Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Infraclass Eutheria, or Placental Mammals
Order Primates

The order Primates comprises about 200 species of mammals exhibiting an exceptional diversity in appearance (Fig. 68). Their body length ranges from 9-12 cm to 2 m, yet despite external differences, they share many common structural features. For instance, the braincase is relatively large, the orbits are directed almost entirely forward, the thumb is opposable to the other digits, and the digits bear Nails.

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Fig. 68 Primates:

1 — tarsier; 2 — black howler monkey; 3 — chacma baboon; 4 — chimpanzee.

The hind limbs are plantigrade, while the forelimbs are grasping. The nipples (from 1 to 3 pairs) are located on the chest. All dental groups are present, with two pairs of incisors. The order is divided into two suborders: Prosimii (Prosimians) and Anthropoidea (Simians/Monkeys). Their representatives inhabit tropical and subtropical regions.

Suborder Prosimii, or Lower Primates. Prosimians include primates ranging in length from 9 to 106 cm. In some species, the tail is long and densely furred, though not prehensile, while in others it is sparsely furred or absent. The most primitive among them are treeshrews (Tupaiidae), which somewhat resemble squirrels and are structurally close to the most ancient insectivorous mammals. Lemurs and tarsiers possess a more advanced Organization than treeshrews. These animals are arboreal and primarily nocturnal. Typical Examples of lemurs include the ruffed lemur and the slender loris, while tarsiers are represented by the tarsier.

Suborder Anthropoidea, or Higher Primates (Simians). Simians unite the most highly organized animals on our planet.

Their Brain is relatively large, and in most species, the cerebral hemispheres feature numerous sulci and gyri. The tail is absent or developed to varying degrees. The thumb is opposable to the other digits. Body length ranges from 15 to 200 cm. About 140 species are known.

Among the higher primates, Old World monkeys and apes are of the greatest interest. Old World monkeys possess well-developed cheek pouches and ischial callosities, and their tail is usually long. Many of them are arboreal, but they also forage for food on the ground. These are highly active, noisy animals that vocalize on any occasion. They move on the ground and along thick branches on all fours, supporting their weight on the palms of their hands and the entire soles of their hind feet. The Cercopithecidae family includes typical monkeys, baboons, and macaques.

Apes (anthropoid apes) comprise the highest Representatives of the order. Their braincase is particularly well developed. The cerebral hemispheres feature complex sulci and gyri. They lack tails and ischial callosities. All apes live in forests, climb trees with ease, and show imperfect adaptations for terrestrial locomotion. When walking, they do not place their entire sole on the ground, but rather walk on the outer edge of the FOOT. Long arms help them move and maintain balance, with the animals supporting their weight on the DORSAL SIDE OF their folded fingers. They live solitary lives or in groups. There are 11 known species of apes; among them are the orangutan, chimpanzee, gorilla, and several forms of gibbons.

Humans also belong to the higher primates. However, humans differ qualitatively from all other animals through The Development of labor activity, articulate speech, social life, and consciousness.

QUESTIONS FOR SELF-check and review:

1. What structural features characterize mammals?

2. What habitats do mammals occupy?

3. How is thermoregulation carried out in mammals?

4. What reproductive features allow mammals to be divided into monotremes, marsupials, and placentals?

5. What structural and lifestyle characteristics are typical of representatives of the subclasses Monotremata, Marsupialia, and the orders of placental mammals (give a few examples)?

6. What is the Significance of mammals in nature and human life?

ORIGIN AND EVOLUTION of Birds and Mammals

Birds and mammals arose independently from distantly related reptile groups, with mammals appearing first, followed by birds.

Mammals diverged from primitive reptiles—therapsids (mammal-like reptiles), which still retained certain amphibian characteristics—during the Triassic period (approximately 215 million years ago).

Fig. 69 Archaeopteryx:

A — impression in shale; B — reconstruction.

The divergence of birds from reptiles probably occurred 190–170 million years ago. However, fossil remains of ancient birds from this period have not yet been found. In the deposits of the Jurassic sea gulf (dating back 150 million years), feather impressions and skeletal remains of the earliest known bird, Archaeopteryx, have been discovered; it is classified into a distinct subclass of lizard-tailed birds (Saururae) (Fig. 69). The ancestors of modern fan-tailed birds (Ornithurae) were likely even more primitive and ancient lizard-tailed birds.

Competition and direct persecution by numerous and diverse reptiles forced primitive birds and mammals to inhabit less favorable, unoccupied, or poorly developed biotopes, and to develop novel adaptations for foraging, defense against predators, and enduring harsh physicochemical conditions.

