Problem 9 - Biology Today and Tomorrow with Physiology

Question

All vertebrates are _________, but only some are ____________. a. tetrapods; mammals b. chordates; amniotes c. amniotes; hominins d. bipedal; australopiths

Step-by-Step Solution

Answer

b. chordates; amniotes

Step 1: Understand the Terminology

Begin by understanding the term "vertebrates." Vertebrates are animals that have a backbone or spinal column. All vertebrates belong to the broader category of chordates.

Step 2: Analyze Option a

Option a suggests that all vertebrates are "tetrapods," which isn't accurate as not all vertebrates have four limbs (e.g., fish are vertebrates but not tetrapods). Therefore, this option is incorrect.

Step 3: Analyze Option b

Option b posits that all vertebrates are "chordates," which is correct since vertebrates are a sub-group within the chordates. It further states "but only some are amniotes." Amniotes are a subgroup of vertebrates that include reptiles, birds, and mammals, thus only some vertebrates are amniotes. This option is correct.

Step 4: Analyze Option c

Option c suggests all vertebrates are amniotes, which is incorrect as not all vertebrates lay amniotic eggs or have amniotic characteristics — fish and amphibians do not. Thus this option is incorrect.

Step 5: Analyze Option d

Option d indicates all vertebrates are bipedal, which is incorrect as bipedal locomotion is specific to certain mammals and birds. Consequently, this option is incorrect.

Key Concepts

Chordates

The term "chordates" refers to a large phylum of animals known as Chordata. Chordates are diverse and include species like fish, mammals, birds, and amphibians. All chordates share certain key characteristics at some stage in their life cycle:

• A notochord, which is a flexible rod-like structure that provides support.
• A dorsal nerve cord, which runs along the back and develops into the central nervous system.
• Pharyngeal slits, which are openings that can develop into gills or parts of the jaw and ear in terrestrial animals.
• A post-anal tail, extending beyond the anus, present at some stage in development.

Vertebrates, which we often think of with animals that have backbones, are a subgroup of chordates. This means that while all vertebrates are chordates, not all chordates are vertebrates. For example, tunicates and lancelets are primitive chordates without a backbone.

Amniotes

Amniotes represent a crucial group of vertebrates that have developed adaptations to lay eggs on land or keep young inside the body in a special amniotic sac, leading to the name "amniotes." This evolutionary adaptation allows reproduction away from water. Amniotes include the following major groups:

• Reptiles: Most lay leathery shelled eggs and include snakes, lizards, turtles, and crocodiles.
• Birds: They are unique for their hard-shelled eggs and feathers aiding in flight.
• Mammals: While most give live birth, monotremes (like the platypus) lay eggs.

Not all vertebrates are amniotes; for instance, amphibians like frogs and salamanders lay their eggs in water. The evolutionary advantage amniotes have is their ability to reproduce on land, expanding their habitats greatly compared to non-amniotic vertebrates.

Tetrapods

The term "tetrapods" comes from Greek, meaning "four feet." Tetrapods are a superclass of vertebrates that have four limbs. These include many familiar animals:

• Amphibians, like frogs and salamanders, which often live in aquatic and terrestrial environments.
• Reptiles and birds, which cover a vast range of environments.
• Mammals, known for their wide range of habitats and diverse forms.

Not all vertebrates are tetrapods, as some, like fish, do not have four limbs. The development of limbs from fins was a major evolutionary step that enabled life on land, although some tetrapods, such as snakes, have lost their limbs over millions of years.

Vertebrate Classification

The classification of vertebrates is an organized system used to understand the relationships and characteristics shared among different animal groups. Vertebrates are part of the larger phylum of chordates and can be classified into several broad groups:

• Fish: The most primitive vertebrates, primarily aquatic, include sharks, rays, and bony fish.
• Amphibians: These transition between aquatic and terrestrial life, including frogs and newts.
• Reptiles: These cold-blooded vertebrates lay eggs on land, like snakes and lizards.
• Birds: Warm-blooded animals known for flight and laying hard-shelled eggs.
• Mammals: Known for live births and milk production, ranging from humans to whales.

Understanding vertebrate classification helps in comprehending the evolutionary history and how different vertebrates adapted to their environments. This classification highlights the path of evolution from simple aquatic life forms to complex terrestrial organisms.