Multituberculata
Multituberculata is an extinct order of rodent-like mammals belonging to the infraclass Allotheria, distinguished by their specialized dentition featuring multiple rows of low cusps (tubercles) on the upper and lower molars, which facilitated efficient grinding of plant material and possibly other foods.[1] These mammals ranged in size from small, mouse-like forms to larger species comparable to beavers, and they exhibited diverse adaptations including burrowing, climbing, and terrestrial locomotion.[1] Originating in the Middle Jurassic around 170–175 million years ago, multituberculates achieved their greatest diversity during the Late Cretaceous and early Paleocene, with over 200 species documented across all continents except Australia and Antarctica.[2] They survived the Cretaceous–Paleogene mass extinction event approximately 66 million years ago but gradually declined in diversity through the Paleogene, ultimately becoming extinct by the late Eocene around 35 million years ago.[2] Phylogenetically positioned as a stem group outside the therian mammals (placentals and marsupials), multituberculates are notable for their prolonged evolutionary history exceeding 130 million years, the longest of any mammalian order, and evidence from bone histology suggests they employed a life history strategy akin to that of placental mammals, involving extended gestation rather than the short pregnancies typical of marsupials.[3] Their decline has been attributed to competitive exclusion by emerging rodents, which possessed superior bite forces and adaptive versatility in exploiting changing Paleogene environments, including the spread of angiosperm-dominated floras.[2] Despite their extinction, multituberculates highlight the ecological success and morphological innovation of early mammals during the Mesozoic era, filling niches later dominated by rodents and lagomorphs.[1]Anatomy and Morphology
Dentition and Skull
Multituberculates possessed a highly specialized dentition characterized by enlarged, ever-growing incisors, a prominent diastema separating the incisors from the cheek teeth, and distinctive molars and premolars adapted for both shearing and grinding. The cheek teeth featured multiple longitudinal rows of cusps, typically two or three, arranged on the occlusal surfaces of the upper and lower molars and premolars, enabling efficient pulverization of food through precise cusp-to-cusp occlusion. A defining feature was the enlarged fourth lower premolar, known as the plagiaulacoid, which formed a blade-like structure with serrated edges and multiple cusps, functioning primarily for shearing tough plant material or possibly small invertebrates. This dentition supported a palinal (posteriorly directed) jaw stroke during mastication, where the lower teeth moved backward relative to the uppers, contrasting with the propalinal motion seen in rodents.[4][2] The skull of multituberculates exhibited an elongated, rodent-like rostrum that housed the procumbent incisors and contributed to the mechanical efficiency of feeding. A substantial diastema between the incisors and the first premolar allowed space for jaw movement without interference, while the zygomatic arches were robust to accommodate powerful masseter muscles. The palate often lacked vacuities in more primitive forms but showed variations, and the braincase was relatively narrow with a prominent petrosal bone forming much of the lateral wall. Recent high-resolution CT scans of specimens, such as the kogaionid Litovoi tholocephalos, reveal an endocast with intermediate complexity: a narrow overall shape reminiscent of early mammals, yet incorporating therian-like features such as expanded paraflocculi and a relatively large cochlea, bridging primitive and derived mammalian neuroanatomy.[5][6] Dentition varied significantly across multituberculate groups, reflecting evolutionary progression from primitive to advanced forms. In the basal Plagiaulacida, the plagiaulacoid premolar was fully developed and blade-like, with premolars and molars bearing simpler, more triangular cusps suited to an omnivorous or insectivorous diet, and low-crowned (brachyodont) teeth overall. In contrast, the more derived Cimolodonta, dominant from the Late Cretaceous onward, showed reduction or modification of the plagiaulacoid, with molars evolving toward hypsodonty—taller crowns with increased occlusal wear surfaces—in later Paleogene taxa, enhancing adaptation to abrasive herbaceous diets. These shifts paralleled broader cranial refinements, such as a more robust rostrum and enhanced jaw musculature in cimolodontans.[7]Postcranial Features
