James A. Hopson

Evolution of the Auditory System in Synapsida (“Mammal-Like Reptiles” and Primitive Mammals) as Seen in the Fossil Record

Based on evidence from comparative anatomy, embryology, and paleontology, it is well established that the middle ear of existing mammals is morphologically unique, the tympanic bone, malleus, incus, and tensor tympani muscle all being homologous with components of the feeding apparatus of other vertebrates (Fig. 28.1, see below for details). Extant mammals are also unique in having a very elongate cochlea, and sensitivity to a broader range of sound frequencies than other vertebrates, usually extending beyond 10,000 Hz. In addition, they are distinctive in generally having a protruding pinna (auricle) and a long, tubular external auditory meatus. The present chapter examines the extensive fossil evidence concerning the nature of the auditory machinery of early mammals and their antecedents. Both authors of the present account have written on this subject, arriving at different initial interpretations in certain regards (Hop-son 1966; Allin 1975).

Reconstruction of the cranial vessels in the Early Cretaceous mammal Vincelestes neuquenianus: implications for the evolution of the mammalian cranial vascular system

ABSTRACT Vincelestes neuquenianus from the Early Cretaceous of Argentina is the earliest therian mammal known from nearly complete crania. Described here is the structure of the petrosal bone and other cranial elements inferred to be associated with the vascular system. From comparisons with Recent amniotes, the major basicranial arteries and veins that we reconstruct for Vincelestes represent a composite of the vascular patterns in monotremes and some placentals (e.g., lipotyphlan insectivorans). Vincelestes had a well-developed stapedial system that was supplied principally via the arteria diploetica magna, a transpromontorial internal carotid artery, and a well-developed lateral head vein that drained the prootic sinus via a prootic canal. As in monotremes and extinct “non-therian” mammals, Vincelestes had an enlarged anterior lamina of the petrosal (=lamina obturans), through the base of which (lateral flange) ran the ramus superior of the stapedial artery. Reconstruction of the major basicranial arte...

The evolution of cranial display structures in hadrosaurian dinosaurs

A theory is presented that cranial crests of hadrosaurs were visual and acoustical display organs. Facial morphology and phylogeny of the Hadrosauridae and earlier theories of crest function are reviewed. The following hypothesis is presented: cranial crests, whether hollow or solid, served as visual signal structures, and hollow lambeosaur crests were also vocal resonators; all crests promoted successful matings within species, i.e. , they served as premating genetic isolating mechanisms. The following predictions are tested and found to support the hypothesis: (1) hadrosaurs had well-developed eyes and ears; (2) external features of crests varied independently of internal structure; (3) crest variations were species-specific and sexually-dimorphic; (4) crest distinctiveness correlates with species diversity; (5) crest size tended to increase through time. The circumnarial depression on the side of the face in hadrosaurines housed an inflatable diverticulum of the nasal passage which served as a visual display organ. Primitive hadrosaurs (kritosaurs) possessed a small nasal horn used as a butting weapon in intraspecific combat. Because the weapon was also used in intimidative displays, narial diverticula evolved to draw attention to it. In the kritosaur Brachylophosaurus fighting was modified to ritualized head-pushing using the flat nasal “shield”. Saurolophines expanded the diverticula on to the elongated nasal horn, converting the weapon to a dominance rank symbol. In non-crested edmontosaurs, enlarged diverticula assumed a vocalization function. Lambeosaurs created resonators by enclosing the diverticula in bone; they further enhanced the resonator function of the nose by forming elongated “organ pipes” in the premaxillae. This “pushed” the olfactory region above the eyes as a conspicuous dome which then was modified to form species-specific visual display organs.

