Successive Diversifications Of Mesozoic Mammals

Most Mesozoic mammal species are represented only by teeth and incomplete jaws. Inadequate as they are for a comprehensive understanding of whole organisms, teeth and jaws nonetheless provide useful information about the taxonomic richness and diverse feeding adaptations of early mammals. Their records show several major episodes of diversification (figure 1.1).

The earliest diversification took place among stem mammals during the Late Triassic to Early Jurassic. From this initial burst of mammalian evolution arose several groups: haramiyidans with multirow, multicuspate, "mul-tituberculate-like" molars; Sinoconodon and morganu-codontans with "triconodont-like" molars showing three main cusps in alignment; and kuehneotheriids with triangulated, "symmetrodont-like" molars. All these mammals possessed the derived mammalian feature of the dentary

figure 1.1. Overview of the temporal distribution and relationships among Mesozoic mammal clades. The great evolutionary bush of diverse Mesozoic mammalian clades is the dominating feature in their taxic evolutionary pattern. The more recent cladistic analyses of dental, cranial, and skeletal characters of all phylogenetically relevant clades have revealed a far greater array of taxa and ranks of clades than could be accommodated by the traditional views of four "long-branch" lineages to the Early Triassic (e.g., Simpson 1928a; figure 1.2A) or two prototherian versus therian divisions extending to the Rhaeto-Liassic (figure 1.2B). The almost fully resolved clado-gram of all Mesozoic mammal groups, together with their improved records of temporal distribution of fossils, suggests that mammalian diversifications occurred episodically during the entire span of Mesozoic mammalian history. The five episodes of diversifications are as follows: (1) The earliest-known episode of diversification occurred in the Late Triassic-Early Jurassic on a global scale, when haramiyidans, morganucodontans, kuehneotheriids, and docodontans (if including Woutersia) appeared. (2) The next episode occurred globally in the Middle Jurassic, characterized by the appearance of shuotheriids, the earliest australosphenidans, eutricon-odontans, putative multituberculates, and amphitheriid "eupantotherians." (3) The Late Jurassic diversification occurred primarily in Laurasia among eutriconodontans, spalacotheriids, dryolestoids, and peramurans. (4) The Early Cretaceous episode saw diversification within australosphenidans and toothed monotremes on the Gondwanan continents, and the basal splits of eutherians and metathe-rians and diversification oftricondontids on the Laurasian continents. (5) The Late Cretaceous episode witnessed diversification within metatherians, within eutherians, and within cimolodontan multituberculates on the northern continents and of gondwanatherians on the southern continents. The earliest diversifications of stem eutherians and stem metatherians, as documented here by the currently available fossil evidence, predate the likely time window estimated by molecular evolutionary studies, which indicates that some superordinal clades of placental mammals may extend back into the Cretaceous (e.g., split of the earliest placental superordinal clades around 108 ± 6 Ma ago; Murphy, Eizirik, Johnson, et al., 2001; Murphy, Eizirik, O'Brien, et al., 2001; and several other studies). Full test of the molecular estimates of the phylogenetic divergence would require a combined analysis of currently available molecular sequence data and morphological data of major extant clades, with the main characters for resolving Mesozoic mammalian relationships coded for modern clades. This has not been done and is beyond the scope of this book. Therefore, for the time being we believe that the issue of the timing of the origin of modern placental superordinal clades is still unresolved. Source: stratigraphic distribution from chapter 2; cladogram based on Luo et al. (2002) and Ji et al. (2002).

condyle to squamosal glenoid in jaw joint,1 but also retained the plesiomorphous ("reptilian") features of an articular-quadrate jaw joint and the attachment of the postdentary bones (homologues of tympanic and middle ear bones of crown mammals) to the mandible. Although haramiyidans, Sinoconodon, morganucodontans, and kuehneotheriids have almost the same mandible design (notwithstanding variations in some individual characters), these groups developed astonishingly different molar structures for different feeding adaptations.

Six more order- or family-level lineages evolved in the next episode of diversification, during the Middle Jurassic. Amphilestids, with triconodont-like molars; eleu-therodontans, with multituberculate-like molars; and am-phitheriids and peramurids, with a triangulate trigonid plus a talonid heel on lower molars were all present on the Laurasian continents. A major innovation of this time was that of grinding molar function, which evolved in at least three emergent lineages: docodontans, with complex shearing and grinding surfaces on the molars; shuotheri-ids, with an anterior pseudotalonid basin; and aus-tralosphenidans from Gondwanan continents, with a fully developed posterior talonid basin for grinding function, in addition to the plesiomorphic shearing features on the molars. It is obvious that the adaptive molar structures related to grinding functions are homoplastic among these groups.

Consistent with this is the fact that, in juxtaposition with their derived, multifunctional molars, docodontans, shuotheriids, and at least some Middle Jurassic aus-tralosphenidans retained the primitive features of the postdentary trough on the mandible. Presence of these primitive mandibular features in the Jurassic southern mammals with multifunctional tribosphenic molars is strong evidence that they are not closely related to bore-osphenidans, which evolved on the northern continents some 25 Ma later, in a separate lineage with far more derived mandibular features (Luo et al., 2002).

The third episode of diversification among early mammals occurred in the Late Jurassic, with five newly emerged clades: Multituberculata (sensu stricto), triconodontids, spalacotheriids, dryolestoids (dryolestids and paurodon-tids), and tinodontids. The most significant apomorphic feature of these emergent clades of the Late Jurassic is the

1 Haramiyavia is the best-represented member of haramiyidans (see chapter 8), and is known by the mandible and parts of the dentition. The posterior end of the mandible in the holotype of Haramiyiavia clemmenseni is not preserved, but Jenkins et al. (1997) inferred from the preserved posterior part of the mandible that this taxon had a dentary condyle. We follow their interpretation.

absence of the postdentary trough on the mandible (Rowe, 1993), a primitive feature retained by almost all Early and Middle Jurassic lineages (except for amphilestids and Hadrocodium). Another striking pattern concerns the structure of the dentary. Despite the fact that eutricon-odontans, multituberculates, and spalacotheriids have very different dentitions, these three clades all have fairly similar characteristics in the posterior part of the mandible: a rounded "angular" region grades into the dentary condyle, and there is often a prominent medial pterygoid crest ("shelf") along the ventral border of the mandible.

The last major episode of diversification occurred in the Early Cretaceous when the stem taxa of metatherians, eutherians, and basal boreosphenidans appeared in Laura-sia, while toothed monotremes and some of their aus-tralosphenidan relatives appeared in Gondwana. The only new major lineage to appear in the Late Cretaceous was that of the gondwanatherians, a poorly known group occurring on several southern landmasses. The existing lineages of eutherians, metatherians, and multituberculates greatly increased in both generic diversity and numerical abundance through the Cretaceous and eventually survived (albeit with varied success) the mass extinction at the end of the period. However, spalacotheriids, eutricon-odontans, and gondwanatherians had declined and went extinct. Dryolestoids (Peligrotherium) survived in South America into the Cenozoic (Gelfo and Pascual, 2001).

Our overall view is that Mesozoic mammal evolution is a great evolutionary bush. The main feature of their taxic macroevolutionary pattern is successive episodes of diversification. This differs from the traditional views of four "long-branch" lineages to Early Triassic (e.g., Simpson, 1928a) (figure 1.2A) or two prototherian versus therian divisions extending to the Rhaeto-Liassic (figure 1.2B).

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