Dental Morphology (Cetacean), Evolution of (marine mammals)


Modern cetaceans exhibit some of the most highly derived dentitions in all of Mammalia. Living odontocetes ‘ ” . exhibit a wide variety of conditions from some of the highly polydont delphinids to the complete lack of teeth in some monodontids. Living mysticetes completely lack teeth as adults. Add to this even more divergent conditions in fossil mysticetes, odontocetes, and their early forebears, the archaeocetes, and the result is a broad range of conditions in which is embedded the story of how modern cetacean dentitions arose from the more typically mammalian teeth, Acreodi (Mesonychia + Triisodontidae) is currently thought to be the sister taxon to Cetacea based on paleontological evidence. Some molecular studies have indicated that Cetacea is nested within Artiodactyla, possibly with Hippopotamidae as its sister taxon. Acreodi are diphyodont and heterodont, with a plesiomorphic dental formula of Their incisors are single rooted and in an arc across the front of the jaw. Mesonychians and cetaceans all have reduced trigonids and talonid basins, and lack an M3 hypoconulid.

I. Archaeocetes

Archaeocetes are diphyodont and heterodont with their incisors in line with the cheek teeth and separated by diastemata. The anterior teeth are conical and single rooted, and the premolar series forms a morphological gradient from the anterior teeth to the posterior premolars along with an increase in size such that P3 or P4 is usually the largest upper or lower tooth. Upper and lower Pj may be single or double rooted, depending on the taxon, and the other premolars are double rooted (but see Albright, 1996). The premolars are generally equilaterally triangular in lateral view. The lower molars have steep mesial edges and more gradually sloping distal edges. The lower molars lack trigo-nid basins and retain hypoconids but have lost the talonid basin. The lower molars also have a vertical groove on their mesial margins. The tooth row is closed in this region such that there are no diastemata between the teeth, and the next anterior tooth rests in the groove of the distally adjacent molar. Archaeocetes often exhibit vertical wear facets on the buccal surfaces of the lower molars that resulted from contact with the lingual surfaces of the upper cheek teeth during chewing (O’Leary and Uhen, 1999).

A. Nonbasilosaurid Archaeocetes

Nonbasilosaurid archaeocetes have a dental formula of that they share with mesonychians and other early mammals and that distinguishes them from the later basilosaurids. Nonbasilosaurid archaeocetes have a paracone (mesial) and metacone (distal) on the buccal margins of their molars, and a protocone projecting lingually. Pakicetids have single-rooted Pjs, which distinguish them from ambulocetids. Ambulocetids have a protocone lobe on the upper molars that is smaller than that in pakicetids and larger than that in rem-ingtonocetids and protocetids.Rem-ingtonocetids have labiolingually narrow incisors and premolars: upper molars large and narrow, sometimes lacking a lingual third root, and crenulations on the cutting edges of the teeth, which distinguishes them from other nonbasilosaurid archaeocetes (Kumar and Sahni, 1986). Protocetids have robust teeth with upper molars having three roots and the cheek teeth lacking accessory denticles. (Fig. 1, but see Georgiacetus later).

B. Basilosauridae

Basilosaurids have a number of synapomorphies that distinguish them from other archaeocetes. Upper and lower first premolars are replaced in dorudontine basilosaurids, but it is unclear how broadly this character is distributed within the archaeocetes (Uhen, 2000). Basilosaurids lack M3, giving them a dental formula of The cheek teeth of basilosaurids have accessory denticles along their mesial and distal margins, flanking an apical cusp. These apical cusps are homologous with the paracone on the uppers and paraconid on the lowers. It is unclear whether any (or all or none) of the accessory denticles are homologous with more primitive mammalian cusps. There are usually no accessory denticles on the mesial margins of the lower molars (for exceptions, see Uhen, 1996). While accessory denticles of the size and number seen in basilosaurids are unknown in earlier cetaceans, Georgiacetus possesses small incipient accessory denticles on some of the cheek teeth. Also, accessory denticles similar to those of basilosaurids can be seen on dP4 of Pappoce-tus lugardi. The lingual posterior root found on the upper posterior cheek teeth of earlier cetaceans is fused to the buccal posterior root on these teeth in basilosaurids, as shown by die presence of a groove in the root and a lingual expansion of enaniel on die crown, which lacks a trigon basin and protocone.

Phylogenetic relationships of cetaceans with examples of dental morphology. For those cetaceans with heterodont teeth, lower molars (posterior cheek teeth) are shown

Figure 1 Phylogenetic relationships of cetaceans with examples of dental morphology. For those cetaceans with heterodont teeth, lower molars (posterior cheek teeth) are shown.

II. Mysticeti + Odontoceti

The fossil record of archaeocetes ends in the Late Eocene. Mysticeti and Odontoceti can be distinguished from archaeocetes based on die presence of some kind of telescoping (Barnes, 1984), but telescoping can hardly be used as a synapomorphy of Mysticeti + Odontoceti, as telescoping is manifested in different ways in each group and is almost certainly not homologous between diem. Monophyodonty may serve as a synapomorphy for Mysticeti + Odontoceti. Even diough modern mysticetes lack teeth as adults, they develop embryonic tooth buds that are later re-sorbed (Karlsen, 1962). In addition, early fossil mysticetes have well-developed teeth as adults (Emlong, 1966; Mitchell, 1989; Barnes and Sanders, 1996a,b). Even a late-occurring archaeocete has been described diat may be monophyodont (Uhen and Gin-gerich, 2000). Early mysticetes and odontocetes are similar to archaeocetes in that they are heterodont, with a morphological gradient from conical, single-rooted anterior teedi to triangular (or rounded) multirooted cheek teeth with accessoiy denticles.

