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ECHINODERMATA
Origins: 538 million years ago (early Cambrian period, Fortunian stage)
Extinction: Still extant
Echinoderms are members of the phyllum Echinodermata. They originated at the cusp of the Cambrian explosion, some 538 million years ago. They are well known for their derived regenerative capabilities, and therefore have been the animals most studied in that area, because of that. Echinoderms have been diversifying plentifully across their entire history, though the more recent diversity probably arose as a response to the Mesozoic marine revolution (MMR), which followed the evolution of many durophagous predators leading echinoderms to develop strategies to evade them, radiating further.
Echinoderms are characterized for a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles and connected by a mesh of collagen fibres. The current phylogenetic understanding of echinoderms places crinoids as the most basal ones, with then the sea urchins being related with the sea cucumbers, and the brittle stars being related with the starfish. There are about 7000 extant species of echinoderms, but up to about 13 000 extinct species known. All echinoderms are marine, ranging from intertidal regions to abyssal depths. The first echinoderms attached themselves to the ocean floor while changing to their adult form, becoming sessile throughout their lives. Eventually echinoderms evolved endoskeletal plates with stereom structure and external cilliary grooves for feeding. The first echinoderms would have had their mouth oriented upwards, using their podia to catch food, a feature still seen in crinoids.
Echinoderms evolved from a bilateral ancestor. During metamorphosis, the echinoderm larvae (which are bilateral) start to develop the left size of their body into the pentaradial shape typical of adults, though there are a few species that break that rule. Most notably, some starfish actually have six arms, breaking the pentaradial symmetry. Other echinoderms may end up being symmetrical in other fashions, including up to 50-200 arms. They have a mesodermal skeleton, and so they have a sponge-like porous structure known as the stereom. Because of this skeletal feature, echinoderm material is often incredibly abundant in the fossil record. Their complex water vascular system typically opens to the exterior through a sieve-like madreporite on the upper surface of the animal, being linked to a slender duct, the stone canal, which then extends to a ring canal that encircles the mouth and the oesophagus. These eventually connect to the tube feet, which can differ depending on the echinoderm group; in crinoids the tube feet lack suckers and are used in a back-and-forth wafting motion to pass the food particles captured by the arms into the mouth. Echinoderms have a simple digestive system, that can differ depending on the species. Some starfish can eject their stomach to digest the food on the outside of their body. Sea cucumbers are detritivores that use large buccal tentacles to sift through the sediment. These have a simple gut, a long coiled intestine, and ends in a cloaca. Crinoids are suspension feeders, catching plankton in the current with their arms. The coelom of echinoderms is complex, and during development it develops into the metacoel, mesocoel and protocoel. They don't have a true heart, and the blood usually lacks any respiratory pigment. Gaseous exchanges occur in many ways, including through their tube feet. They lack specialized excretory organs, so waste usually diffuses out through the respiratory surfaces. Their immune system is composed of immune cells called coelomocytes, that are contained in the coelomic fluid. The coelomocytes are amoebic and phagocytic, engulfing alien particles and infected cells. These cells are large and granular and contribute to an efficient pathogen defense system. The nervous system of echinoderms varies between species, with, for example, starfish having sensory cells in the epithelium, simple eyespots and touch-sensitive tentacle-like tube feet at the tip of their arms. The ancestors of echinoderms likely had just one genital aperture, but since some have evolved multiple gonopores through which the gametes can be released.
Echinoderms are capable of regenerating lost parts, and there are two different methods, epimorphosis and morphallaxis, with the former involving stem cells forming a blastema, generating new tissues, and the latter involves rearranging existing tissues to replace the lost part. Transdifferentiation of one type of tissue to another during tissue replacement can be observed.
