Archaeocyaths are an extinct group of sponges that have a controversial history.
Discoveries in the 1960s and 1970s caused re-examination of sponge phylogeny generally,
and comparisons between archaeocyaths and sponges in particular. The result was
the abandonment of the Phylum Archaeocyatha. Present consensus is that archaeocyaths
represent both a clade and a grade--Class Archaeocyatha and the archaeocyathan
morphological grade--within Phylum Porifera. The first archaeocyaths appear roughly
530 million years ago, during the Lower Cambrian and were very important members
of Lower Cambrian environments. They diversified into hundreds of species during
this time period and some of these species contributed greatly to the creation
of the reef systems that supported a wide variety of organisms. Despite their
great success in terms of numbers, the archaeocyaths were a short-lived group.
They were almost completely non-existent by the middle Cambrian, some 10 to 15
million years after their first appearance.
Chordata comprises those animals most often occupying the top of the food chain,
including the fishes, reptiles(included the extinct dinosaurs), reptiles, birds
and mammals (including humans). The Chordates primary common feature is a notochord,
a rod that extending most of the length of the organism. Lying dorsal to the gut
but ventral to the central nervous system, it stiffens the body and acts as support
during locomotion. More specifically, the notochordat should be present at least
during some part of the organisms development. Other common morphologies include
bilateral symmetry, segmented body, a well-developed coelom, a single, dorsal,
hollow nerve cord, usually with an enlarged anterior end (i.e., brain), a tail
projecting beyond (posterior to) the anus at some stage of development, ventral
heart, with dorsal and ventral blood vessels and a closed blood system, a complete
digestive system, and usually, a bony or cartilaginous endoskeleton.
first multi-celled animals (metazoa) evolved over 600 million years ago
and can be grouped into three categories: sponge-like animals, cnidarians, and
worms. The sponges, and cnidarians (corals and sea anemones), are the most primitive
with about 11 specialized cell types. Worms and higher metazoa have approximately
55 specialized cells.
Sponges are simple multi-celled animals. In general, sponges have open-topped,
sack-like bodies which are fixed to the sea floor. Water is pulled through the
body, and food is filtered out.
2) Cnidarians comprise corals, sea anemones, and jellyfish.Their body plan
resembles a sack with an end that functions as a mouth that can be opened and
closed with tentacles that can be extended to move food to the mouth.
3) Among the simple metazoa, worms are the most advanced, and it is conjectures
that their body was the basis for the evolution of all more advanced life forms.
Worms have a fluid-filled cavity called a coelom inside the body and advanced
variations of this cavity can be seen in all higher animals. In many animals,
it has become the sack which holds the internal organs.Insects
earliest known insects are tiny wingless forms from the early and middle Devonian.
Insect flight developed with suddenness resembling the Cambrian explosion during
the middle Carboniferous, apparently the result of the significant survival advantage
that was accrued. By the end of the Carboniferous, the insects had evolved into
a large number of distinct orders, including many dragonfly-like forms. This was
the hey-day of the
the Permian, new insects
forms appeared. Blattoid and Orthopteroid orders attained
their greatest diversity, and new groups like the Psocoptera, Homopteran, Hemiptera,
Mecoptera and Coleoptera became ubiquitous and diverse.
Permian extinction wiped out nine orders of insects, and more orders disappeared
in the Triassic or the early Jurassic. However, surviving orders such as Neuroptera,
Mecoptera, and Diptera, and Coleoptera underwent further radiation establishing
many families extant in modern times.
From the Upper Cretaceous, insects have
largely been of modern type. This is illustrated by the many recent families and
genera that have been found preserved in Baltic amber and elsewhere.
the orders recorded with certainty for the first time from Tertiary beds, the
Strepsiptera and Siphonaptera may have been relatively late developments, but
the others were probably more ancient.
of the sure signs that a fossil site is a Lagerstatt is when insects are well
preserved. Insect preservation generally requires rapid sedimentary coverage before
decay and predation occur. Liaoning is notes for its excellent insect preservation.
are layered mounds, columns, and sheets found in the rock. They were originally
built by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing
microbe growing on a sea floor. Cyanobacteria are prokaryotic cells (the simplest
form of modern carbon-based life) that lack a DNA-containing nucleus. This simple
organism was the only life on Earth for some 2 billion years, and dominated life
on earth between 1 and 2 billion years ago. Though uncommon, extant stromatolites
are found in a few places such as Shark Bay, Australia; the salinity of the water
is very high in this bay, such that the only life that can survive is cyanobacteria.
structures are formed when the bacteria precipitate or trap and bind layers of
sediment to make accretionary formations that present in different forms, including
domes, cones and complex branching configurations. Structures sizes are highly
variable from twig size to Mack truck size. Especially when polished, stromatolite
can be most beautiful.
interesting controversies persist in the origins of stromatolites.
the fossil remains of extinct life on Earth, the trilobites are the most ubiquitous.
