Marine amniotes II: synapsids
Mammalia (Cretaceous - Recent) The most recent common ancestor of modern mammals and all of its descendants. In broad strokes, its phylogeny is very clear:
As with Aves, you have many sources of a huge variety of information on mammalian biology, paleo- and otherwise. This lecture concentrates on one biomechanical issue: The synapsid return to the oceans.
- Monotremata The egg-laying platypus and echidna
- Theria (Cretaceous - Recent) Recognized by various synapomoprhies including live birth. Includes two major clades:
Although many synapsids have been fresh-water aquatic (E.G. the Jurassic mammal-relative Castorocauda), only one group, the placental mammals, have ever returned to the oceans as exclusively marine creatures. This probably has a lot to do with reproductive physiology.
- Monotremes inherit the plesiomorphy of egg-laying, but share with other mammals the synapomorphy of mammary glands and intensive care of young. Thus, they could neither give live birth, like plesiosaurs or mosasaurs, nor lay batches of eggs and abandon them to their fates, like turtles. (These limits probably constrained more primitive synapsids, also)
- Marsupials give birth to an embryo at an early developmental stage that must make its way, on its own power, from the mother's vagina to her pouch. It's difficult to imagine a marsupial neonate accomplishing this underwater.
- Placentals are the only synapsids that give live birth to a newborn that might conceivably be able to survive birth in the water.
The diversity of marine placentals: Just as the extinction of large theropods on land created an ecological vacuum into which mammals, birds, and crocodylians attmepted to radiate; the extinction of predatory marine reptiles in the oceans opened up the "marine amniote" niche. During the Paleogene, placental mammals aggressively invaded this niche. Their diversity includes:
- Sirenians (Middle Paleogene - Recent) Sea Cows: herbivorous relatives of elephants.
- Desmostylians (Late Paleogene - Early Neogene) Semiaquatic herbivorous (?) relatives of elephants.
- Pinnipeds (Latest Paleogene - Recent) Carnivorans related to bears.
- Cetacea (Paleogene - Recent) Yet more predatory relatives of even-toed ungulates.
Of these, the earliest group to get their feet wet, and the most transformed for aquatic life are Cetacea, so this lecture concentrates on them:
Cetacean phylogeny: The phylogeny of living Cetacea is straightforward:
- Odontoceti: (Paleogene - Recent) Toothed-whales. Specialized for hunting by active sonar echolocation. This is accomplished using a complex system of nasal diverticuli and the melon, a body of fatty liquid in the "forehead" (actually the upper lip!) to generate and focus high-frequency sound. Note, the presence of teeth is a plesiomorphy!
- Mysticeti: (Paleogene - Recent) Baleen-whales. Specialized for ram-suspension feeding in which prey is filtered through keratinous sheets of baleen.
Derived features of living whales:
- Accoustic isolation of the middle ear to optimize hearing underwater. Achieved by the enclosure of the middle ear inside a bony Auditory bulla that is only loosely attached to the rest of the skull.
- Telescoping of the bones of the skull to strengthen the region between the eye-sockets. (Compare a gray fox and bottlenose dolphin
- Retraction of the nostrils to the top of the head.
- Shortening and reduction of cervical vertebrae (+ shortening of neck)
- Modification of tail and trunk for axial swimming using dorsoventral (up and down) flexion. (Increase in number of lumbar and caudal vertebrae.)
- Reduction of hindlimbs to vestigial state or absence.
These are remarkable adaptations, but all the more so when one considers the starting point for this evolutionary trend: a primitive even-toed ungulate. Today, we think of these creatures as swift herbivores, but during the early Paleogene, artiodactyls experimented with a variety of life styles including:
Competing hypotheses of phylogeny:
- The mesonychid hypothesis: The anatomical similarity of the skulls of the earliest fossil whales to mesonychids, early artiodactyl carnivores, has long fueled speculation that whales and artiodactyls were closely related.
- The "whippo" hypothesis: In the 1990s, repeated molecular phylogenetic analyses yielded very robust support for a close relationship between whales and hippopotamuses. At first, this seemed bizarre, however as the relationships of hippos to other ancient artiodactyls became clear, the connection started to seem plausible, even to morphologists who had previously favored mesonychids. At this time, the whippo hypothesis seems to have carried the day.
The "whippo" hypothesis carries an interesting implication: Whales are not related to artiodactyls, whales ARE artiodactyls.
So, how did we get from a hoofed terrestrial creature to living whales?
