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

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 studies provide convincing evidence that animals originate from a common ROOT, demonstrating the monophyletic Evolution of the entire animal kingdom. Consequently, the Phylogenetic relationships among animals of various taxonomic groups can be represented as a branching tree, rooted in primitive unicellular animals (Fig. 82).

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

1) comparative Study of the Morphology of extant animals (comparative morphology);

2) investigation of their Embryology, also from a comparative perspective (comparative embryology);

3) examination of the fossil remains of animals from past geological epochs (comparative paleontology).

The comparative morphological method of studying animals reveals varying degrees of structural complexity and uncovers specific Similarities and differences, helping to form concepts of possible transitional forms and hypothetical ancestors of a given animal group.

For instance, it has been established that flat ctenophores—animals closely related to Coelenterates—simultaneously share A number of features with turbellarians belonging to the flatworm phylum, combining the characteristics of radially symmetric diploblastic animals with those of bilaterally symmetric triploblastic animals. Such similarities suggest a phylogenetic link between Flatworms and coelenterates.

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Fig. 82 Phylogenetic Tree of the animal kingdom (diagram)

The application of comparative embryology data is based on the biogenetic law (rule), according to which the past does not vanish without a trace in animal evolution, and animals in their individual development repeat to varying degrees the stages of their historical development—ontogeny recapitulates phylogeny. This law was formulated by the renowned German Darwinist E. Haeckel, drawing upon extensive research by luminaries of Russian 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 serve as an additional argument when resolving questions of phylogenetic relationships.

The paleontological approach to studying animal phylogeny also provides compelling evidence for reconstructing the Lineage of those animals whose remains have been preserved in the earth's 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 entities of a significantly simpler, non-Cellular Organization. The earliest organisms possessed The ability to synthesize living bodily matter through the assimilation of non-biological inorganic and organic substances from their environment. These gave rise to unicellular animals, possibly of amoeboid Structure.

Among flagellates, organisms emerged capable of building their bodies from Inorganic Compounds using solar energy via Photosynthesis. Green flagellates, combining animal and plant features 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 retained by the majority. Ancient amoeboid organisms gave rise to modern sarcodines and parasitic cnidosporidians. One branch of the phylogenetic tree leads from flagellates to Ciliates—the most complexly structured animals—while another leads to parasitic Sporozoans, although there is no consensus regarding THE ORIGIN OF sporozoans and cnidosporidians.

The evolution of the unicellular animal world led, via colonial flagellates, to Multicellular animals. Flagellates exhibit the greatest diversity among Protozoa, along with complex adaptations for Nutrition and survival in diverse environments. They include simple and complex colonial organisms that gave rise to two branches of modern multicellular animals: Sponges and all other multicellular forms, via primary diploblastic ancestors.

Several hypotheses exist regarding the origin of multicellularity, all sharing the consensus that Multicellular Organisms evolved from colonial flagellates. Sponges originated from choanoflagellates; their sessile lifestyle did not favor progressive development, causing them to retain many primitive structural traits. Other colonial flagellates gave rise to the diploblastic ancestors of coelenterates. Most of these transitioned to a sessile or sluggish existence, as seen in modern coelenterates.

A comparative study of diploblastic coelenterates leads to insights into the possible 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 Developmental Stages of the third germ layer—the mesoderm. The rudiments of the mesoderm found in coelenterates are even more pronounced in ctenophores, animals closely related to coelenterates in terms of origin and organization.

The question of flatworm origins remains a subject of debate. Lang's hypothesis regarding their phylogenetic affinity with crawling flat ctenophores has been challenged, yet a number of newer hypotheses fail to resolve the issue either. Lang based his hypothesis on the morphological similarities between ctenophores and turbellarians. Like certain flat ctenophores, the latter possess a flattened body covered with ciliated epithelium, a Nervous system taking the form of a nerve Cell net or aggregated nerve cords, and a gut lacking an anus. In some turbellarians, alongside bilateral Symmetry, traces of radial symmetry persist in the arrangement of certain 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 cerebral ganglion positioned nearly above the pharynx with nerve cords radiating from it.

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

Coelomate animals (coelomata), quite justifiably termed higher multicellular animals, branched off from the ancestors of modern pseudocoelomates. This Classification is supported by organizational features such as the concentration of Nerve Cells into ganglia—reflecting the progressive evolution of The Nervous System—along with The Development of a vascular Circulatory system, metanehridial excretory organs, and body segmentation.

