum can be used as an indicator of Epizootic Rabbit Enteropathy, and suggest that strategies designed to control its proliferation in the caecum might help to limit fattening mortality in rabbit fed diets not-medicated with antibiotics.     

The association between Harris lines and enamel hypoplasia in prehistoric California Indians

Hypoplastic defects of tooth enamel and Harris lines in the long bones have been heralded as potentially useful indicators of health conditions in prehistoric populations. Both result from temporary cessation of growth processes due to similar types of disease, malnutrition, or other metabolic insult. An association test for the first six years of life was conducted on a large series of prehistoric California Indians, using femora and canines from young adults. No significant association was found. This is ascribable to differences in etiology and stability.     

Variations in enzyme activity in stomach and pancreatic tissue and digesta in piglets around weaning

A study was performed to investigate the effect of weaning at 4 weeks of age on the activity of digestive enzymes in the stomach and pancreatic tissue and in digesta from 3 days prior to weaning to 9 days postweaning in 64 piglets. In stomach tissue the activity of pepsin and gastric lipase was determined. Pepsin activity declined abruptly after weaning but 5 days postweaning the weaning level was regained and in the gastric contents no change in pepsin activity was observed. Weaning did not influence the activity of gastric lipase. The activity of eight enzymes and a cofactor was measured in pancreatic tissue. The effect of weaning on the enzyme activity was highly significant for all enzymes except elastase. The activity of all enzymes remained at the weaning level during day 1–2 postweaning followed by a reduction of the activity. The activity of trypsin, carboxypeptidase A, amylase and lipase exhibited minimum activity 5 days postweaning. Trypsin activity increased to the preweaning level on day 7–9 whereas the activity of the others increased but did not reach the preweaning level. The activity of chymotrypsin, carboxypeptidase B and carboxyl ester hydrolase decreased during the entire experimental period. In digesta no effect of weaning was observed on the activity of amylase and trypsin. The activity of chymotrypsin was reduced after weaning in the proximal third of the small intestine and lipase and carboxyl ester hydrolase activity was reduced in the middle and distal parts of the small intestine after weaning. The present study shows that the activities of the digestive enzymes in the pancreatic tissue are affected by weaning. Even though the pancreatic secretion cannot be judged from these results they show that the enzymes respond differently to weaning. In general the activity of the digestive enzymes in pancreatic tissue is low on day 5 postweaning which in interaction with other factors may increase the risk of developing postweaning diarrhoea.     

Animal performance and stress at weaning when dairy cows suckle their calves for short versus long daily durations

Calves in most dairy farms are separated from their dams either immediately or within a few hours after birth, prompting increasing concern of the society for reasons of animal welfare. The aim of this study was to identify systems to maintain cow-calf contact (CCC) that balance the benefits for calf growth and health against the negative impacts on sellable milk and stress at weaning. We tested reuniting cows and calves for 20 min before (Before-group) or 2.5 h after (After-group) morning milking (in Trial 1) or for a 9 h period between the morning and evening milkings (Half-day-group, in Trial 2). In Control-groups, calves were separated from their dam at birth and fed with artificial nipple with tank milk provided daily at 13% (Trial 1) and 14% (Trial 2) of their BW. In both trials, each practice was applied on a group of 14 dam–calf pairs (7 Holstein [Ho] and 7 Montbéliarde [Mo]). All calves were weaned at a BW of at least 100 kg. In Trial 1, the After-group was prematurely stopped when the calves were eight weeks of age as calf growth became limited (340 g/d) due to low milk intakes (2.97 kg/d). During the first eight weeks of lactation, milk yield at the parlour was 29%, 51% and 42% lower in After-, Before- and Half-day-cows respectively compared to Controls. From week 14 to 16 when all calves were separated from their dam, Before-cows still produced 25% less milk than Control-cows while Half-day-cows reached the milk yield of Control-cows within a week. There were no significant differences in milk somatic cell count and in frequency of health disorders (cows and calves) between suckling and Control-groups. Compared to Control-calves, calf growth until weaning was higher in the suckling calves in Trial 1 (861 vs 699 g/d) and similar in Trial 2 (943 vs 929 g/d). At weaning, Before- and Half-day-calves started to vocalise earlier and continued to vocalise longer than Controls. In conclusion, the best compromise between cow milk yield and calf growth is a long period of CCC (9 h) between the morning and evening milkings. Still abrupt weaning stresses both cows and calves even if CCC has been restricted before separation.     