Landmarks in cetacean evolution: Watch for the following trends:
- Modification of tooth row
- Retraction of nostrils and telescoping of skull
- Lengthening of torso and tail
- Reduction of hindlimbs
Pakicetus: The best known primitive cetacean (Eocene epoch - Paleogene) Shares with more derived cetaceans an elongate snout in front of the nostrils, with incisors and canines arrayed in a V when viewed from below. Incisors and canines are simple cones, while post-canine teeth remain complex.
Pakicetus is primarily known from the skull. Given its similarity to more derived whales, it was originally reconstructed as aquatic. The lack of aquatic specializations in the ear, however, moves most current researchers to regard it as primarily terrestrial (as below), although it may have hunted in shallow water or near the water's edge.
Ambulocetus: Roughly contemporaneous with Pakicetus (Eocene epoch - Paleogene), Ambulocetus provides post-cranial remains that shed light on its locomotor adaptations. Ambulocetus remains are found in near-shore and estuarine deposits.
The shape of its skull is reminiscent of a crocodylian, but its manner of axial swimming, involving both the vertebral column and the hindlimbs, is reminiscent of an otter. Perhaps it could also prey on land animals.
Rhodocetus: (Eocene epoch - Paleogene), Rhodocetus is also a limb and trunk propelled otter-style swimmer. It is slightly derived in the elongation of the trunk and the retraction of the nostrils compared to Ambulocetus.
Of particular interest is its well-preserved ankle. This shows two items of note:
- In all placental mammals, the astragalus (a tarsal) articulates with the shin across a spool shaped surface. This is the cylindrical joint around which the foot rotates. Only in artiodactyls, the distal surface of the astragalus is also spool shaped, giving the foot a double-cylindrical articulation with the shin. This greatly extends the range of rotation of the artiodactyl ankle. Rhodocetus clearly preserves this double-spool astragalus shape, lending force to the argument that cetaceans are artiodactyls.
- Moreover, the foot is large and the ankle has considerable leverage (compare with the very cursorial pronghorn (Antilocapra). Rhodocetus used its hindlimbs.
Basilosaurus: (Late Eocene epoch - Paleogene) By the end of the Eocene Epoch, profound changes in the locomotion and paleobiology of whales had occurred, as indicated by Basilosaurus (originally mistaken for a reptile, hence the misleading name.)
- The tooth row is reduced to anterior conical teeth and posterior blade-like shearing teeth, superficialy resembling those of sharks.
- The nostrils are significantly retracted, but still in front of the eyes.
- The torso is very long and serpentine.
- The hindlimbs are reduced to the point of uselessness for swimming, but were still present and robust. Copulatory guides?
- Here, the dorso-ventral undulation of the torso was the power source for swimming, as it is in sea cows. The presence of tail flukes is unclear.
Dorudon: Roughly contemporaneous with Basilosaurus Late (Eocene epoch - Paleogene), Dorudon approaches the biomechanical condition of living whales in:
- The condition of its inner ears. Modern whales have highly reduced semicircular canals.
- Its general proportions.
- The reduction of its hindlimbs.
- Its skull broadly resembled that of Basilosaurus
Mysteceti:: The origins of baleen whales are somewhat mysterious, however indications are provided by fossils from the Oligocene Epoch (Late Paleogene). These include:
LLanocetus, whose teeth are widely spaced and bear exaggerated serrations, reminiscent of the crab-eater seal - used for straining small arthropods from the water. In life, could the spaces between its teeth have been occupied by baleen?
Janjucetus, which, although toothed, shares other synapomorphies of the skull and jaw with baleen whales
Odontoceti:: Are more straightforward. Primitive forms are intermediate between Dorudon and living members in teeth, but show extreme retraction of the nostrils in the manner of living odontocetes, as in the Miocene (Neogene) Squalodon. While primitive in dentition, it, like other fossil odontocetes, shows the osteological correlates of the melon and diverticula of the nasal passages used in echolocation. Living odontocetes differ in that:
- their nasal passages are invariably asymmetrical
- Their teeth are uniformly conical.
Odobenocetops:: For the record, one very strange departure in odontocete evolution is Odobenocetops, a toothed whale "trying to be" a walrus. Derived features include:
The oral apparatus most closely resembles that of a walrus (a pinniped) which:
- uses whiskers detect prey
- uses tusks to root prey up
- grasps with muscular lips
- uses tongue used to generate suction that pulls prey soft tissues from shell