Martinez
Rodriguez
Lopez
Hernandez
Perez
Sanchez
Gomez
Ramirez
Diaz
Ortiz
Cruz
Why do so many Hispanic last names end with a Z?
-ez, -az, -is, -oz, and -es are common indicators of patronyms in Iberian languages.
Thus:
Martinez - son of Martin
Rodriguez - son of Rodrigo
Hernandez - son of Hernando
etc.
Indeed, several generations of the early kings of Navarre were named, alternately, Ramiro and Sancho, and listed as Ramiro Sanchez and Sancho Ramirez. Like Martin, John’s son becoming Martin Johnson, the patronymic ‘fossilized’ into a family name.
I was going to say something about Zorro, but I see your explanation is much better. Incidentally, my wife’s maiden name is “jacobsen” and her family name is closely related to :Jacobson" (they are of scandinavian origin) and they both have similar origin: son of Jacob.
I am Italian, not in the sense most people in the US are “Italian”, but I was actually born and raised there (I have been in the US 30 years) and I have been trying to think of similar last names in the Italian language unsuccessfully.
As I said, it has been 30 years (although I am still very fluent since I both read and converse in Italian regularly thanks to Skype and Italian media).
I think in Italian it’s a lot more subtle. For instance, I know the family name “Esposito”, originally from the Naples area, meant “Found one” or “orphan”.
Other last names imply the provenance from a family, such as “Dei medici” or “De’ Medici” meaning “coming from the Medici family” and therefore son of someone. But I cannot think of a last name that actually means “son of someone” when translated literally, it’s more like “part of that clan”.
If I am wrong, please set me straight.
There were two main ways Italian did patronymics. The first is to add di, so that Pietro’s son Marco becomes Marco di Petro. The second way, is to change the final o to i, so that he becomes Marco Petri.
I haven’t heard of any patronymics in Italian, and the Wikipedia article on patronymics doesn’t mention Italy either.
Just becauez.
Re: Italian, this site backs up the claim, which was news to me: http://www.italyworldclub.com/genealogy/surnames/
I thought all those were placename-related.
Since it’s bound to come up, we might as well get the Icelandic naming system into this thread. Still in use today, one adds -son or -dotir to the end of one’s father’s first name to get your last name (although not everyone still does this). It will be left as exercise for the reader to guess what “-dotir” means. 
A can’t make the accent symbol, but “dotir” should have an accent on the “i”.
ETA: In the OP, “Cruz” does not fit. It just means “cross” in spanish.
I seem to recall reading that this ending is of Germanic origin.
Possibly from the same source as -s names in English (a Germanic language)?
e.g.
Williams = son of William
Andrews = son of Andrew
Phillips = son of Phillip
I know a lot of Hispanic names that are ES instead of EZ
Like Martines not Martinez or Rodrigues not Rodriguez etc.
Is this just a personal preference of spelling?
-es names are usually Portuguese: Gomes, Lopes, etc.
Is that the convention in Galicia, too?
No idea on that one.
Minor note: Cruz is “cross” in Spanish and Portuguese. It comes from the Latin crux. It is equivalent to Croix in French, Croce in Italian, and Kreuz in German.
I believe it is “dóttir.”
What is Perez? Is there a name like “Pere” in Spanish?
Some of these patronymics get slightly condensed over time. “Perez” is “son of Pedro”, I believe.
My bad. And to further correct my original post, it’s really -sson and -sdottir that is added, with the first “s” being the possessive. Erik’s son => Eriksson.