Under such conditions, the best adaptation—alongside restructuring in the locomotor, digestive, and other Organ Systems—was the acquisition of more flexible behavior driven by the complexification of the Central Nervous system and Sense Organs.

In both classes, analogous adaptations emerged independently, elevating the physiological activity of birds and mammals to a higher level compared to other vertebrates. Among this complex suite of adaptations, the following should be highlighted:

✵ mechanisms maintaining a stable body Temperature in the face of fluctuating environmental temperatures;

✵ advancement of The Nervous System and sense organs, accompanied by increasingly complex behavior;

✵ expansion of interactions among individuals, forming the basis for complex forms of intraspecific organization that enhance the species' competitive edge in the Struggle for Existence.

THE ORIGIN OF Chordates

Fossil remains of chordates have not been preserved. Consequently, reconstructing the Cytology/cytology/16.html">Early stages of their evolution relies heavily on indirect evidence: comparing the anatomy of adult forms and conducting comparative embryological studies.

Ancestors of chordate animals have been sought among various animal groups, including annelid worms. It was hypothesized that these hypothetical ancestors transitioned to an active lifestyle but began moving on what was originally the dorsal side of the body. The precursor of the notochord might have been a Connective Tissue cord located within the ventral musculature of some polychaetes (though these ingenious ideas found no confirmation).

Another group of zoologists suggests that the precursors of chordates were likely coelomate animals that shifted to a sedentary lifestyle, leading to a reduction in the number of body segments and The formation of a secondary Mouth. Echinoderms and pogonophorans—a distinct group of filter-feeding animals—diverged from such organisms. A third evolutionary branch led to The Emergence of chordates. At the very outset, hemichordates branched off, a group now accorded phylum rank. The emergence of the chordate phylum and its subsequent evolution are associated with increased mobility, accompanied by a general intensification of physiological Functions and, above all, improved foraging strategies.

Two hypotheses regarding the origin of chordates are the most widely accepted.

According to Garstang's hypothesis (1928), the ancestors of chordates—closely related to hemichordates—switched to a sessile lifestyle, refining The Mechanism of Water filtration through a Pharynx perforated by gill slits. Motile larvae ensured geographical dispersal. Such larvae could have acquired the capacity for neoteny (reproduction at the larval stage), while the adult phase might have been lost. This is how a mobile chordate ancestor could have arisen, similar to ascidian larvae.

According to the hypothesis of other researchers (A. N. Severtsov, 1912, 1939; N. A. Livanov, 1958), chordates may have originated from worm-like crawling or burrowing ancestors, outwardly resembling modern hemichordates, which developed a notochord. In their pharynx, perforated by gill slits, an endostyle arose—an organ that secretes mucus and captures food particles from the filtered water. This transformed the pharynx into a powerful filtration apparatus, much like that of modern amphioxi.

Both hypotheses require further development. They are mutually exclusive, yet both agree that the ancestors of chordates relied on passive feeding. Such a feeding strategy is tied to a sluggish lifestyle. Of the two pathways to a mobile existence and active feeding—via neoteny or the emergence of burrowing animals—the second appears more probable, given that ascidian larvae and neotenic larvae do not feed independently.

Chordates in the Phylogenetic Tree of the Animal Kingdom

The Study of extensive zoological data leads to a materialistic and dialectical understanding of the developmental pathways of living nature.

Comprehensive research confirms the derivation of all animals from a common ROOT, supporting the monophyletic Development of the entire animal kingdom. Consequently, the Phylogenetic relationships among animals of various taxonomic groups can be depicted as a branching tree, at the base of which lie primitive unicellular animals (Fig. 70).

Three avenues of research contribute to establishing animal phylogeny and uncovering their evolutionary affinities:

✵ comparative studies of the Morphology of extant animals (comparative morphology);

✵ studies of their Embryology, also from a comparative perspective (comparative embryology);

✵ studies of fossil remains from past geological epochs (comparative paleontology).

The comparative morphological method in zoology allows researchers to uncover varying degrees of structural complexity and detect Similarities and differences, helping to shape hypotheses regarding potential transitional forms and ancestral groups within the animal kingdom.

For instance, it has been established that flat ctenophores—animals related to Coelenterates—also share A number of features with turbellarians from the flatworm phylum, combining the traits of radially symmetrical diploblasts with those of bilaterally symmetrical triploblasts. Such similarities suggest a phylogenetic link between Flatworms and coelenterates.

Fig. 70 Phylogenetic tree of the animal kingdom (diagram)

The Use of comparative embryology data is grounded in the biogenetic law (rule), according to which the past leaves a trace in animal evolution, and animals in their individual development repeat to varying degrees the stages of their historical development—ontogeny mirrors phylogeny. This law was formulated by the renowned German Darwinist E. Haeckel, drawing upon extensive research by luminaries of our domestic science: A. O. Kovalevsky and I. I. Mechnikov on the embryology of invertebrates and lower chordates, and K. M. Baer on the embryonic development of vertebrates. Similarities in Animal Embryonic Development can serve as an additional argument when resolving questions regarding phylogenetic relationships.

The paleontological approach to studying animal phylogeny also provides compelling evidence for establishing the lineages of those animals whose remains have been preserved in sedimentary strata.

Unicellular organisms, possessing all their characteristic complex structural features—such as Cytoplasm, a Nucleus, and various Organelles performing vital functions—were preceded by living beings of a significantly simpler organization and non-cellular Structure. The first organisms possessed The ability to synthesize living bodily matter through the assimilation of non-biological inorganic and organic substances from the environment. They gave rise to unicellular animals, potentially of an amoeboid structure.

Among flagellates, organisms emerged that were capable of building their bodies from Inorganic Compounds utilizing solar energy via Photosynthesis. Green flagellates, combining traits of both animals and plants in their structure and biology, indicate that the two kingdoms of organic nature originated from common ancestors.

The evolution of unicellular organisms proceeded in various directions, yet the unicellular body organization was preserved in the majority. Ancient amoeboid organisms gave rise to modern sarcodines and parasitic Cnidosporidia. One branch of the flagellate genealogical tree leads to Ciliates—animals of the most complex structure—while another leads to parasitic Sporozoans, although opinions regarding the origin of sporozoans and Cnidosporidia remain divided.

The development of the unicellular animal world led, via colonial flagellates, to Multicellular animals. Among Protozoa, flagellates stand out for their greatest diversity and The complexity of their adaptations for feeding and living in diverse conditions. They include simple and complex colonial organisms that gave rise to two branches of modern multicellular animals: Sponges and all other Multicellular Organisms via primary two-layered ancestors.

Several hypotheses exist regarding the origins of multicellularity, and they are all united by the common recognition that multicellular animals descended from colonial flagellates. Sponges originated from choanoflagellates; their sedentary lifestyle did not favor progressive development, which is why they have retained many primitive structural traits. Other colonial flagellates gave rise to the two-layered ancestors of coelenterates. Most of them transitioned to a sessile or sluggish existence, as observed in modern coelenterates.

A comparative study of two-layered coelenterates leads to insights into the potential pathways of tissue origin and evolution, functional differentiation, and specialization. Coelenterates possess cellular structures of varying complexity and purpose within their mesoglea, which help illustrate the Selection/3.html">Stages of development of the third germ layer—the mesoderm. The rudiments of the mesoderm found in coelenterates are even more distinctly expressed in ctenophores—animals closely related to coelenterates in terms of origin and organization. The question regarding the origin of flatworms remains a subject of debate. Lang's hypothesis regarding their phylogenetic affinity with flat, crawling ctenophores has been challenged, yet a number of newer hypotheses fail to resolve the issue either. Lang bases his hypothesis on the morphological similarities between ctenophores and turbellarians. The latter, much like certain flat ctenophores, feature a flattened body covered with ciliated epithelium, a nervous system shaped as a net of Nerve Cells or gathered into nerve cords, and a gut lacking an anus. In some turbellarians, alongside bilateral Symmetry, certain radial symmetry traits are preserved in the arrangement of individual organs: the central position of the mouth on the ventral side, the radial divergence of gut branches from the pharynx, and the Location OF THE nerve ganglion nearly above the pharynx with nerve cords radiating from it.

Roundworms and spiny-headed worms (acanthocephalans) are closely related to flatworms. The phylum Nemathelminthes includes the class Gastrotricha, representatives of which—both in the adult state and during embryonic development—share a number of structural similarities with ciliated flatworms (such as the presence of ciliated epithelium on the ventral side, typical protonephridia, etc.). This provides a basis for grouping roundworms closely with flatworms.

Coelomate animals (coelomates)—fully justified in being called higher multicellular animals—branched off from the ancestors of modern pseudocoelomates. This is justified by such organizational features as the concentration of nerve cells into ganglia (the Progressive development of the nervous system), the formation of a vascular Circulatory system, metanephridial-type excretory organs, and body segmentation.

