Achille De Zigno was not only a famous palaeobotanist; he also devoted his research to fossil vertebrates, mainly those from various stratigraphic intervals of Veneto. Particularly, he focused his research activity on Triassic fish and Palaeogene fish, like those from the Eocene of Mount Bolca, which had been renowned for centuries. Considering the quantity and the quality of his studies on this subject, Achille De Zigno can be considered one of the most important palaeoichthyologists of the 19th century, together with Oronzo Gabriele Costa (1789-1867), Gaetano Giorgio Gemmellaro (1832-1904), Francesco Bassani (1853-1916).

De Zigno was the first Venetian palaeontologist to complete a synthesis work on fossil vertebrates (fish and reptiles) found in the Mesozoic succession in Veneto. Reference: De Zigno, A. (1883). Sui vertebrati fossili dei terreni mesozoici delle Alpi Venete. Nuovi saggi della Regia Accademia di Scienze Lettere ed Arti in Padova, 9, 315–326.

Draft of a plate reproducing Cretaceous vertebrate remains from Veneto, taken from the materials of the unpublished work by Achille De Zigno Fossiles de la Vénétie. Biblioteca di Geoscienze dell'Università di Padova.

Many of De Zigno publications on fossil vertebrates are devoted to research on mammals, especially marine mammals, in particular sirenians.
In one of his field notebooks, De Zigno describes his excursion on Mount Zuello (now Duello) near Roncà (Verona) in August 1874, with Mr Cerato, who was a member of the family of quarrymen of Mount Bolca.

In the Bartonian (Middle Eocene) sedimentary rocks of Mount Duello, he commissioned excavations that brought to light fossil remains of sirenians, vertebrae, ribs and four skulls that he attributed to three new species of Halitherium. Most of those fossil remains belong to the new species called Halitherium veronense. De Zigno described and illustrated immediately his finds in one of his Annotazioni paleontologiche. Other publications followed that one.

Pl. XVIII from: De Zigno, Achille (1874). Annotazioni paleontologiche. Sirenii fossili trovati nel Veneto. Memorie del Reale Istituto veneto di scienze, lettere ed arti, 18, [427]-453.

Those remains were initially attributed to the genus Halitherium and later to the genus Prototherium, which is considered a primitive kind of dugong, a group that originated and then diversified in the Tethyan area.

They are the most ancient and the best-preserved fossil sirenians in the world. As well as in Veneto, remains of Prototherium were found also in Germany and in Spain.

Of the four skulls found on Mount Duello, two of them, attributed to Prototherium veronense are still kept in the Section of Geology and Paleontology of the Museum of Nature and Humankind of the University of Padua (MNU).

In the Eocene site of Mount Duello many other remains of vertebrates have been found, such as fish, reptiles (among them the extraordinary skull of the crocodile Megadontosuchus arduini MGP-PD 1Z.) and bird remains. All of them were studied by De Zigno and are now kept in the Section of Geology and Paleontology of the Museum of Nature and Humankind of the University of Padua (MNU).

Pl. I from: De Zigno, A. (1880). Sopra un cranio di coccodrillo scoperto nel terreno eoceno del veronese. Atti della Reale Accademia dei Lincei. Memorie. Classe di scienze fisiche, matematiche e naturali, 5, 65–72, 2 tavv. Biblioteca di Geoscienze dell'Università di Padova.

Draft of a plate reproducing the remains of the crocodiles of Mount Duello, taken from the materials of the unpublished work by Achille De Zigno Fossiles de la Vénétie. Biblioteca di Geoscienze dell'Università di Padova.

On Mount Duello, vertebrae of the snake Palaeophis oweni were found, among others. Palaeophis oweni is considered in a recent study (Georgalis et al., 2020) the biggest fossil snake found in Italy up to now.

Vertebra of Palaeophis oweni from Mount Duello, shown in different views, that now is kept in the Section of Geology and Paleontology of the Museum of Nature and Humankind of the University of Padua (MNU) (MGP-PD 6981Za), and the original label.

Pl. XV from: De Zigno, A. (1879). Annotazioni paleontologiche. Nuove aggiunte alla fauna eocena del Veneto. Memorie del Reale Istituto veneto di scienze, lettere ed arti, 21, [775]-789.

The drawings from no. 9 to no.11 represent the vertebrae of Palaeophis oweni.

The drawing no. 16 represents the rostrum of a Cylindracanthus rectus (MGP-PD 6982Z), again from Mount Duello. The Cylindracanthus rectus is an extinct fish, similar to a present-day Marlin. The fossil rostrum has a cylindrical-conical shape. It is almost complete. It measures 31 cm.

Draft of the plate that represents the rostrum of Cylindracanthus rectus and other fossils. The plate is part of the materials of the unpublished work by Achille De Zigno Fossiles de la Vénétie. Biblioteca di Geoscienze dell'Università di Padova.