7/27/2019 10 - Skeleton II

1/25

Lecture 10The Dermatocranium and Axial SkeletonKK Chapter 7,8; H&G Chapter 8,9

The dermatocranium changes relatively little from early bony fishes through to

mammals. Many of the changes involve simplification by loss of bones,

especially the back of the skull, in the transition from fish to tetrapod.

(Fish have no necks, tetrapods do!)

However, the dermatocranium changes more among teleost fishes where

adaptations for feeding cause extensive modifications. We will not cover

these changes.

We will start by looking at the skull ofAmia, a primitive bony fish, and followthe changes to mammals.

7/27/2019 10 - Skeleton II

2/25

Amia KK 7.23, H&G 8.11

Amia has a skull typical of early Osteichthyes, and not unlike

Crossopterygii. Many of the dermatocranium elements are homologouswith those of tetrapods. Early tetrapods lack the opercular bones and the

bones connecting the pectoral girdle to the head and in that location they

have instead a flexible neck. The bones shaded in pink are part of the

pectoral girdle. The red line marks the part of the skull that is lost with the

gills.

7/27/2019 10 - Skeleton II

3/25

Dermal Bones of a Tetrapod

The major difference relative

to Amia is the loss of bones

at the back of the head. Thistrend will continue with loss

of postparietals and

temporals, most of the orbital

series, and simplification of

the lower jaw.

KK 7.10 H&G 8.6

TopView Palate

7/27/2019 10 - Skeleton II

4/25

Early Amphibian (KK 7.37, H&G 8.12) Early Reptile (KK 7.33)

Turtle (KK 7.37, H&G 8.15)

7/27/2019 10 - Skeleton II

5/25

The dermal skull roof changes relatively little in tetrapods,

except for appearance of openings in the temple. In many

mammal and birds the temple opening converges with the orbit.

KK 7.34,H&G 8.18

What are

euryapsids?

7/27/2019 10 - Skeleton II

6/25

Temporal Openings in Tetrapods

Temporal openings allow for muscles closing the jaws to bulge, andprovide better points of attachment.

Note the zygomatic arch. Some mammals have a sagittal and/or nuchal

crest on the top or back of the head to provide even better attachment

points.

KK 7.35, H&G 8.17 Frontal sections.

7/27/2019 10 - Skeleton II

7/25

The temple opening in

humans. Humans are

among the mammals

(apes, artiodactyls)

where the temple

opening is not

confluent with the

orbit.

7/27/2019 10 - Skeleton II

8/25

From Internal Nares to Secondary Palate

Choanate vertebrates (Sarcopterygii, tetrapods)

have a connection between the external naris

and the mouth. In mammals, this connection

between respiratory and digestive pathways

moves to the back of the mouth because of the

hard and soft palate.

early reptile (Parareptilia)

KK 7.57, H&G 8.19

KK 7.27

7/27/2019 10 - Skeleton II

9/25

The Secondary Palate

The secondary palate is formed by

folds in the marginal bones of the

palate (premaxilla, maxilla and

palatine).

The space between the original andsecondary palate in mammals

contains turbinate bones.

The secondary palate is also found in

turtles and crocodiles.

Why did this evolve? And what arethe implications for the evolution

of endothermy?

Palatine fissures?

KK 7.58, H&G 8.19

7/27/2019 10 - Skeleton II

10/25

Review: the skull of the

opossum, Didelphis

KK 7.51

7/27/2019 10 - Skeleton II

11/25

The Axial SkeletonKK Chapter 8, H&G Chapter 9

7/27/2019 10 - Skeleton II

12/25

Notochord

Vertebral Column

Ribs, sternum, gastralia

Median Fins (caudal, dorsal, anal)

Parts of the Axial Skeleton

Embryological origins

notochord, schlerotome andmesenchyme

7/27/2019 10 - Skeleton II

13/25

The Notochord

found in all Chordata

found in all vertebrate embryos, plus adults of

Agnatha, Placodermi, Acanthodii, Holocephali, some

Actinopterygii (not teleosts), many Sarcopterygii, and

the first Amphibia mechanically important for swimming

7/27/2019 10 - Skeleton II

14/25

Centra in bony fish provide the same mechanicalproperties as the notochord.

