Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and archaeocyatha appear ; at the first appearance of a complex feeding burrow called Trichophycus pedum ; or at the first appearance of a group of small , generally disarticulated , armored forms termed the small shelly fauna . Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and archaeocyatha appear ; at the first appearance of a complex feeding burrow called Treptichnus pedum ; or at the first appearance of a group of small , generally disarticulated , armored forms termed ` the small shelly fauna ' .