Multituberculates displayed considerable variation in body size, ranging from diminutive, shrew-like forms to large, beaver-sized individuals, reflecting their adaptation to diverse ecological niches across the Mesozoic and Cenozoic eras. The earliest known multituberculate, Rugosodon eurasiaticus from the Middle Jurassic of China, represents one of the smallest, with a total body length of approximately 20 cm and an estimated mass under 100 g, comparable to modern shrews.[8] In contrast, the Paleocene taeniolabidoid Taeniolabis taoensis from North America achieved the largest size among all multituberculates, with estimated body masses of up to approximately 40 kg based on cranial measurements, akin to a large beaver, and postcranial elements indicating a robust build consistent with this size.[9][10] This size disparity, spanning over three orders of magnitude, underscores the group's evolutionary flexibility in response to environmental changes.[11] Limb morphology in multituberculates was diverse, supporting a range of locomotor strategies from terrestrial cursoriality to potential scansorial or saltatorial behaviors. In taxa like the Cretaceous Kryptobaatar dashzevegi from Mongolia, the forelimbs were robust with abducted humeri and strong muscular attachments, suggesting adaptations for digging or burrowing activities, as evidenced by the twisted humeral shaft and large deltopectoral crest for powerful forelimb retraction. Hindlimbs in many forms, such as the Paleocene Ptilodus gracilis, featured relatively elongated femora and tibiae with a large greater trochanter and pronounced peroneal process on the calcaneus, indicating saltatorial capabilities for leaping or possibly arboreal climbing, though phalangeal robusticity points more toward generalized terrestrial locomotion than specialized arborealism.[12] Overall, limb proportions reflect a predominantly sprawling posture with abducted limbs, but with parasagittal elements in the hindlimb for efficient forward propulsion during asymmetrical gaits.[13] Vertebral and pelvic structures provided support for dynamic movement and reproductive strategies. The vertebral column in Asian multituberculates like Kryptobaatar included long, craniodorsally sloping lumbar spinous processes, facilitating an asymmetrical gait with short jumps and enhanced spinal flexibility for maneuvering. Pelvic adaptations, such as a deep acetabulum with a large iliosacral angle of 35–37° and dorsoventral contact between ilium and sacrum, suggest stability for weight-bearing during locomotion and potential for live birth, as the broad pelvic canal accommodated passage of young without the narrow constraints seen in monotremes. Recent bone histology analyses further support a placental-like reproductive strategy with extended gestation, implying live birth to relatively developed offspring rather than marsupial-style pouch rearing. In Taeniolabis, the robust pelvis and short caudal vertebrae indicate a stable base for its large body mass, with fossorial elements in the calcaneus hinting at digging behaviors.[10] Fossil evidence for soft tissues includes tail and fur impressions, offering insights into sensory and thermoregulatory functions. Caudal vertebrae in forms like Catopsbaatar catopsaloides were numerous and elongated, suggesting a long, flexible tail possibly used for balance during jumping or as a sensory organ, similar to modern rodents.[14] Fur impressions preserved in coprolites of the Eocene Lambdopsalis bulla reveal a dense pelage with guard hairs and underfur, indicating homeothermy and protection against environmental stresses, with no evidence for prehensile capabilities but potential for insulation in varied habitats. These features align with the group's inferred warm-blooded physiology and active lifestyles.Taxonomy and Phylogeny
Historical Classification
The first recognized multituberculate fossils were teeth from the Early Cretaceous Purbeck Group of England, described as the genus Plagiaulax by Hugh Falconer in 1857.[15] Falconer interpreted these specimens as belonging to an extinct herbivorous marsupial rodent, based on their multicusped premolars and overall dental morphology. This initial classification reflected the limited understanding of Mesozoic mammals at the time, with the unusual tuberculate teeth leading to comparisons with both reptilian and marsupial forms rather than recognizing their distinct mammalian affinities. Subsequent work by Richard Owen in 1871 expanded on these discoveries through a monograph detailing Mesozoic mammals from the British Museum collections, including additional Plagiaulax material and new genera like Bolodon. Owen confirmed their mammalian nature but emphasized their aberrant dentition, placing them in a provisional group separate from typical therian mammals and suggesting possible links to monotremes due to shared primitive features. In the late 19th century, Edward Drinker Cope advanced the taxonomy by naming numerous North American Paleogene species, such as Taeniolabis in 1882, and formally establishing the order Multituberculata in 1884 as a suborder within Marsupialia, highlighting similarities in their specialized cheek teeth to those of rodents. Cope's framework sparked ongoing debates about multituberculate affinities, with some researchers, including Henry Fairfield Osborn in his studies of early 20th-century Paleocene faunas, proposing closer ties to rodents based