The novel (and subsequently movie) Jurassic Park featured a dinosaur called Dilophosaurus, that was purported to be venomous and had an ability similar to that of the extant spitting cobra. Unfortunately, the whole setup isn't real.
This got me wondering though. Modern venomous snakes, to give a particular example, ought to have had ancestors that already had appropriately modified salivary glands, and similarly for most other extant venomous species. (I suppose I'll restrict myself to vertebrates so as not to be too broad.)
I now ask: have fossils of animals that are likely to have been venomous ever been discovered?
Extant venomous snakes do have venomous ancestors. Fry et al. (2006) reported on finding venom toxins more broadly within Reptilia, beyond the well-known venomous snakes and the helodermatid lizards. They show that varanid and iguanid lizards also have venom toxins.
The same group of authors (Fry et al., 2009) then reported on the use of venom in the extant varanid lizard, Varanus komodoensis (Komodo monitor). This is a different hypothesis than the usual "toxic bacteria" hypothesis for predation in these animals. They then reinterpret the skull of the giant (~5.5 m) varanid Varanus (Megalania) priscus. They conclude, based on anatomical similarity and the close phylogenetic relationship between it and V. komodoensis, that V. priscus' predatory mode was more similar to other varanid lizards, likely involving venom delivery.
Discovery of a venomous clade within squamates, including snakes and several clades of lizards, does not imply venom more broadly within Reptilia. Specifically, it would not parsimoniously reconstruct the presence of venom in Archosauria, the clade that includes dinosaurs. However, Gong et al. (2009) interpret the teeth of the dromaeosaurid theropod dinosaur, Sinornithosaurus as having venom grooves. These grooves are similar to grooves in the teeth of Uatchitodon, a Triassic squamate from Arizona.
There is currently no solid evidence that archosaurs could spit, nor is there ever likely to be.
Venomous extant mammals include the platypus (Ornithorhynchus), Solenodon, and some species of shrews. The venom gland in the platypus is a modified sweat gland on the hind limb, which includes an ossified spur. A similar structure has also been found in the basal mammalian taxa Gobiconodon and Zhangheotherium (Hurum et al., 2006). Hurum et al. (2006) described a similar structure in the multituberculates. As the most basal clade of Mammalia, the authors conclude that an ossified spur is apomorphic for Mammalia and that the ancestral mammal may have used it for venom delivery. This structure would have been retained more-or-less unmodified in platypus and lost in other mammalian lineages. The authors' conclusions are supported by the presences of a non-venomous spur in the other clade of monotremes, the echidnas.
Fox and Scott (2005) report that the Palaeocene (~60 Ma) pantolestid Bisonalveus browni also possessed teeth with venom delivery grooves.
Szaniawski, 2009 used similar logic based on grooved "tooth" elements to conclude that some extinct jawless chordate conodonts may also have been venomous.