One branch of coelomate animals is made up of organisms that retain a primary mouth in their adult state (protostomes). Earlier than other groups, the branch leading to THE PHYLUM MOLLUSCA split off from the primary coelomates. Despite the fact that Mollusks feature an unsegmented body at all Stages of Ontogeny, their affinity with other coelenterates remains unquestionable. Both groups possess nerve ganglia, a circulatory system, and metanephridial excretory organs. Such profound organizational similarity cannot be coincidental. Furthermore, the veliger larva—typical of marine mollusks—bears a resemblance to the trochophore larva of polychaete Annelids.

Among segmented coelomate animals, the ancestral group should be considered the ancient primary polychaetes, which are close to the phylum Annelida. They gave rise to oligochaetes, from which leeches subsequently evolved.

A comparison of annelids and Arthropods reveals a striking structural similarity: both possess a secondary body cavity (coelom), metamerism, a nervous system consisting of supraesophageal ganglia, a circumesophageal ring, and a ventral nerve cord, a circulatory system with a main vessel on the dorsal side, and other shared features. There is also a segmental arrangement of appendages, which in polychaete annelids appear as unjointed parapodia, whereas in arthropods, alongside the acquisition of a hard cuticle, they transformed into jointed legs. The shared Structural Features of annelids and arthropods point to a close phylogenetic relationship between the two. Arthropods represent a progressive branch originating from a common ancestor shared with modern polychaete annelids.

Among arthropods, crustaceans continue to inhabit the aquatic environments where their ancestors lived, having retained many primitive structural traits that link them to annelids.

Chelicerates share many similarities with crustaceans and descend from ancient branchiate arthropods. Tracheate arthropods, among which insects are the most progressive group, are also closely related to their ancestral annelids—the annelids.

Another branch of the animal kingdom's genealogical tree leads from ancient coelomate worm-like animals to deuterostomes—echinoderms and chordates.

Echinoderms are an ancient group of animals that secondarily acquired radial body symmetry. This symmetry emerged in Paleozoic representatives of this phylum in connection with a transition to a sessile lifestyle. Echinoderm larvae possess a bilaterally symmetrical body.

Lower chordates are linked to echinoderms by the secondary Formation of the mouth during ontogeny, similarities in the formation of the secondary body cavity (coelom), the two-layered structure of their integument, and a number of other traits.

Chordates represent one of the most highly organized groups in the animal kingdom. The exact origins of chordates remain obscured in details. They may have evolved from a common root shared with echinoderms.

As research by A. N. Severtsov demonstrated, primary chordates were creatures quite similar to modern amphioxus. These were coelomate, deuterostome animals exhibiting signs of internal segmentation, possessing a notochord, a dorsal nerve tube, and gill slits in the pharyngeal walls. These primitive primary chordates gave rise, on the one hand, to modern Acrania (amphioxus and allied forms) and, on the other hand, to craniates (vertebrates), in which the anterior portion of the Spinal Cord transformed into a brain protected by a newly formed Skull. Urochordates form a lateral branch of lower chordates; the organization of adult urochordates underwent regression linked to a transition to a sessile lifestyle combined with passive feeding.

In vertebrate evolution, one branch led to the emergence of jawless fish (lampreys and hagfishes), while another gave rise to Gnathostomes, which encompass all other classes of Vertebrata. Driven by active foraging and a general surge in behavioral activity, animals of this branch developed paired limbs, a jaw apparatus, a complex Skeleton, and various other structural complexities. All of this ensured their development and widespread distribution. The earliest representatives of gnathostome vertebrates were fish, primarily cartilaginous fish (sharks and rays). However, by the end of the Paleozoic era, various Fishes with bony or cartilaginous-bony skeletons became widespread. Lungfishes and lobe-finned fishes trace their ancestry back to ancient Paleozoic fish. The latter must be considered the progenitors of terrestrial vertebrates, the oldest of which were stegocephalians—labyrinthodont amphibians with armored heads. Both modern amphibians and the first Paleozoic reptiles, which achieved vast morphological diversity during the Mesozoic era, evolved from ancient primitive amphibians.

During the Mesozoic, two higher classes of vertebrates branched off from reptiles: Mammals and Birds. As previously mentioned, remains of peculiar creatures known as Archaeopteryx have been discovered in Jurassic deposits, combining the traits of both reptiles and birds. This represents a lateral branch in avian evolution. True birds emerged during the Cretaceous period and achieved immense diversity during the Paleogene and Neogene periods of the Cenozoic era.

Mammals evolved from ancient reptiles at the very beginning of the Mesozoic era—that is, earlier than birds. However, the evolutionary radiation leading to the modern richness of forms within this vertebrate class dates, much like that of birds, to the Cenozoic era.