graptolite
Concise Oxford English Dictionary © 2008 Oxford University Press:
graptolite /ˈgraptəlʌɪt/
▶noun a fossil planktonic invertebrate animal, believed to be related to the pterobranchs.
– origin C19: from Gk graptos ‘marked with letters’ + -lite: so named because impressions left on hard shales resemble slate pencil marks.
'graptolite' also found in these Oxford entries:

