27 They arrived again in Jerusalem, and while Jesus was walking in the temple courts, the chief priests, the teachers of the law and the elders came to him. 28 “By what authority are you doing these things?” they asked. “And who gave you authority to do this?”
29 Jesus replied, “I will ask you one question. Answer me, and I will tell you by what authority I am doing these things. 30 John’s baptism—was it from heaven, or of human origin? Tell me!”
31 They discussed it among themselves and said, “If we say, ‘From heaven,’ he will ask, ‘Then why didn’t you believe him?’ 32 But if we say, ‘Of human origin’ …” (They feared the people, for everyone held that John really was a prophet.)
33 So they answered Jesus, “We don’t know.”
Jesus said, “Neither will I tell you by what authority I am doing these things.”
29 Jesus replied, “I will ask you one question. Answer me, and I will tell you by what authority I am doing these things. 30 John’s baptism—was it from heaven, or of human origin? Tell me!”
31 They discussed it among themselves and said, “If we say, ‘From heaven,’ he will ask, ‘Then why didn’t you believe him?’ 32 But if we say, ‘Of human origin’ …” (They feared the people, for everyone held that John really was a prophet.)
33 So they answered Jesus, “We don’t know.”
Jesus said, “Neither will I tell you by what authority I am doing these things.”
But really he had replied, since the answer is made explicit in Matthew 12:
Then some of the Pharisees and teachers of the law said to him, “Teacher, we want to see a sign from you.”
39 He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah. 40 For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth
39 He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah. 40 For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth
(Matthew 12)
In Mark Jesus refers to the enigma "John the Baptist".
In Matthew, ceteris paribus, Jesus refers to the enigma "the sign of Jonah".
The two enigmas are one and the same: the god fish, aka Oannes.
At Babylon there was (in these times) a great resort of people of various nations, who inhabited Chaldæa, and lived in a lawless manner like the beasts of the field. In the first year there appeared, from that part of the Erythræan sea which borders upon Babylonia, an animal destitute of reason, by name Oannes, whose whole body (according to the account of Apollodorus) was that of a fish; that under the fish’s head he had another head, with feet also below, similar to those of a man, subjoined to the fish’s tail. His voice too, and language, was articulate and human; and a representation of him is preserved even to this day.
This Being was accustomed to pass the day among men; but took no food at that season; and he gave them an insight into letters and sciences, and arts of every kind. He taught them to construct cities, to found temples, to compile laws, and explained to them the principles of geometrical knowledge. He made them distinguish the seeds of the earth, and shewed them how to collect the fruits; in short, he instructed them in every thing which could tend to soften manners and humanize their lives. From that time, nothing material has been added by way of improvement to his instructions. And when the sun had set, this Being Oannes, retired again into the sea, and passed the night in the deep; for he was amphibious. After this there appeared other animals like Oannes, of which Berossus proposes to give an account when he comes to the history of the kings. Moreover Oannes wrote concerning the generation of mankind; and of their civil polity; and the following is the purport of what he said: . . . .
This Being was accustomed to pass the day among men; but took no food at that season; and he gave them an insight into letters and sciences, and arts of every kind. He taught them to construct cities, to found temples, to compile laws, and explained to them the principles of geometrical knowledge. He made them distinguish the seeds of the earth, and shewed them how to collect the fruits; in short, he instructed them in every thing which could tend to soften manners and humanize their lives. From that time, nothing material has been added by way of improvement to his instructions. And when the sun had set, this Being Oannes, retired again into the sea, and passed the night in the deep; for he was amphibious. After this there appeared other animals like Oannes, of which Berossus proposes to give an account when he comes to the history of the kings. Moreover Oannes wrote concerning the generation of mankind; and of their civil polity; and the following is the purport of what he said: . . . .
http://www.sacred-texts.com/cla/af/af02.htm