One branch of coelomate animals comprises organisms that retain a primary mouth in the adult stage (protostomes). The branch leading to The Emergence of the mollusk phylum diverged from ancestral coelomates earlier than the others. Despite the fact that various Mollusks maintain an unsegmented body throughout all Stages of Ontogeny, their affinity with other coelomates is unquestionable. Both groups possess nerve ganglia, a circulatory system, and metanephridial excretory organs. Such profound organizational similarities cannot be coincidental. Furthermore, the veliger larva—typical of marine mollusks—resembles the trochophore larva of polychaete Annelids.

Among segmented coelomates, the ancestral group is considered to be ancient primary polychaetes, closely related to the phylum Annelida. These gave rise to oligochaetes, from which leeches subsequently evolved.

A comparison of annelids and Arthropods reveals striking structural similarities: both possess a secondary body cavity (coelom), metamerism, a nervous system consisting of supraesophageal ganglia, a circumesophageal ring, and a ventral nerve cord, as well as a circulatory system with a dorsal main vessel, among other shared features. This includes the segmental arrangement of appendages, which appear as unjointed parapodia in polychaetes, whereas in arthropods, alongside the acquisition of a hard exoskeleton, they transformed into jointed legs. These shared structural traits indicate a close affinity between annelids and arthropods. Arthropods represent a progressive branch originating from a common ancestor shared with modern polychaete annelids.

Among arthropods, crustaceans continue to inhabit aquatic environments—where their ancestors also lived—and have retained numerous primitive structural features that link them to annelids.

Arachnids share many similarities with crustaceans, originating from ancient branchiate arthropods. The tracheate arthropods—among which insects represent the most progressive group—are likewise related to ancestral annelids.

Another branch of the animal kingdom's evolutionary tree leads from ancient coelomate worm-like animals to deuterostomes, namely Echinoderms and chordates.

Echinoderms are an ancient group of animals that secondarily acquired radial body symmetry. This symmetry was present in paleozoic representatives of this phylum due to their transition to a sessile lifestyle. Echinoderm larvae possess bilaterally symmetrical bodies.

Lower chordates are linked to echinoderms by the secondary Formation of the mouth during ontogeny, similarities in coelom formation, a two-layered integumentary structure, and several other features.

Chordates represent one of the most highly organized groups in the animal kingdom. The exact origins of chordates remain somewhat unclear in detail; they may have shared a common evolutionary root with echinoderms.

As research by A. N. Severtsov demonstrated, early chordates were creatures quite similar to modern amphioxus. These were coelomate, deuterostome organisms exhibiting internal segmentation, possessing a notochord, a dorsal nerve tube, and pharyngeal gill slits. These primitive early chordates gave rise, on the one hand, to modern Acrania (amphioxus and closely related forms), and on the other, to craniates (vertebrates), in which the anterior portion of the Spinal Cord evolved into a Brain protected by a newly formed Skull. A lateral branch of lower chordates is formed by tunicates (urochordates), whose adult forms underwent anatomical regression associated with a transition to a sessile lifestyle and passive feeding.

In vertebrate evolution, one lineage led to the emergence of jawless fish (lampreys and hagfish), while another gave rise to Gnathostomes, encompassing all other classes of Vertebrata. Driven by active foraging and a general increase in behavioral activity, the animals in this lineage developed paired appendages, a jaw apparatus, a complex Skeleton, and various other structural advancements. All of these factors ensured their successful development and widespread distribution. The earliest representatives of Jawless vertebrates were fish, primarily cartilaginous fish (sharks and rays). However, by the end of the Paleozoic era, various fish with bony or cartilaginous-bony skeletons became widespread. Lungfish and lobe-finned fish trace their ancestry back to ancient Paleozoic fish. The latter are considered the ancestors of terrestrial vertebrates, the oldest of which were the armored amphibians known as stegocephalians. Both modern amphibians and the first Paleozoic reptiles—which achieved a vast diversity of forms during the Mesozoic era—originated from these ancient, primitive amphibians.

During the Mesozoic era, the two higher vertebrate classes—Mammals and Birds—branched off from reptiles. As previously mentioned, fossil remains of peculiar creatures known as Archaeopteryx have been discovered in Jurassic deposits, combining features of both reptiles and birds. These represent a side 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, predating birds. However, the evolutionary radiation that led to the modern Abundance of forms within this vertebrate class dates back to the Cenozoic era, much like birds.