Basicranial Evidence for Early Mammal Phylogeny

The distribution of thirty-eight basicranial characters is considered among monotremes, marsupials, placentals, and the following extinct taxa—Tritheledontidae, Tritylodontidae, Sinoconodon, Morganucodontidae, Haldanodon, Triconodontidae, Multituberculata, and Vincelestes. PAUP analysis of the ensuing data matrix supports the following conclusions: 1. Marsupialia and Placentalia form a clade supported by an anterior lamina of the petrosal that is greatly reduced or absent, a cavum epiptericum floored primarily by the alisphenoid, major basicranial drainage via the postglenoid foramen, and a squamosal contributing broadly to the cranial wall. 2. A clade with Vincelestes from the Early Cretaceous of Argentina as the sister taxon to Marsupialia plus Placentalia is supported by a caudal tympanic process of the petrosal, a post-promontorial tympanic sinus, a true cochlear aqueduct, and a cochlear duct coiled through at least 270 degrees. 3. A clade comprising the taxa in (2) along with Multituberculata plus Monotremata is supported by loss of support for the ventromedial part of incus on the cranium and a greatly reduced quadrate ramus of the alisphenoid. A clade with Multituberculata and Monotremata is supported by a common tympanic aperture for the prootic canal and pterygoparoccipital foramen. 4. A clade including Triconodontidae and the taxa in (3) is supported by loss of the vascular foramen in the periotic lateral flange and the suspension of the postdentary bones from the cranium. 5. A clade with Haldanodon and the taxa in (4) is based on the loss of the quadratojugal notch in the squamosal. 6. A clade including the taxa in (5) plus Sinoconodon and Morganucodontidae is supported by an anterior lamina of the petrosal expanded forward dorsal to the exit of the maxillary and mandibular nerves, a cavum epiptericum partially floored by the petrosal, facial ganglion floored by petrosal, stapes length less than 5.5% of skull length, ossified base of the pila antotica absent, major basicranial drainage via a large prootic canal that opens endocranially, a paroccipital process with a distinct ventrally directed projection for muscle attachment, a petrosal promontorium, and a well-developed dentary-squamosal contact. 7. Tritylodontidae shares support of the ventromedial part of the quadrate on the cranium via a convex surface on the crista parotica of the petrosal and a fossa for the stapedius muscle on the petrosal with the taxa in (6).

Reexamination of the morphological evidence for the cohort Epitheria (Mammalia, Eutheria)

Novacek and co-workers recognized a monophyletic clade Epitheria, comprising all eutherians except edentates and the extinct palaeoryctoids, on the basis of two synapomorphies: a stirrupshaped stapes and a foramen ovale enclosed within the alisphenoid. To evaluate this phylogenetic hypothesis, we reexamined the distributions of stapedial morphologies and positions of the foramen ovale across Recent and extinct mammals and nonmammalian cynodonts. The states and distributions of the stapes and forament ovale characters used by Novacek and coworkers were modified by recognizing two stapedial characters (one relating to shape of the crura, the other to the nature of the foramen) and a single, multistate foramen ovale character (within, behind, and lateral to the alisphenoid). The taxon-character matrix used by Novacek (1989, 1992b), substituting our amended stapedial and foramen ovale characters and adding several previously unscored extinct taxa and three new characters, was subjected to a series of PAUP manipulations. Identified among the most parsimonious trees were three major topologies for the base of Eutheria: (1) a polytomy including an Edentata/Ungulata clade, (2) a polytomy with Edentata and Ungulata as separate clades, and (3) Edentata and (when included) Palaeoryctoidea as the successive outgroups to a monophyletic Epitheria. We conclude that topology 2 best reflects the current state of knowledge. An edentate/ungulate clade is supported by three characters (from the mastoid region and subarcuate fossa); however, other morphological studies require modification of the distributions of these characters in xenarthrans and bassal ungulates, thereby eliminating support for this clade. In nearly all manipulations, obtaining a monophyletic Epitheria required that one or two steps be added to the most parsimonious trees. When a monophyletic Epitheria was obtained, it was supported by a triangular stapes and, in some trees, the reappearance of a stapedial artery (lost earlier at the level of Recent therians) and a transpromontorial internal carotid artery. In the most parsimonious trees, a foramen ovale within the alisphenoid was an equivocal synapomorphy of Recent therians or cutherians, and a stapes with strongly convex crura (our state closest to the stirrup-shaped state of Novacek and co-workers) appeared independently within various eutherian lineages. The reduction or loss of the stapedial foramen was identified as an independent event in monotremes and within marsupials and various eutherian lineages.

Homologies of the Prootic Canal in Mammals and Non-mammalian Cynodonts

ABSTRACT The term prootic canal has been used to describe vascular canals in the adult petrosal bones of extant monotremes and marsupials and presumed vascular canals in the petrosals of extinct “non-tribosphenic” mammals (i.e., morganucodontids, docodontids, triconodontids, multitubercu- lates, Vincelestes). Study of the ontogeny of these canals in extant mammals and of their morphology in extinct mammals reveals that the term prootic canal has actually been applied to four different arrangements of canals. The prootic canal variously encloses the prootic sinus (most extinct “non- tribosphenic” mammals), the prootic sinus in company with the ramus superior of the stapedial artery (the platypus Ornithorhynchus and most multituberculates), the prootic sinus and the lateral head vein (the echidna Tachyglossus), and the lateral head vein alone (didelphid, caenolestid, and some dasyurid marsupials). We propose restricting the term prootic canal to the canal enclosing the prootic sinus (middle cerebral vein) a...

