what is the second mentioned?
They are a group of elasmobranch fish
Shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. 
what group of the periodic table is potassium in Alkali metals what is this metal? They are all shiny, soft, highly reactive metals at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1 what is it on the planet earth? The alkali metals, due to their high reactivity, do not occur naturally in pure form in nature what is its compound? NaCl, NaO, NaS, NaP, NaAs, NaSb, NaBi where is it applied? Lithium is often used in lithium-ion batteries, and lithium oxide can help process silica how is this metal produced and isolated? It is produced by thermal decomposition of lithium peroxide at 300-400°C what are the derivatives and ammonium? The ammonium ion has very similar properties to the heavier alkali metals, acting as an alkali metal intermediate between potassium and rubidium, and is often considered a close relative what is this above mentioned ion? It is a positively charged polyatomic ion with the chemical formula what are its properties based on acid? It is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule what is the biological trend behind this? They are a waste product of the metabolism of animals metabolism of which animals? Mammals, sharks, and amphibians
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Shark [SEP] Introduction
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as "Cladoselache" and "Xenacanthus", as well as other Chondrichthyes such as the holocephalid eugenedontidans. Under this broader definition, the earliest known sharks date back to more than 420 million years ago.