Prevalence and age at development of enamel hypoplasias in Mexican children

Enamel hypoplasias, deficiencies in enamel thickness resulting from disturbances during the secretory phase of enamel development, are generally believed to result from nonspecific metabolic and nutritional disruptions. However, data are scare on the prevalence and chronological distributions. of hypoplasias in populations experiencing mild to moderate malnutrition. The purpose of this article is to present baseline data on the prevalences and chronological distributions of enamel hypoplasias, by sex and for all deciduous and permanent anterior teeth, in 300 5 to 15-year-old rural Mexican children. Identification of hypoplasias was aided by comparison to a published standard (Federation Dentaire Internationale: Int. Dent. J. 32(2):159-167, 1982). The location of defects, by transverse sixths of tooth crowns, was used to construct distributions of defects by age at development. One or more hypoplasias were detected in 46.7% (95% CI = 40.9-52.5%) of children. Among the unworn and completely erupted teeth, the highest prevalence of defects was found on the permanent maxillary central incisors (44.4% with one or more hypoplasias), followed by the permanent maxillary canine (28.0%) and the remaining permanent teeth (26.2 to 22.2%) Only 6.1% of the completely erupted and unworn deciduous teeth were hypoplastic. The prevalence of enamel defects on the permanent teeth was up to tenfold greater than that found in studies of less marginal populations that used the FDI method. The prevalence of defects in transverse zones suggests a peak frequency of hypoplasias during the second and third years for the permanent teeth, corresponding to the age at weaning in this group. In the deciduous teeth, a smaller peak occurs between 30 and 40 weeks post gestation. The frequency of defects after three years of age is slightly higher in females than males, suggesting a sex difference in access to critical resources.     

Evidence of spinal tuberculosis at the beginning of the fourth millennium BC from Arene Candide cave (Liguria, Italy)

This paper describes tuberculosislike lesions affecting the spine of Neolithic skeleton found in Arene Candide cave (Liguria, Italy) and representing one of the earliest cases of this disease in Europe. The burial is attributed to middle Neolithic (square-mouthed pottery culture) whose layers date, by 14C, to the first half of the fourth millennium BC. The skeleton, well preserved in the postcranial portion, belongs to a male aged about 15 years having a very light body build. Resorptive lesions localized to the lower thoracic and upper lumbar vertebral bodies, collapse and angular kyphosis, resulting from complete destruction of T11 and T12 and partial destruction of T10 and L1, are the main pathologic features. The morphology of the lesions, sites involved, and age of the specimen are consistent with a diagnosis of tuberculous spondylitis (Pott's disease). The extreme gracility of the skeleton, along with other signs of stress (Harris lines, enamel hypoplasia) suffered during the last years of life, and the limited bony regeneration point to a disease process in its late phase having arisen at the beginning of adolescence. The funerary ritual suggests that the social integration of this adolescent was not prevented by the severe infirmity.     