on ecological and dental parallels. George Gaylord Simpson's 1928 catalogue of Mesozoic mammals synthesized these early findings and provided the first comprehensive classification, dividing Multituberculata into three suborders: the primitive Plagiaulacida (encompassing Jurassic and Early Cretaceous forms like Plagiaulax), and the more derived Ptilodontoidea and Taeniolabidoidea (dominating Late Cretaceous and Paleogene assemblages). Simpson's scheme underscored their evolutionary distinctiveness while maintaining debates on rodent-like adaptations, such as grinding dentition suited to herbivory. By the mid-20th century, Zofia Kielan-Jaworowska's expeditions to the Gobi Desert in the 1960s and 1970s uncovered exceptionally preserved Late Cretaceous specimens, including skulls and postcrania, which demonstrated unique anatomical features like inflected angular processes and specialized jaw mechanics, solidifying Multituberculata's status as a separate mammalian order rather than a marsupial or rodent offshoot. These contributions culminated in the 1970s with the broader recognition of Multituberculata within the subclass Allotheria, a grouping initially proposed by Cope but refined to encompass their basal position relative to other mammals, based on shared dental and cranial traits with extinct Jurassic haramiyidans. This historical progression from misclassification as marsupials or reptiles to an independent clade laid the groundwork for understanding their long evolutionary history.Modern Suborders and Families
The modern taxonomic framework for Multituberculata recognizes two primary suborders based on dental morphology, postcranial features, and stratigraphic distribution, as outlined in the comprehensive revision by Kielan-Jaworowska, Cifelli, and Luo (2001).[16] The suborder Plagiaulacida is considered paraphyletic, representing a grade of early, primitive forms that bridge the basal multituberculates to more derived lineages, while Cimolodonta forms a monophyletic clade defined by shared apomorphies such as advanced occlusal patterns in the lower premolars.[16] This classification has remained influential, though recent phylogenetic studies using tip-dating methods have refined relationships among early allotherians, rejecting a strict monophyly of Allotheria and suggesting some haramiyidan-like forms may nest within multituberculate diversity, with Gondwanatheria positioned outside the group.[17] Plagiaulacida spans the Middle Jurassic to Early Cretaceous and is characterized by simpler tuberculate dentition suited to insectivory or omnivory, with fossils primarily from Laurasian continents. Key families include Plagiaulacidae (e.g., Plagiaulax from the Early Cretaceous of Europe), Albionbaataridae (e.g., Albionbaatar from the Early Cretaceous of England), and Paulchoffatiidae (e.g., Paulchoffatia from the Late Jurassic of Portugal). The earliest definitive multituberculates date to the Middle Jurassic Bathonian stage, such as Hahnotherium from England. An important early multituberculate, Rugosodon eurasiaticus, from the Late Jurassic Tiaojishan Formation of China, exemplifies early dental specializations like multiple premolars. These taxa exhibit low diversity, with fewer than 50 described species, reflecting their role as stem-group forms before the cimolodontan radiation. Cimolodonta, the dominant suborder, ranges from the Early Cretaceous to the late Eocene and achieved peak diversity in the Late Cretaceous and Paleogene of North America and Asia, with over 150 species described. This suborder is divided into several superfamilies, including Ptilodontoidea (e.g., families Ptilodontidae with Ptilodus from the Paleocene of North America, known for its rodent-like grinding dentition) and Taeniolabidoidea (e.g., Taeniolabididae with Taeniolabis, the largest known multituberculate at up to 50 kg body mass from the Paleocene of North America). Other notable families include Djadochtatheriidae (e.g., Kryptobaatar from the Late Cretaceous of Mongolia, renowned for well-preserved skeletons showing cursorial adaptations) and Eucosmodontidae (e.g., Eucosmodon from the Late Cretaceous of North America). Cimolodontans occupied diverse niches, from arboreal to terrestrial, across Laurasia, with isolated records in Europe and Asia. The following table summarizes major families within each suborder, highlighting temporal and geographic distributions:| Suborder | Family | Temporal Range | Geographic Range | Key Genera Examples |
|---|---|---|---|---|
| Plagiaulacida | Plagiaulacidae | Late Jurassic–Early Cretaceous | Europe | Plagiaulax, Bolodon |
| Plagiaulacida | Albionbaataridae | Early Cretaceous | Europe | Albionbaatar |
| Plagiaulacida | Paulchoffatiidae | Late Jurassic | Europe | Paulchoffatia |
| Cimolodonta | Ptilodontidae | Late Cretaceous–Eocene | North America | Ptilodus, Baiotomeus |
| Cimolodonta | Taeniolabididae | Paleocene–Eocene | North America | Taeniolabis, Catopsalis |
| Cimolodonta | Djadochtatheriidae | Late Cretaceous | Asia | Kryptobaatar, Nemegtbaatar |