Earlier, in my discussion of the anatomy of a forged decretal, I talked a bit about the dates. From the first letter of Evaristus, I said, Pseudo-Isidore concludes each of the letters he forges with a date: the day of the month, followed by the consuls who served in the year the letter was issued. Pseudo-Isidore gets his consuls' names from the Liber Pontificalis, and so I rushed passed them to consider the distribution of calendar dates, which Pseudo-Isidore invents outright. I even had a groovy chart.
In the process, I made the error of assuming that the outright fabrications of Pseudo-Isidore are somehow more significant than his borrowings. I should have been thinking about the consuls, which are far more interesting. They've been bothering me for some time, but it was only today that I made the trek down to the fourth floor of the library to unearth the Liber Pontificalis. I have now compared all the consular dates that Pseudo-Isidore gives in Part I of his decretals to the consular dates on offer in the Liber Pontificalis (in the course of which work I also found this helpful online list of Roman consuls, which has freed me from my Cappelli).I have tabulated the results in an Excel spreadhseet, and am finally ready to pass the distilled version on to you.
The Liber Pontificalis gets its dates for the early popes from the Liberian Catalogue, which provides consuls for pontificates through the time of Pope Liberius (d. 366). The compiler of the Liber Pontificalis handles this source pretty loosely. He typically gives the consuls for the beginning of pontificates, but sometimes omits the concluding dates, or vice versa (and the Liberian Catalogue is itself not entirely consistent). This means that Pseudo-Isidore is constrained to dating most of his letters to either the first year or the last year of every pontificate. Now the Liber Pontificalis enjoyed reasonably wide circulation in Carolingian Europe, so you'd think that Pseudo-Isidore would have thought twice about forging dates from its pages. How likely is it, after all, that absolutely every early pontiff should have issued absolutely every decretal* on either his first or last year in office? And because his source is so limited, less than half of the decretals in Part I of Pseudo-Isidore have unique consular dates; the remaining letters with dating clauses share their consuls with at least one other epistle. Surely that would have looked odd to any enterprising, attentive reader.
Pseudo-Isidore compounds these difficulties with remarkable carelesness. Given the paucity of information that the Liber Pontificalis provides about Roman consuls, I would have bet that our forgers would like to change their dates up as much as possible -- that is, provide different consular dates whenever possible. They do this in the letters ascribed to Denis, Eutichian, Victor, Evaristus, and Sother: Each of these popes gets two letters, and in each case the first letter has consuls from the beginning of that pope's pontificate, and the second letter has consuls from the end of it. But in three other cases (Viginus, Zepherinus and Felix), two sets of consuls are on offer and Pseudo-Isidore uses only one of them for both letters -- the first set in each case. As if to balance this out, two of the three popes who have only one letter in Pseudo-Isidore, but two sets of consul dates in the Liber Pontificalis (Gaius and Elutherius), write their decretals on their last year in office (that is, they get the second set of consuls). In the case of Anicitus, however, Pseudo-Isidore mixes the consuls, and gives us one from the beginning and one from the end of the pontificate.
Nor is that the only error. Mixed consuls occur again in a letter ascribed to Fabian, and on four further occasions in Part I (letters in the name of Alexander, Sixtus, Telesphorus and Felix), Pseudo-Isidore mistakes emperors for one or both of the consuls (the Liberian Catalogue, and thus the Liber Pontificalis, frequently supplied emperors who held power during various pontificates). Interestingly, Alexander, Felix and Fabian are also three of the four popes who contribute three letters in Part I -- everyone else getrs only one or two. It's like something goes haywire with Pseudo-Isidore's dating process when he has to deal with more than two letters in one pope's name.
A final tidbit: Some recensions of the Liber Pontificalis fail to give consular dates for Urban, and all recensions lack consular dates for Cornelius. In both cases our forgers borrow them from the preceding pope's biography -- Calixtus in Urban's case, and Fabian in Cornelius's case (where Pseudo-Isidore again provides a mixed set of consuls).
*The prefatory material claims that Pseudo-Isidore has all papal decretals from the Clement through Damascus.