Ontogenic and nutritional modifications in the intestinal fucosylation process at the weaning period. Influence of dietary fibers

In the rat small intestine, the glycosylation changes which normally take place at the weaning period are characterized by a shift from sialylation of fucosylation. The introduction of dietary fibers at weaning is one of the more striking nutritional modification so that some authors have suggested that the presence of fibers and the development of colonic fermentation might be important for the development of the small intestine, as for the colon. In order to define the respective contribution of ontogenic and nutritional factors to the intestinal glycosylation changes at this period, some aspects of the intestinal glycosylation were studied in five groups of rats (16-day-old suckling rats, prolonged nursing 23-day-old rats, 23-day-old rats weaned at day 19 with either a fiber-free, a cellulose or a pectin diet). Intestinal glycoproteins of suckling rats are characterized by a low fucose content and a high proportion of mannose. The amounts of the neutral sugars (fucose, mannose and galactose), expressed either per gram of intestine or for one intestine, are alwars higher in the fiber-fed groups than in the prolonged-nursing group or the group fed the fiber-free diet. Activities which promote fucosylation process (GDP-fucose production and fucosyltransferase activities) and those which are opposed to fucosylation (endogenous inhibitor of fucosyltransferase and GDP-fucose pyrophosphatase) are strongly modified in opposite ways at day 23 as compared to day 16. These modifications depend on the age of the animal (ontogenic factors) with additional modifications induced by the dietary factors. In particular, similar sugar contents and patterns are obtained with cellulose and pectin diests though the enzymatic activities of the fucosylation pathway are very different. No correlation was found between the caecal content of short chain fatty acids and any of the parameters under study. Thus, dietary fibers induce metabolic changes in the small intestine glycosylation in short-term experiments independently of colonic fermentation. Besides, these results point out that the consideration of fucosyl-transferase activities alone are not sufficient to predict glycoprotein fucose content and that other regulatory sites are involved. Dietary manipulations at the weaning period could represent a good model for the study of glycosylation regulation.     

Health and differential survival in prehistoric populations: prenatal dental defects.

Linear hypoplasia of the deciduous teeth is rare in most human populations, but common where nutritional status is poor. Deciduous enamel hypoplasia, hypocalcification, and hypoplasia-related caries are described in Middle and Late Woodland skeletal series from the Lower Illinois Valley. Gross enamel defects that can be referred to pre-natal development are found in 83 of 170 children under six years of age at death. Circular caries secondary to hypoplasia is significantly more common in the Late Woodland series, reflecting the apparent higher cariogenicity of Late Woodland diets. There is a significant association between prenatal dental defects and bony evidence for anemia and infectious disease. Children with enamel defects show relatively higher weaning age mortality than those without. These relationships suggest that at least moderate levels of malnutrition existed in Illinois Woodland populations.     

Updating and Recoding Enamel Microstructure in Mesozoic Mammals: In Search of Discrete Characters for Phylogenetic Reconstruction

The previously unknown enamel microstructure of a variety of Mesozoic and Paleogene mammals ranging from monotremes and docodonts to therians is described and characterized here. The novel information is used to explore the structural diversity of enamel in early mammals and to explore the impact of the new information for systematics. It is presently unclear whether enamel prisms arose several times during mammalian evolution or arose only once with several reversals to prismless structure. At least two undisputed reversions or simplifications are known—in the monotreme clade from Obdurodon to Ornithorhynchus (via Monotrematum?), and (perhaps more than once) within the clade from archaeocete to a variety of odontocete whales. Similarly, both prismatic and nonprismatic enamel is present among docodonts. Seven discrete characters showing enough morphological diversity to be of potential importance in phylogenetic reconstructions may be identified as a more appropriate summary of enamel microstructural diversity among mammaliaforms than the single character “prismatic enamel-present/absent” employed in recent matrices. Inclusion of five of these characters in the matrix of Luo et al. (2002) modifies the original topology by collapsing several nodes involving triconodonts and other nontribosphenic taxa. There is considerable support for prismatic enamel as a synapomorphy of trithelodonts plus Mammaliamorpha, and multituberculates appear to have small or “normal” sized prisms as the ancestral condition, with some (as yet) enigmatic changes to nonprismatic structure in some basal members of the group and the appearance of “gigantoprismatic” structure as an autapomorphic state of less inclusive clades. Other potential qualitative characters and the need for attaining appropriate methods to incorporate quantitative features may be important for future analyses.