KK 8.19

intervertebral

ligament

7/27/2019 10 - Skeleton II

15/25

Fish with notochords have bony or cartilaginous pieces (neural or hemal

arches or spines) associated with the notochord. Fish with vertebrae tend

to have biconcave (amphicoelous) centra. Tetrapod centra tend to be

acoelous (platyan); more stout. Procoelous or opisthocoelous centra may

be found where more flexibility is required, e.g. tail or neck.

KK 8.1, H&G 9.4

KK 8.4, H&G 9.2

7/27/2019 10 - Skeleton II

16/25

Vertebrae of Fish

The ribs are between the muscle blocks (myomeres) of the fish trunk.

The dorsal ribs follow the horizontal septum between the epaxial and

hypaxial muscles.

KK 8.6

7/27/2019 10 - Skeleton II

17/25

Development of the Vertebral Column

Vertebrae arise

from thescherotome, and

begin as two

elements per

segment. In most

cases they fuse to

make a singlecentrum, but do so

in an inter-

segmental fashion.

Where they

remain separate,

they are calledpleurocentrum and

intercentrum.

KK 8.10 & 8.13, H&G 9.3

7/27/2019 10 - Skeleton II

18/25

Evolution of the vertebra in the Amphibia

A labyrinthodont with equal inter-and pleurocentra (above). In the

lineage leading to amniotes, the

pleurocentra dominate.

KK 8.24, 8.26. H&G 9.7

7/27/2019 10 - Skeleton II

19/25

Caudal Fins

The tail fin issupported by

flattened neural

and especially

hemal arches.

The tail of fishes

with homocercal

tails begin as

heterocercal.

KK 8.20,H&G

9.14

7/27/2019 10 - Skeleton II

20/25

The impact of terrestrial life on the axial skeleton

Centra expand, fuse and replace the notochord.

Intervertebral joints and processes

(zygapophyeses) develop that restrict or facilitate

movement depending on the location Processes to increase the leverage of muscles

Connection to limb girdles.

More flexible relationship with the head via

cervical vertebrae + atlas (and axis).

7/27/2019 10 - Skeleton II

21/25

Tetrapod

Vertebrae

Zygapophyses resist the tendency of

the trunk to sag, and ribs now have two

attachments to the vertebral column

and meet at the ventral mid-line

(sternum).

Tetrapods do not have dorsal ribs, but

their ribs often have two heads and

attach to the sternum.

KK 8.7, H&G 9.1

R i l S i li ti f V t b

7/27/2019 10 - Skeleton II

22/25

Regional Specialization of Vertebrae

cervicals - small and flexible, with ribs small or absent

thoracics - ribs, no hemal arches, reach sternum in tetrapods

lumbars - no ribs, very stout

sacrals - with heavy sacral ribs attaching to pelvic girdle

caudals - flexible, with ribs small or absent. May be hemal

arches, intercentra.

R i l S i li ti f V t b

7/27/2019 10 - Skeleton II

23/25

Regional Specialization of Vertebrae

Fishes:thoracics and caudals only

Amphibia:atlas, 1 cervical,thoracics, 1 sacral, andcaudals Reptilia:atlas, axis, cervicals,thoracics, 2+ sacrals, caudals

Aves:atlas, axis, cervicals,thoracics, synsacrum,pygostyle

Mammalia:atlas, axis, cervicals,thoracics, lumbars, 3+ sacrals,

caudals

7/27/2019 10 - Skeleton II

24/25

Atlas and Axis

In amniotes, the first twovertebrae (atlas and axis) are

specialized for nodding the head

(occipital condyles articulate with

the atlas) and rotating the head

(atlas rotates on the axis on the

axis).

The latter joint is actually

between the pleurocentrum of the

atlas, now fused to the axis, and

the intercentrum of the atlas.

KK 8.27, H&G 9.10

7/27/2019 10 - Skeleton II

25/25

Bull Skeleton