A. Odontoceti

Modem odontocetes can be generally characterized as being polydont and monophyodont, with single-rooted teeth that grow throughout life, but there are numerous counterexamples to these generalizations in both modem and fossil cetaceans (discussed later). The earliest odontocetes show only limited polydonty, with

Agorophius having 8 and Xenorophus with 10 teeth (Fordyce, 1982 and references therein). Polydonty could have originated by intercalation of deciduous and permanent teeth and could have been further increased in later odontocetes by die addition of supernumerary teedi, but it would be difficult to explain why no odontocete is known diat exceeds diree teeth in the premaxilla under this scenario (Fordyce, 1982). The recent description of early mysticetes with one extra molar in each quadrant (Barnes and Sanders, 1996a,b) may indicate diat polydonty arose prior to die split of Mysticeti and Odontoceti by the terminal addition of teeth, although many other toothed mysticetes appear not to possess polydonty at all (Emlong, 1966; Fordyce, 1982).

All modem and fossil odontocetes are diought to be monophyodont (Fordyce, 1982). It is unclear whether the teeth of modem odontocetes are homologous widi die deciduous or adult teeth (or bodi) of archaeocetes. A newly described archaeocete that appears to possess adult teedi in a skeletally juvenile individual suggests diat die teedi of all Mysticeti + Odontoceti may be adult teeth, but Karlsen (1962) suggested that die embiyonic toodi buds of mysticetes were homologous widi die deciduous dentition of more primitive mammals.

Early odontocetes Agorophiidae (including Xenorophus), Simocetidae, Waipatiidae, and Squalodontidae are all considerably heterodont. Their anterior teeth are conical with long, single roots. These teeth grade into teeth with a more triangular shape with multiple accessory denticles on the anterior and posterior edges of the teeth and two roots. The most posterior lower cheek teeth may lack accessory denticles on their anterior edges. These differences in tooth form are reminiscent of the differences along the tooth rows of archaeocetes, but the teeth are smaller compared to the size of the skull.

Physeteroidea includes two genera, Phi/seter and Kogia. Modern physeteroids usually lack upper teeth (or have tiny upper teeth that do not erupt), but some fossil representatives (such as Scaldicetus) have similarly sized teeth in the lower and upper jaws (Hirota and Barnes, 1994).

Ziphiidae are thought to have secondarily reduced the dentition from a primitively polydont condition. One modern species, Tasmacetus shepherdi, and additional fossil species are polydont. In some other modern species, the females usually lack erupted teeth, and males have one tooth on each side oi the lower jaw and none in the upper jaw. The teeth of male ziphiids take on different forms in different species.

Iniidae have rugose heterodont teeth that are expanded lin-gually in the posterior portion of the jaw. Monodontidae have very reduced dentitions. Male narwhals (Monodon monoceros) have a single, long, spiral tusk, usually on the left side with a small unerupted tooth on the right. Females usually have two unerupted teeth. Both male and female belugas (Delphinaptertis leucas) generally have nine teeth in each upper and eight in each lower row. One related fossil, Odobenocetops, has a walrus-like skull with one large tusk-like tooth and another shorter tusk on the opposite side (de Muizon, 1993). Members of the extinct Kentri-odontidae have small conical teeth that are generally homodont, a feature shared widi one of their likely descendants, the Delphinidae, but not with another, the Phocoenidae, which have numerous small spatulate teeth or (in one species) peglike vestigial teeth that may be embedded in gum tissue.

B. Mysticeti

Despite the lack of teeth in modern mysticetes, they are thought to have evolved from toothed ancestors in part because they develop tooth buds as embryos that are later resorbed (Karlsen, 1962), and many tooth-bearing early mysticetes are known from the Oligocene epoch (Fordyce and Barnes, 1994). All post-Oligocene mysticetes are baleen-bearing and lack teeth (Fordvce and Barnes, 1994).

One group of toothed mysticetes includes species with ar-chaeocete-like teeth (Fordyce, 1989; Barnes and Sanders, 1996a,b). This group could be delimited to include Llanocetus denticrenatus from the late Eocene of Antarctica (Mitchell, 1989). Mitchell (1989) placed Llanocetus in the family Llanoceti-dae, which he included in a new infraorder Crenataceti from which he excluded all other toothed mysticetes known at the time. Kekenodon onamata and Phococetus vasconum may also be members of this group (Fordyce, 1992). The teeth of this group are similar in size to those of archaeocetes, and the cheek teeth are similarly double rooted and have accessory denticles. Some species have an extra molar in each quadrant over the count of basilosaurids, yielding three upper and four lower molars (Barnes and Sanders, 1996a,b). These teeth may have been used to filter food from seawater, as the teeth generally lack wear resulting from tooth/food contact (Mitchell, 1989).

The second group of toothed mysticetes includes species that all have tiny teeth that are conical anteriorly and have tiny denticles posteriorly, but are all single rooted. All of these species have been placed in the Aetiocetidae. Some aetiocetids have the plesiomorphic number of teeth for archaeocetes (11), whereas others are polydont. Aetiocetids are not thought to have had baleen and may have used their small teeth for filter feeding. The small, toothed mysticete Mammalodon from the Late Oligocene or early Miocene of Australia may be related to the aetiocetids, but it is difficult to compare in part because it is known only from the type specimen, and its teeth are highly worn (Pritchard, 1939; Fordyce and Barnes, 1994).

The earliest baleen-bearing mysticetes that lack teeth are placed in the Cetotheriidae, but this taxon has long been recognized as a wastebasket taxon in need of serious revision. The earliest cetotheres are from the Late Oligocene (contemporaneous with toothed mysticetes) and have worldwide distribution. Cetotheres may be ancestral to balaenopterids, but their relationship to other modern mysticete families is unclear.

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