there is a limit to individual cell size, hence larger organisms need to be multi-cellular with cells grouped together and usually with specialisation of cells (eg. structural support, digesting food and transporting substances such as oxygen and CO₂):
bigger cells tend to have less surface area per unit of volume which means that the natural movement (diffusion) of molecules of gases, nutrients and wastes in and out of the cell isn't enough to keep things running without a transport system and these molecules also have further to travel in larger cells
the alternative is to become flat or threadlike (like horsehair worms) or thin and flat (such as flatworms) -these animals don't need an internal transport system because none of their cells (or their contents) are far from the surrounding air or water
fossils from 518mya at Chengjiang formation in China
a massive fossil find in 518mya old rocks on the bank of the Danshui river in Hubei province in southern China in 2019 has unearthed 4,351 separate fossils so far represent 101 species, 53 of them new to science, where well preserved primitive forms of jellyfish, sponges, algae, anemones, worms and arthropods with thin whip-like feelers, including 4cm long mud dragons and numerous comb jellies were entombed in an ancient underwater mudslide that swept them into deeper, colder water where they were buried in fine sediment halting the usual process of decay
fossils from the Burgess Shale, a 508m-year-old rock formation in Canada
by now, a gene duplication event (400-1000mya and before vertebrates had evolved) had created the two growth hormone releasing factor peptide super-families from a common ancestor
14):
the insulin and insulin-like growth factor (IGF) signaling pathway is highly conserved among the metazoans - many invertebrates have large numbers of insulin-like peptides (ILPs)
15)some vertebrate hormones may have had a role in neural tissues in invertebrates
16)the ancestral steroid receptor was likely present before the separation between protostomes and deuterostome, however, the oestrogen receptor may have been secondarily lost in taxa such as arthropod and nematode protostomes (Maglich et al. 2001), or the urochordate and echinoderm deuterostomes
adrenal and sex steroid receptors are not found in echinoderms and hemichordates
17)the common octopus, Octopus vulgaris, is the first invertebrate species shown to possess representatives of three classes of sex-steroids found in vertebrates (progestins, androgens and estrogens) as well as binding proteins for these steroids
receptors for vertebrate oestrogens and 3-ketosteroids first appeared in basal chordates (cephalochordates: amphioxus)
an ancestral progesterone receptor and an ancestral corticoid receptor, the common ancestor of the glucocorticoid and mineralocorticoid receptors, evolved in jawless vertebrates (cyclostomes: lampreys, hagfish)
evolution of an androgen receptor and distinct glucocorticoid and mineralocorticoid receptors arose in cartilaginous fishes (gnathostomes: sharks)
18)all animal species lines started out as small animals, some species evolved to become larger in size as this may make it easier to evade predators (elephants and whales have few enemies other than humans), hunt prey, out-compete rivals, better at conserving heat (because of their relatively smaller surface area) greater potential for intelligence, and have larger energy stores to endure temporary hardships but they are very specialized, reproduce more slowly and evolve more slowly so are unable to survive major environmental upheavals.
global mass extinction event of 65-66mya at the end of the Cretaceous Period when 75% of animal species died out including all the dinosaurs and anything larger than a domestic cat - this is now confirmed to be due to an asteroid hitting Mexico's Yucatan Peninsula where it created a 200km crater and it is thought the global winter event due to 2,000 gigatonnes of dust (11x weight of Mt Everest) being expelled into the atmosphere was the eventual cause of the mass extinction.