In terms of popularity, they are probably second only to the dinosaurs. Trilobites
were marine invertebrates that emerged in the Cambrian Explosion, a time that
superimposed abundant oxygen on earth with the evolutionary emergence of cellular
mechanisms to use oxygen for energy (see related article of cell biology). Diversity
of life exploded and trilobites shared in the chain reaction. Trilobites rapidly
evolved functional complexity and, particularly, sophisticated vision systems.
Size, a relatively easy adaptation in evolution, also exploded. The entire Paleozoic
Era is sometimes called the age of trilobites and for these 300 million years,
trilobite evolution resulted in an estimated 15,000 different species exhibiting
enormous diversity of forms most wonderful and beautiful. They vary in size from
a few millimeters to a couple of feet. Trilobites have been called the "Butterflies
of the Seas", a name we find most hard to dispute.
are members of the
which includes modern-day insects, scorpions, centipedes, shrimps,
lobsters and crabs, and other jointed-legged animals. They lived
throughout the crucial period just before the modern arthropod
groups are known to have appeared. It is sound conjecture that
trilobites descended from a single ancestor. But, by the time
trilobites appeared in the Lower Cambrian
fossil record, they were already a highly
diverse and advanced sub-phylum of animals. Interestingly, Darwin
thought trilobites most closely related to Crustacea, and the
famous Walcott thought it was an arachnid. Setting aside one hypothesis
in favor of the other yet alludes science.
trilobite was encased in an exoskeleton composed of calcite, a hard mineral, just
like a modern-day clamshell. As an evolutionary adaptation that emerged in the
Cambrian, exoskeletons served as a means of containment and support for internal
organs. In fact, the appearance of exoskeletons in the Cambrian Explosion was
undoubtedly a critical evolutionary innovation. In short, this support system
was a prerequisite for an explosion of trilobite size. Natural selection would
also have favored a development of harder and thicker shells due to the protection
they provided. The trilobite evolved to be a veritable armored tank on the topside,
but on the underside it was soft and vulnerable, and no currently living animals
have such a configuration.
the trilobite is divided into three segments, the cephalon (or head), the thorax
(middle) and pygidium (or tail). It may not surprise you that the organs contained
in these segments correspond well to those of human head, middle and tail. The
middle segment of the trilobite was further divided into a number of "thoracic
segments", each connected fore and aft to adjacent segments by weaker tissue.
These linked thoracic segments rendered the trilobite flexible, and hence more
mobile, but also made them more vulnerable to predation.
particularly flourished in the oceans of the Cambrian and Ordovician periods,
beginning around 540 million years ago, with a diminishing of families persisting
until the Permian. The number of families actually peaked in the Late Cambrian
when an extinction event removed many. The morphological diversity actually peaked
in the Ordovician. Many more families were removed at the end of the Ordovician
440 million years ago during a great ice age where ice sheets advanced to the
equator. The diminished number of trilobite families that survived to the Silurian
radiated into new and exotic forms, and still more exotic spiny and pustulose
forms in the Devonian. The Devonian was punctuated by periods of rising seas that
disrupted the reef systems where the trilobites flourished forcing selective adaptation.
The end of the Devonian saw the Frasnian-Famennian event where only Proteus survived
into the Carboniferous. Despite reduced ancestry, with decent with modification
ruling, these trilobites filled the same ecological niches repeated the forms
of their extinguished relatives. The genetic path was assuredly different, but
the evolved forms had discernable specificity with those long extinct. However,
trilobites never truly recovered in the Carboniferous, with but a handful of genera
extant by the Permian. Failing to adapt to deep-water habitats, their vulnerability
to climatic change remained and led to their disappearance prior to yet another
great mass extinction at the end of the Permian. The age of the trilobite yielded
to the age of the
are animals with backbones that comprise one subphylum of the phylum
remainder of Chordata are the invertebrates that do not have backbones. Vertebrates
encompass many of the familiar animals on Earth: birds; fishes; frogs; reptiles;
snakes; and mammals, including we Homo sapiens. The vertebrate fossil record dates
back some 500 million years ago to the upper Cambrian. Vertebrates first evolved
in the ocean. The invertebrate ancestors of vertebrates had gill slits that were
primarily used for filter feeding, with oxygen uptake being through their skin.
These early vertebrates lacked jaws, as does extant hagfish and lampreys. Jawed
vertebrates appeared 100 million years later in Silurian time. In terms of number
of species, vertebrates are a tiny per cent of the living creatures.