The only certain historical mention of Marcello (the surname Tegalliano is a late-fourteenth-century invention of the chroniclers Nicolò Trevisan and Enrico Dandolo) is the Pactum Lotharii stipulated in 840 between the emperor Lothair I and the doge Pietro Tradonico.

The text mentions the so-called terminatio liutprandina, an agreement on the delimitation of the borders around Cittanova concluded under the king of the Lombards Liutprand between Duke Paoluccio and the magister militum Marcello and still in force at the time of the Eastern Roman Empire.

[2] The Pactum was repeatedly confirmed in the following centuries and was therefore well known to Giovanni Diacono, the first chronicler who, around the year 1000, identified Paoluccio and Marcello respectively as the first and second doges of Venice.

[2] Carlo Guido Mor and Stefano Gasparri hypothesized instead that Paoluccio was the Lombard duke of Treviso, while Marcello had been his interlocutor on the Venetian side.

In 723, Pope Gregory II sent a letter to the bishops of the ecclesiastical province of Venice and Istria in which he reprimanded Sereno of Aquileia about respecting the rights of Donato of Grado.