The Emergence of Whales, Chp. 9

Synopsis of Chapter 9: Homology and Transformation of Cetacean Ectoympanic Structures

by Zhexi Luo Section of Vertebrate Paleontology, Carnegie Museum of Natural History, Pittsburgh, PA 15213-4080

Section 1. Introduction

This chapter is detailed to the max (and has the requisite jargon to go with it). The primary issue will be the ectotympanic bulla, which is more familiar to us as the “middle ear”. Luo notes up front that the “evolutionary adaptation to underwater hearing is only one of several factors that could have determined the anatomical structure of the cetacean bullae”. It had to change to enable hearing underwater, because sound travels differently in water than in air. So:

“Several specialized bullar structures of cetaceans clearly have acoustic significance for underwater hearing. These bullar characters of cetaceans may thus be attributable to evolutionary adaptation (!!).”

So, in this chapter, he’s going to (1) “evaluate the embryogenesis of major bullar structures in extant mysticetes and odonotocetes”, attempting to establish the homology “or lack thereof” between the bullar structures of adult cetaceans and “their ungulate relatives”; (2) “describe the bullar structures of pakicetids, the transitional (!!) group between terrestrial ungulates and the more derived whales, and the protocetids that were amphibious”; (3) survey the diversity of bullar structures in the main cetacean groups (basilosaurids, mysticetes, odontocetes), and (I love this part) (4) “elucidate the pattern of evolutionary transformation by distinguishing the cetacean apomorphies that could have derived from adaptive evolution (!!) from the apomorphies with no apparent adaptive value.”

For those of you paying some attention to why I started doing this, the force driving “adaptive evolution” would be: NATURAL SELECTION.

2. Development

Figure 1 illustrates what he describes, the embryonic development of the ectotympanic bulla. He devotes several pages and figures 1-3 to this. Seriously, though quite detailed, I can’t tell what’s going to be really important later, though some structures will be compared. I did notice that “relative to the anterior pedicle of adult mysticetes, the accessory ossicle of adult odontocetes is a neotenic feature.” He then lists the differences between ungulates, mysticetes and odontocetes. Here’s a sample of why I’m not getting a lot from this section:

Comparing adult odontocetes to mysticetes: “But the accessory ossicle differs from the pedicle of mysticetes in that the former is not incorporated into the tegmen tympani”, etc.

But further on:

“The external auditory openings in the bullae of basilosaurids, odontocetes, and mysticetes differ from those of terrestrial ungulates in possessing a conical apophysis, a triangular projection protruding into the external auditory meatal opening.” Because it’s often tightly pressed to the base of the sigmoid process, it blocks much of the auditory meatal opening, so the external auditory opening is extremely small in proportion to the whole bulla.

In all seriousness, I can’t do justice to this. At all. And I sincerely apologize to the remarkable Dr. Luo, if he ever encounters this. The discussion is detailed, the diagrams are intricate and detailed. He discusses the differences in each group (compared to the other groups. The level of expertise is great; unfortunately it’s not my area of expertise.

3.1 Pakicetids

Covers Pakicetus and Icthyolestes. Figure 7 is a schematic diagram of the transformation from ungulates through pakicetids, basilosaurids, balaenopterids (mysticetes/baleen whales), and delphinids (odontocetes/ toothed whales). From the caption:

“The transformation of a functional planar tympanic membrane is correlated to the reduction of the tympanic annulus and the tubular portion of external auditory meatus. Transformation of a planar tympanic membrane into the conical tympanic ligament occurred as the annulus for membrane suspension was reduced and the ectotympanic ring is folded into the conical apophysis (medial process).” Yes, I can see this in the diagram. For pakicetids, the tympanic membrane is folding, and the conical tympanic ligament forms, forced into that shape by the enlargement of the bulla, and the appearance of the conical apophysis. Yes, I CAN SEE IT! (Wish I could reproduce the diagram.)

Unfortunately, though he discusses it, he doesn’t have this diagram. So you can read his short research descriptions or ignore them. Dr. Luo is accomplished and prolific.]

Moving on:

3.2 Protocetids

Luo notes that protocetids are really a grade, not a group. Meaning that they aren’t monophyletic. Some protocetids are more closely related to the more derived whales than other protocetids.

2 apomorphies of the bulla:
1. Enlargement of the posterior process of the tympanic.
2. Posterior process is joined to the bullar body by two pedicles. This is also seen in basilosaurids.