Depending on the species, echinoderms usually become sexually mature at around two or three years of age. Some species may aggregate in large numbers during the breeding season, to increase the chances of successful fertilisation, as the gametes are released in the open water. Even in abyssal depths, echinoderms can synchronize their reproductive activity, despite the lack of sunlight. Although they're capable of sexual reproduction, sometimes they can also attain assexual reproduction, with cases observed of parthenogenesis being applied too. The larvae of some echinoderms are capable of assexual reproduction. Parts that are autotomised (lost from their original body) can develop into fully formed larvae. The larvae of some sand dollars can clone themselves when they detect fish mucus, indicating that predators are nearby, triggering their need to replicate. Echinoderms are deuterostomes, so larvae start out by developing the anus first and then the mouth, as is typical of these types of animals, and the mesoderm migrates inwards, before it develops into the echinoderm skeleton. The secondary body cavity, the coelom, forms by the partitioning of the three body cavities. Echinoderm larvae are often planktonic, but some species retain the eggs inside the female, and some may even brood the larvae. The names of echinoderm larvae are different for each taxonomic group, for example, sea urchin larvae are called "echinopluteus" (in reference to the Echinoidea group, which includes the sea urchins) and brittle star larvae are called "ophiopluteus" (Ophiuroidea is the clade that includes brittle stars).
Motile echinoderms usually move with their tube feet, sometimes being rather sticky, using a mucus to enhance adhesion. Brittle stars have more slender arms, and use the arms as a whole to move around, making them the most agile of echinoderms. Crinoids can be motile but others tend to be sessile. Their diet is very variable, ranging from the herbivorous sea urchins, the detritivorous sea cucumbers and the carnivorous and predatory starfish. Basket stars, types of brittle stars, are suspension feeders, like crinoids, while other brittle stars consume other foods. Many brittle stars are scavengers or detritivores. Sea urchins tend to eat flora, but some can eat smaller animals, capturing them with their tube feet. Star fish have flexible arms, using their tube feet to capture their prey and then engulfing it by arching its back and going over its meal. The starfish then inserts the stomach into its prey and excretes digestive enzymes to slowly digest its meal. Although echinoderms also developed plenty of anti-predator defenses, in response to prehistoric durophages, crinoids are an exception for having little to no predatory preoccupations, having remained as they are for the longest time. Many echinoderms can have spines for defense, sometimes even injecting toxins to whatever is stung by them. Sea cucumbers can rupture their body and discharge their gut and internal organs outside as a defense mechanism. Some starfish can even swim away from danger.
Echinoderms are important regulators of the carbon cycle of the ocean, sequestring about 0.1 gigatonnes of carbone dioxide per year as calcium carbonate for their skeletons. In the present, echinoderms can however become a plague, as the disappearance of its predators causes overgrowing populations of these invertebrates, creating damage to local ecosystems.
Humans harvest over 100 thousand tonnes of echinoderms every year. They can be eaten, with sea cucumbers being frequent delicacies. The gonads of sea urchins are also consumed, with their taste being described as quite a pleasant soft mixture of seafood and fruit. Humans also use echinoderms for the purpose of studying their regenerative powers, in the hopes of treating our own neurodegenerative diseases. The skeletons of echinoderms are also quite an object of fascination for humans, for their incredible symmetry, providing us great aspirations.
main source: Wikipedia
Phylogeny
Echinodermata
- Praetetracrinus kutscheri
- Pentacrinites dichotomus
- Seirocrinus subangularis
NAME: Praetetracrinus kutscheri
SIZE: Variable
DESCRIBED BY: Jäger, 1995
CLOSEST LIVING RELATIVES: Cyrtocrinid sea lilies (potentially)
DEPICTED IN: The Life of a Temnodontosaurus episodes 2 and 3
Praetetracrinus kutscheri was a typical species of crinoid. It was a suspension feeder, extending its arms in the water column to catch floating debris and organic matter, such as plankton or marine snow. It did not have cirri, which is used by other crinoids for better holding onto the sediment and even for walking. Instead the expanded base of the stem attaches directly to the substrate. The stem is composed of a column of highly porous ossicles, which are connected by ligamentary tissue.
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