Foot posture in a primitive pterosaur

The nature of the hindlimb posture and gait of pterosaurs has been controversial, partly because most of the pterosaur skeletons that have been found were flattened in thin-bedded rocks, therefore obscuring three-dimensional anatomy. A major controversy concerns the extent to which pterosaurs move on the ground; they have been variously interpreted as ranging from sprawling, quadrupedal walkers to erect, bird-like bipedal cursors. Study of pelvis and femur material from the derived group Pterodactyloidea has resolved which movements are possible at the hip, but the lack of three-dimensional, articulated pterosaur feet has prevented examination of all of the movements that are possible within the foot. We have found a large, uncrushed, partial skeleton of a new species of the basal pterosaur Dimorphodon in thick-bedded deposits of Tamaulipas, Mexico; this material includes such a three-dimensional foot. The nature of this skeleton contradicts an important part of the cursorial interpretation, that is, that only the toes contacted the ground during terrestrial locomotion. The flattened metatarsal–phalangeal joint at the base of the first four toes of this specimen would not allow such a digitigrade posture without separating most of the joints. A flat-footed stance is consistent with presumed footprints of pterosaurs that show impressions of the entire sole of the foot.

Patterns of evolution in the manus and pes of non-mammalian therapsids

ABSTRACT In the transition from early synapsids to mammals, the phalangeal formula was reduced from 2-3-4-5-3 (manus) and 2-3-4-5-4 (pes) to 2-3-3-3-3. The standard view is that certain phalanges in digits III and IV were reduced in length to form disc-like elements which were ultimately lost within each non-biarmosuchian therapsid group. This implies at least five separate origins of the mammalian phalangeal pattern. Rowe and van den Heever (1986) argue for a simpler interpretation in which the mammalian formula originated once within therapsids directly from the unreduced sphenacodont condition, with disc-like elements in gorgonopsians and cynodonts being partial reversals or neomorphs. New evidence on the structure and phylogenetic distribution of disc-like phalangeal elements supports the view that they are homologous with unmodified phalanges and that they were reduced in size in the common ancestry of all therapsids. The phylogenetic distribution of discs in early cynodonts suggests that the hypothe...

THE ORIGIN AND ADAPTIVE RADIATION OF MAMMAL-LIKE REPTILES AND NONTHERIAN MAMMALS†

The evolutionary origin of the ancestors of the higher mammals from a primitive reptilian source is better documented in the fossil record than is the origin of any other major category of organisms. The Early Cretaceous to Recent radiation of modern therian mammals, the Metatheria and Eutheria, discussed elsewhere in this monograph by McKenna, was the end product of a series of four earlier terrestrial radiations spanning a period of some 200 million years, from the Early Pennsylvanian to the Early Cretaceous. The groups involved in these radiations, each representing a higher level of advancement toward modern mammals than the preceding one, were: (1) the Pennsylvanian and Early Permian pelycosaurs; (2) the Permo-Triassic noncynodont therapsids; (3) the primarily Triassic cynodonts; and (4) the primarily Jurassic nontherian and primitive therian mammals. In reviewing the course of evolution of the reptilian and early mammalian predecessors of the therian mammals, I shall emphasize the principal adaptations (where known) of the basal members of each radiation to indicate why they possessed the potential for giving rise to a major adaptive radiation. I shall also discuss the main adaptive trends within the subgroups of certain radiations in order to show that the adaptations acquired by some lineages to meet the demands of their particular ways of life put definite limits on further “progressive” evolution, whereas the adaptations acquired by other lineages provided them with the potential to form the bases of new radiations at higher organizational levels. With the exception of the modern therians, the only living survivors of the synapsid and Jurassic mammal radiations are the monotremes. I shall discuss them in some detail because they provide the only source of material for experimental research on the entire nontherian radiation of mammals and, as such, are the closest approach we can make to the study of the detailed structure of the very earliest mammals and their therapsid progenitors.

The Classification of Nontherian Mammals

On the basis of a unique specialization of the braincase the four nontherian orders of mammals—the living Monotremata and the extinct Tricono-donta, Docodonta, and Multituberculata—are united into a single subclass, Prototheria. The Triconodonta and Docodonta are closely related and are grouped together in the infraclass Eotheria. The Monotremata and Multituberculata are only remotely allied to all other mammals and are each placed in a separate infraclass—Ornithodelphia for the monotremes and Allotheria for the Multituberculates. A classification of prototherian mammals to family is presented.