ZOOL 403 Chapter Notes - Chapter 5: Pulmonary Alveolus, Saltwater Crocodile, Lark
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ZOOL 403 Fall 2018
Systematics of the Archosauria
1. To examine the common classification scheme for the extant archosaurs
(and the opportunity to examine local representatives).
2. To be able to identify the major features of bird groups
Crocodylia- The Crocodiles, Alligators And Gavials
Although not as diverse or numerous as other reptilian groups, the crocodylians represent some
of the oldest living “reptilian” forms. Early crocodylians arose in the Triassic period and modern
forms have existed since the Cretaceous. Different taxa resemble each other closely, except in
the shape of the skull. (Features of the skull are used frequently in the determination of
phylogenetic relationships). All live in an aquatic environment. As such, the nostrils are placed
dorsally and can be closed when the individual is submerged. Crocodylians have increased
respiratory efﬁciency, achieved by a partition between the pectoral and abdominal cavities
(analogous to the mammalian diaphragm) and by well-developed pulmonary alveoli. These
characteristics are useful when long periods of time are spent underwater.
Diversity and Classiﬁcation of Crocodylians
As the different groups of crocodylians exhibit substantial morphological similarity, they are
believed to be a monophyletic group that is divided into three clades. As stated earlier, aside
from body size the main differences between these clades is in the structure of the skull and in
the patterns and locations of scutes on the body.
Figure 8.1 presents a current cladogram of the Crocodylia. Refer to this diagram as you examine
ZOOL 403 Fall 2018
This group contains two genera and thirteen species that are native to tropical areas of North,
Central and South America, Africa, India, Asia, Indonesia and Australia. Both fresh and saltwater
forms exist. They are moderate to large in size (between 2 and 7 m in absolute length), with
variable snout shapes. However, all members of the Crocodylidae are easily distinguished by
the distinctive fourth mandibular tooth; visible from the side of the closed mouth. There are no
osteoderms on the venter (belly). Two species, Crocodylus porosus (Saltwater crocodile) and
Crocodylus niloticus (Nile crocodile), have been known to prey on humans.
Perhaps the most distinguishing feature of this group is their very elongate, narrow, snout. All
are almost exclusively piscivorous. When the jaw is closed, the fourth mandibular tooth and all
teeth cranial to it lie outside the upper jaw. The many teeth are very closely spaced.
All gavialids possess dorsal, but no ventral, armour. There are two species placed in this taxon,
each within its own genus – Gavialis and Tomistoma. Gavialis is possibly the most aquatic of the
crocodylians. They are limited in distribution to the river systems of Asia.
In alligatorids, the teeth of the lower jaw ﬁt into pits in the upper jaw and thus cannot be seen
when the mouth is closed. Belly osteoderms may be present or absent. They are largely
restricted to the American tropics. Most are conﬁned to freshwater habitats – primarily larger
rivers, lakes, swamps, and lagoons. Some of the smaller alligatorids (such as Paleosuchus
trigonatus) prefer small streams. One species, Alligator mississipiensis, inhabits temperate
areas where freezing of surface waters may occur regularly during the winter months. It was
previously thought that alligators sought refuge from the cold in underground dens. However,
radio telemetry studies have shown that the alligators move into shallow water and position their
nostrils so that they remain above the surface ice. In this way, they are still able to breathe while
in a state of torpor. Crocodylians are unable to rely exclusively on energy gleaned from
Aves - birds
All modern birds belong to the clade Neornithes, diagnosed by the absence of teeth and
gastralia. See the cladogram in Figure 8.2. Despite a tremendous diversity and an easily
recognizable morphology, avian phylogeny remains largely unresolved, even using DNA.
Neornithes may be further subdivided into the Palaeognathae and the Neognathae.
Both clades are typically diagnosed on the structure of the palate (although other characters
also exist): palaeognaths have an immobile palate, with a robust vomer (sagittal element along
the palate) fused to the braincase; neognaths have a reduced vomer (sometimes lost
completely) and subsequently the palate is highly mobile.
The Palaeognathae includes all the large ﬂightless birds known as ratites (ostriches, rhea,
cassowaries, emu, and kiwis) and the South American tinamous. All other birds are members of
the Neognathae. Synapomorphies for the Ratites are listed below.
ZOOL 403 Fall 2018
Figure 8.2 Phylogeny of Aves. There are more recent molecular phylogenies
that make some of these groupings paraphyletic (see Hackett et al 2008), but
the groups you will see are all still used in ﬁeld guides.