2 other apomorphies are related to the expansion of the involucrum.

3.3 Basilosaurids and dorudontines

Most significant apomorphies are associated with the modification of the external auditory opening of the ectotympanic:

The annulus of the ectotympanic is greatly reduced in size and modified in morphology; and a conical apophysis has formed.

The sigmoid process differs from pakicetids.

Basilosaurids share several similarities in the external auditory opening with extant cetaceans, so it has been hypothesized that they also had a conical tympanic ligament, like extant cetaceans.

3.4 Cetacean crown group
2 bullar apomorphies:

1. Processus tubarius has lost its contact with the entoglenoid process of the squamosal.
2. Absence of the double pedicles (I think). Note to authors: when you say “First”, it makes it easier on the reader if you also say “Second,”!

3.5 Odontocetes
2 bullar apomorphies

1. Accessory ossicle
2. Thin edge of the Eustachian tube opening

3.6 Mysticetes A striking bullar apomorphy:

*Complete fusion of the anterior process to the anterior process of the petrosal.

(By the way, there is also a schematic diagram of this, too.)

4. Character Evolution

4.1 Tympanic membrane and Homologues

“As an adaptation (!!) to the aquatic environment, extant cetaceans have extensive modifications to their external ears. As a result, the tympanic membrane is so highly modified that it becomes nonfunctional for hearing…”

Some workers argue that the external auditory meatus and the tympanic ligament have some capacity for hearing.

There is a consistent relationship between the ectotympanic annulus and the tympanic membrane in all extant mammals. So the condition of the tympanic membrane in fossil mammals can be reliably inferred from the annulus as preserved in the fossils.

(I did not know that, either.)

Mesonyx — typical annulus of terrestrial mammal Pakicetids — about the same as Mesonyx and extant artiodactyls, though some indications of changes Basilosaurids — significantly reduced external auditory opening. Probably had a conical tympanic ligament; differ significantly from extant artiodactyls. Implies same specialization in tympanic structures as extant whales.

4.2 Sigmoid Process

There are two prominent differences in the bulla between mesonychids and cetaceans. Take my word for it, I won’t bother to describe them. Luo describes three hypotheses for the homology of the cetacean sigmoid process.

4.3 Posterior Process

The enlarged posterior process in extant cetaceans is hypothesized to be a neomorphic character state of the posterior crus of the ectotympanic ring of other mammals.

4.4 Involucrum [Here’s some good stuff!]

The presence of the involucrum in all cetaceans, including pakicetids, suggests that this is a major apomorphy of cetaceans. It therefore is an “unequivocal” diagnostic character for the cetaceans, including Pakicetus. (Some people wonder how the early archaeocetes are determined to be whales: this is how.)

“The involucrum marks the beginning [good word!] of the development of pachyostosis (massiveness and hypertrophy) and pachyosclerosis (high density and heavy mineralization) of the tympanic complex in cetaceans.” The massive and dense bone of the involucrum can serve as a crude barrier to acoustic interference via bone conduction.

Further down:

“The increased mineralization of the tympanic bulla in cetaceans gives the bulla a greater density, and a better stiffness that results in a high modulus of elasticity.” … “… the bone of the bulla is more isotropic. Isotropic bones transmit sound vibration without distortion.” Bulla hardness borders on enamel, resulting in better fossil preservation.

Next paragraph describes how pachyosclerosis of the ear bones is correlated with underwater hearing (and also enables balance and buoyancy).

4.5 Hearing of Pakicetids

Ahem:

“Pakicetids have a crucial position in the phylogenetic transition from the terrestrial ungulate ancestry to fully aquatic whales.” Some features of the ear region are the primitive characteristics of terrestrial mammals. Pakicetus could hear airborne sound. Pakicetus had an enlarged bullar cavity, enabling low-frequency hearing (not suitable for hearing high-frequency echolocation sound). Thus, Pakicetus didn’t yet have the specialized structures for underwater hearing. But Pakicetus did have the involucrum, which is the first step. I.e., the process of adapting to an aquatic existence had commenced with Pakicetus.

CONCLUSIONS:

Embryonic hearing structures allow homology recognition with other mammals;

Bullar characters are useful for cetaceans systematics;

The earliest cetaceans could still hear airborne sounds.